CWE

Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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Scoring CWEs

Common Weakness Scoring System (CWSS™)

The MITRE Corporation
Copyright © 2014
http://cwe.mitre.org/cwss/

CWSS version: 1.0.1

Document version: 1.0.1

Revision Date: September 5, 2014

Project Coordinator:

Bob Martin (MITRE)

Document Editor:

Steve Christey Coley (MITRE)
Table of Contents
(1) Introduction

The Common Weakness Scoring System (CWSS) provides a mechanism for prioritizing software weaknesses in a consistent, flexible, open manner. It is a collaborative, community-based effort that is addressing the needs of its stakeholders across government, academia, and industry.

Software developers often face hundreds or thousands of individual bug reports for weaknesses that are discovered in their code. In certain circumstances, a software weakness can even lead to an exploitable vulnerability. Due to this high volume of reported weaknesses, stakeholders are often forced to prioritize which issues they should investigate and fix first, often using incomplete information. In short, people need to be able to reason and communicate about the relative importance of different weaknesses. While various scoring methods are used today, they are either ad hoc or inappropriate for application to the still-imprecise evaluation of software security.

Software developers, managers, testers, security vendors and service suppliers, buyers, application vendors, and researchers must identify and assess weaknesses in software that could manifest as vulnerabilities when the software is used. They then need to be able to prioritize these weaknesses and determine which to remediate based on which of them pose the greatest risk. When there are so many weaknesses to fix, with each being scored using different scales, and often operating with incomplete information, the various community members, managers, testers, buyers, and developers are left to their own methodologies to find some way of comparing disparate weaknesses and translating them into actionable information.

Because CWSS standardizes the approach for characterizing weaknesses, users of CWSS can invoke attack surface and environmental metrics to apply contextual information that more accurately reflects the risk to the software capability, given the unique business context it will function within and the unique business capability it is meant to provide. This allows stakeholders to make more informed decisions when trying to mitigate risks posed by weaknesses.

CWSS is distinct from - but not a competitor to - the Common Vulnerability Scoring System (CVSS). These efforts have different roles, and they can be leveraged together.

CWSS offers:

  • Quantitative Measurements: CWSS provides a quantitative measurement of the unfixed weaknesses that are present within a software application.
  • Common Framework: CWSS provides a common framework for prioritizing security errors ("weaknesses") that are discovered in software applications.
  • Customized Prioritization: in conjunction with the Common Weakness Risk Analysis Framework (CWRAF), CWSS can be used by consumers to identify the most important types of weaknesses for their business domains, in order to inform their acquisition and protection activities as one part of the larger process of achieving software assurance.

1.1 What is CWSS?

CWSS is organized into three metric groups: Base Finding, Attack Surface, and Environmental. Each group contains multiple metrics - also known as factors - that are used to compute a CWSS score for a weakness.

  • Base Finding metric group: captures the inherent risk of the weakness, confidence in the accuracy of the finding, and strength of controls.
  • Attack Surface metric group: the barriers that an attacker must overcome in order to exploit the weakness.
  • Environmental metric group: characteristics of the weakness that are specific to a particular environment or operational context.

CWSS 1.0 Metric Groups

Figure 1: CWSS Metric Groups
(A larger picture is available.)

1.2 Other weakness scoring systems

Various weakness scoring systems have been used or proposed over the years. Automated tools such as source code scanners typically perform their own custom scoring; as a result, multiple tools can produce inconsistent scores for the same weakness.

The Common Vulnerability Scoring System (CVSS) is perhaps the most similar scoring system. However, it has some important limitations that make it difficult to adapt to software security assessment. A more detailed comparison is in Appendix A.

More details on other scoring systems, including adaptations to CVSS, are provided in Appendix B.

1.3 How does CWSS work?

1.3.1 Score Calculation

Each factor in the Base Finding metric group is assigned a value. These values are converted to associated weights, and a Base Finding subscore is calculated. The Base Finding subscore can range between 0 and 100. The same method is applied to the Attack Surface and Environmental metric group; their subscores can range between 0 and 1. Finally, the three subscores are multiplied together, which produces a CWSS score between 0 and 100.

CWSS 1.0 Scoring Summary

Figure 2: CWSS Scoring
(A larger picture is available.)

1.3.2 Scoring Methods within CWSS

The stakeholder community is collaborating with MITRE to investigate several different scoring methods that might need to be supported within the CWSS framework.

MethodNotes
Targeted

Score individual weaknesses that are discovered in the design or implementation of a specific ("targeted") software package, e.g. a buffer overflow in the username of an authentication routine in line 1234 of server.c in an FTP server package.

Automated tools and software security consultants use targeted methods when evaluating the security of a software package in terms of the weaknesses that are contained within the package.

Generalized

Score classes of weaknesses independent of any particular software package, in order to prioritize them relative to each other (e.g. "buffer overflows are higher priority than memory leaks"). This approach is used by the CWE/SANS Top 25, OWASP Top Ten, and similar efforts, but also by some automated code scanners.

The generalized scores could vary significantly from the targeted scores that would result from a full analysis of the individual occurrences of the weakness class within a specific software package. For example, while the class of buffer overflows remains very important to many developers, individual buffer overflow bugs might be considered less important if they cannot be directly triggered by an attacker and their impact is reduced due to OS-level protection mechanisms such as ASLR.

Context-adjusted

Modify scores in accordance with the needs of a specific analytical context that may integrate business/mission priorities, threat environments, risk tolerance, etc. These needs are captured using vignettes that link inherent characteristics of weaknesses with higher-level business considerations. This method could be applied to both targeted and generalized scoring.

Aggregated

Combine the results of multiple, lower-level weakness scores to produce a single, overall score (or "grade"). While aggregation might be most applicable to the targeted method, it could also be used in generalized scoring, as occurred in the 2010 CWE/SANS Top 25.

Note that the current focus for CWSS is on the Targeted scoring method and a framework for context-adjusted scoring. Methods for aggregated scoring will follow. Generalized scoring is being developed separately, primarily as part of the 2011 Top 25 and CWRAF.

1.4 Who performs the scoring?

CWSS scores can be automatically calculated, e.g. by a code analysis tool, or they can be manually calculated by a software security consultant or developer. Since automated analysis is not likely to have certain information available - such as the application's operating environment - CWSS scoring could possibly be conducted in multiple rounds: a tool first automatically calculates CWSS scores, then a human analyst manually adds additional details and recalculates the scores.

1.5 Who owns CWSS?

CWSS is a part of the Common Weakness Enumeration (CWE) project, co-sponsored by the Software Assurance program in the office of Cybersecurity and Communications of the U.S. Department of Homeland Security (DHS).

1.6 Who is using CWSS?

To be most effective, CWSS supports multiple usage scenarios by different stakeholders who all have an interest in a consistent scoring system for prioritizing software weaknesses that could introduce risks to products, systems, networks and services. Some of the primary stakeholders are listed below.

StakeholderDescription
Software developers Developers often operate within limited time frames, due to release cycles and limited resources. As a result, they are unable to investigate and fix every reported weakness. They may choose to concentrate on the worst problems, the easiest-to-fix. In the case of automatic weakness findings, they might choose to focus on the findings that are least likely to be false positives.
Software development managers Development managers create strategies for prioritizing and removing entire classes of weaknesses from the entire code base, or at least the portion that is deemed to be most at risk, possibly by defining custom "Top-N" lists. They must understand the security implications of integrating third-party software, which may contain its own weaknesses. They may need to support distinct security requirements and prioritization for each product line.
Software acquirers Customers, including acquisition personnel, want to obtain third-party software with a reasonable level of assurance that the software provider has performed due diligence in removing or avoiding weaknesses that are most critical to the acquirer's business and mission. Related stakeholders include CIOs, CSOs, system administrators, and end users of the software.
Enterprise security managers Enterprise security managers seek to minimize risk within their enterprise, both for well-known vulnerabilities in third-party products, as well as vulnerabilities (or weaknesses) in their own in-house software. They may wish to use a scoring mechanism that can be integrated with other security management processes, such as combining third-party vulnerability scanning results (for known third-party vulnerabilities) with custom application analysis (for in-house software) to help assess the overall risk to an asset.
Code analysis vendors and consultants Vendors and consultants often have their own custom scoring techniques, but they want to provide a consistent, community-vetted scoring mechanism for different customers.
Evaluators of code analysis capabilities Evaluators analyze and measure the capabilities of code analysis techniques (e.g., NIST SAMATE). They could use a consistent weakness scoring mechanism to support sampling of reported findings, as well as understanding the severity of these findings without depending on ad hoc scoring methods that may vary widely by tool/technique.
Other stakeholders Other stakeholders may include vulnerability researchers, advocates of secure development, and compliance-based analysts (e.g., PCI DSS).

As of July 2014 (when CWSS 0.8 was active), there are several real-world implementations of CWSS. The primary users have been code analysis vendors and software security consultants.

(2) Metric Groups

2.1 Metric Group Factors

CWSS contains the following factors, organized based on their metric group:

GroupNameSummary
Base Finding Technical Impact (TI) The potential result that can be produced by the weakness, assuming that the weakness can be successfully reached and exploited.
Base Finding Acquired Privilege (AP) The type of privileges that are obtained by an attacker who can successfully exploit the weakness.
Base Finding Acquired Privilege Layer (AL) The operational layer to which the attacker gains privileges by successfully exploiting the weakness.
Base Finding Internal Control Effectiveness (IC) the ability of the control to render the weakness unable to be exploited by an attacker.
Base Finding Finding Confidence (FC) the confidence that the reported issue is a weakness that can be utilized by an attacker
Attack Surface Required Privilege (RP) The type of privileges that an attacker must already have in order to reach the code/functionality that contains the weakness.
Attack Surface Required Privilege Layer (RL) The operational layer to which the attacker must have privileges in order to attempt to attack the weakness.
Attack Surface Access Vector (AV) The channel through which an attacker must communicate to reach the code or functionality that contains the weakness.
Attack Surface Authentication Strength (AS) The strength of the authentication routine that protects the code/functionality that contains the weakness.
Attack Surface Level of Interaction (IN) the actions that are required by the human victim(s) to enable a successful attack to take place.
Attack Surface Deployment Scope (SC) Whether the weakness is present in all deployable instances of the software, or if it is limited to a subset of platforms and/or configurations.
Environmental Business Impact (BI) The potential impact to the business or mission if the weakness can be successfully exploited.
Environmental Likelihood of Discovery (DI) The likelihood that an attacker can discover the weakness
Environmental Likelihood of Exploit (EX) the likelihood that, if the weakness is discovered, an attacker with the required privileges/authentication/access would be able to successfully exploit it.
Environmental External Control Effectiveness (EC) the capability of controls or mitigations outside of the software that may render the weakness more difficult for an attacker to reach and/or trigger.
Environmental Prevalence (P) How frequently this type of weakness appears in software.

Each factor is described in more detail in subsequent sections.

2.2 Values for Uncertainty and Flexibility

CWSS can be used in cases where there is little information at first, but the quality of information can improve over time. It is anticipated that in many use-cases, the CWSS score for an individual weakness finding may change frequently, as more information is discovered. Different entities may determine separate factors at different points in time.

As such, every CWSS factor effectively has "environmental" or "temporal" characteristics, so it is not particularly useful to adopt the same types of metric groups as are used in CVSS.

Most factors have these four values in common:

ValueUsage
Unknown

The entity calculating the score does not have enough information to provide a value for the factor. This can be a signal for further investigation. For example, an automated code scanner might be able to find certain weaknesses, but be unable to detect whether any authentication mechanisms are in place.

The use of "Unknown" emphasizes that the score is incomplete or estimated, and further analysis may be necessary. This makes it easier to model incomplete information, and for the Business Value Context to influence final scores that were generated using incomplete information.

The weight for this value is 0.5 for all factors, which generally produces a lower score; the addition of new information (i.e., changing some factors from "Unknown" to another value) will then adjust the score upward or downward based on the new information.

Not Applicable

The factor is being explicitly ignored in the score calculation. This effectively allows the Business Value Context to dictate whether a factor is relevant to the final score. For example, a customer-focused CWSS scoring method might ignore the remediation effort, and a high-assurance environment might require investigation of all reported findings, even if there is low confidence in their accuracy.

For a set of weakness findings for an individual software package, it is expected that all findings would have the same "Not Applicable" value for the factor that is being ignored.

Quantified

The factor can be weighted using a quantified, continuous range of 0.0 through 1.0, instead of the factor's defined set of discrete values. Not all factors are quantifiable in this way, but it allows for additional customization of the metric.

Default

The factor's weight can be set to a default value. Labeling the factor as a default allows for investigation and possible modification at a later time.

2.3 Base Finding Metric Group

The Base Finding metric group consists of the following factors:

  • Technical Impact (TI)
  • Acquired Privilege (AP)
  • Acquired Privilege Layer (AL)
  • Internal Control Effectiveness (IC)
  • Finding Confidence (FC)

The combination of values from Technical Impact, Acquired Privilege, and Acquired Privilege Layer gives the user some expressive power. For example, the user can characterize "High" Technical Impact with "Administrator" privilege at the "Application" layer.

2.3.1 Technical Impact (TI)

Technical Impact is the potential result that can be produced by the weakness, assuming that the weakness can be successfully reached and exploited. This is expressed in terms that are more fine-grained than confidentiality, integrity, and availability.

The Technical Impact should be evaluated relative to the Acquired Privilege (AP) and Acquired Privilege Layer (AL).

ValueCodeWeightDescription
Critical C 1.0 Complete control over the software being analyzed, to the point where operations cannot take place.
High H 0.9 Significant control over the software being analyzed, or access to critical information can be obtained.
Medium M 0.6 Moderate control over the software being analyzed, or access to moderately important information can be obtained.
Low L 0.3 Minimal control over the software being analyzed, or only access to relatively unimportant information can be obtained.
None N 0.0 There is no technical impact to the software being analyzed at all. In other words, this does not lead to a vulnerability.
Default D 0.6 The Default weight is the median of the weights for Critical, High, Medium, Low, and None.
Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses. This factor might not be applicable in an environment with high assurance requirements; the user might want to investigate every weakness finding of interest, regardless of confidence.
Quantified Q This factor could be quantified with custom weights.

If this set of values is not precise enough, CWSS users can use their own Quantified methods to derive a subscore. One such method involves using the Common Weakness Risk Analysis Framework (CWRAF) to define a vignette and a Technical Impact Scorecard. The Impact weight is calculated using vignette-specific Importance ratings for different technical impacts that could arise from exploitation of the weakness, such as modification of sensitive data, gaining privileges, resource consumption, etc.

2.3.2 Acquired Privilege (AP)

The Acquired Privilege identifies the type of privileges that are obtained by an attacker who can successfully exploit the weakness.

Notice that the values are the same as those for Required Privilege, but the weights are different.

In some cases, the value for Acquired Privileges may be the same as for Required Privileges, which implies either (1) "horizontal" privilege escalation (e.g. from one unprivileged user to another) or (2) privilege escalation within a sandbox, such as an FTP-only user who can escape to the shell.

ValueCode*WeightDescription
Administrator A 1.0

The attacker gains access to an entity with administrator, root, SYSTEM, or equivalent privileges that imply full control over the software under analysis; or, the attacker can raise their own (lower) privileges to an administrator.

Partially-Privileged User P 0.9

The attacker gains access to an entity with some special privileges, but not enough privileges that are equivalent to an administrator; or, the attacker can raise their own (lower) privileges to a partially-privileged user. For example, a user might have privileges to make backups, but not to modify the software's configuration or install updates.

Regular User RU 0.7

The attacker gains access to an entity that is a regular user who has no special privileges; or, the attacker can raise their own (lower) privileges to that of a regular user.

Limited / Guest L 0.6

The attacker gains access to an entity with limited or "guest" privileges that can significantly restrict allowable activities; or, the attacker can raise their own (lower) privileges to a guest. Note: this value does not refer to the "guest operating system" concept in virtualized hosts.

None N 0.1

The attacker cannot gain access to any extra privileges beyond those that are already available to the attacker. (Note that this value can be useful in limited circumstances in which the attacker can escape a sandbox or other restrictive environment but still cannot gain extra privileges, or obtain access as other users.)

Default D 0.7

Median of the weights for None, Guest, Regular User, Partially-Privileged User, and Administrator.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0

This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses. This factor might not be applicable in an environment with high assurance requirements that wants strict enforcement of privilege separation, even between already-privileged users.

Quantified Q

This factor could be quantified with custom weights. Note that Quantified values are supported for completeness; however, since privileges and users are discrete entities, there might be limited circumstances in which a quantified model would be useful.

* A mnemonic for the main values in this factor is "RUNLAP" (Regular User, None, Limited, Admin, Partially-Privileged).

2.3.3 Acquired Privilege Layer (AL)

The Acquired Privilege Layer identifies the operational layer to which the attacker gains privileges by successfully exploiting the weakness.

ValueCode*WeightDescription
Application A 1.0

The attacker acquires privileges that are supported within the software under analysis itself. (If the software under analysis is an essential part of the underlying system, such as an operating system kernel, then the System value may be more appropriate.)

System S 0.9

The attacker acquires privileges to the underlying system or physical host that is being used to run the software under analysis.

Network N 0.7

The attacker acquires privileges to access the network.

Enterprise Infrastructure E 1.0

The attacker acquires privileges to a critical piece of enterprise infrastructure, such as a router, switch, DNS, domain controller, firewall, identity server, etc.

Default D 0.9

Median of the weights for Application, System, Network, and Enterprise Infrastructure.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.

This factor might not be applicable in an environment with high assurance requirements that wants strict enforcement of privilege separation, even between already-privileged users.

Quantified Q

This factor could be quantified with custom weights. Note that Quantified values are supported for completeness; however, since privilege layers are discrete entities, there might be limited circumstances in which a quantified model would be useful.

* A mnemonic for the main values in this factor is "SANE" (System, Application, Network, Enterprise Infrastructure).

2.3.4 Internal Control Effectiveness (IC)

An Internal Control is a control, protection mechanism, or mitigation that has been explicitly built into the software (whether through architecture, design, or implementation). Internal Control Effectiveness measures the ability of the control to render the weakness unable to be exploited by an attacker. For example, an input validation routine that restricts input length to 15 characters might be moderately effective against XSS attacks by reducing the size of the XSS exploit that can be attempted.

When there are multiple internal controls, or multiple code paths that can reach the same weakness, then the following guidance applies:

  • For each code path, analyze each internal control that exists along the code path, and choose the Value with the lowest weight (i.e., the strongest internal control along the code path). This is called the Code Path Value.
  • Collect all Code Path Values.
  • Select the Code Path Value that has the highest weight (i.e., is the weakest control).

This method evaluates each code path in terms of the code path's strongest control (since an attacker would have to bypass that control), then selects the weakest code path (i.e., the easiest route for the attacker to take).

ValueCodeWeightDescription
None N 1.0

No controls exist.

Limited L 0.9

There are simplistic methods or accidental restrictions that might prevent a casual attacker from exploiting the issue.

Moderate M 0.7

The protection mechanism is commonly used but has known limitations that might be bypassed with some effort by a knowledgeable attacker. For example, the use of HTML entity encoding to prevent XSS attacks may be bypassed when the output is placed into another context such as a Cascading Style Sheet or HTML tag attribute.

Indirect (Defense-in-Depth) I 0.5

The control does not specifically protect against exploitation of the weakness, but it indirectly reduces the impact when a successful attack is launched, or otherwise makes it more difficult to construct a functional exploit. For example, a validation routine might indirectly limit the size of an input, which might make it difficult for an attacker to construct a payload for an XSS or SQL injection attack.

Best-Available B 0.3

The control follows best current practices, although it may have some limitations that can be overcome by a skilled, determined attacker, possibly requiring the presence of other weaknesses. For example, the double-submit method for CSRF protection is considered one of the strongest available, but it can be defeated in conjunction with behaviors of certain functionality that can read raw HTTP headers.

Complete C 0.0

The control is completely effective against the weakness, i.e., there is no bug or vulnerability, and no adverse consequence of exploiting the issue. For example, a buffer copy operation that ensures that the destination buffer is always larger than the source (plus any indirect expansion of the original source size) will not cause an overflow.

Default D 0.6

Median of the weights for Complete, Best-Available, Indirect, Moderate, Limited, and None.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.
Quantified Q This factor could be quantified with custom weights.

2.3.5 Finding Confidence (FC)

Finding Confidence is the confidence that the reported issue:

  1. is a weakness, and
  2. can be triggered or utilized by an attacker

ValueCodeWeightDescription
Proven True T 1.0

The weakness is reachable by the attacker.

Proven Locally True LT 0.8

The weakness occurs within an individual function or component whose design relies on safe invocation of that function, but attacker reachability to that function is unknown or not present. For example, a utility function might construct a database query without encoding its inputs, but if it is only called with constant strings, the finding is locally true.

Proven False F 0.0

The finding is erroneous (i.e. the finding is a false positive and there is no weakness), and/or there is no possible attacker role.

Default D 0.8

Median of the weights for Proven True, Proven Locally True, and Proven False.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.

This factor might not be applicable in an environment with high assurance requirements; the user might want to investigate every weakness finding of interest, regardless of confidence.

Quantified Q

This factor could be quantified with custom weights. Some code analysis tools have precise measurements of the accuracy of specific detection patterns.

2.4 Attack Surface Metric Group

The Attack Surface metric group consists of the following factors:

  • Required Privilege (RP)
  • Required Privilege Layer (RL)
  • Access Vector (AV)
  • Authentication Strength (AS)
  • Level of Interaction (IN)
  • Deployment Scope (SC)

2.4.1 Required Privilege (RP)

The Required Privilege identifies the type of privileges that an attacker must already have in order to reach the code/functionality that contains the weakness.

ValueCode*WeightDescription
None N 1.0

No privileges are required. For example, a web-based search engine may not require any privileges for an entity to enter a search term and view results.

Limited / Guest L 0.9

The entity has limited or "guest" privileges that can significantly restrict allowed activities; the entity might be able to register or create a new account without any special requirements or proof of identity. For example, a web blog might allow participants to create a user name and submit a valid email address before entering comments. Note: this value does not refer to the "guest operating system" concept in virtualized hosts.

Regular User RU 0.7

The entity is a regular user who has no special privileges.

Partially-Privileged User P 0.6

The entity is a valid user with some special privileges, but not enough privileges that are equivalent to an administrator. For example, a user might have privileges to make backups, but not to modify the software's configuration or install updates.

Administrator A 0.1

The entity has administrator, root, SYSTEM, or equivalent privileges that imply full control over the software or the underlying OS.

Default D 0.7

Median of the weights for None, Limited, Regular User, Partially-Privileged User, and Administrator.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.

This factor might not be applicable in an environment with high assurance requirements that wants strict enforcement of privilege separation, even between already-privileged users.

Quantified Q

This factor could be quantified with custom weights. Note that Quantified values are supported for completeness; however, since privileges and users are discrete entities, there might be limited circumstances in which a quantified model would be useful.

* A mnemonic for the main values in this factor is "RUNLAP" (Regular User, None, Limited, Admin, Partially-Privileged).

2.4.2 Required Privilege Layer (RL)

The Required Privilege Layer identifies the operational layer to which the attacker must have privileges in order to attempt to attack the weakness.

ValueCode*WeightDescription
Application A 1.0

The attacker must have privileges that are supported within the software under analysis itself. (If the software under analysis is an essential part of the underlying system, such as an operating system kernel, then the System value may be more appropriate.)

System S 0.9

The attacker must have privileges to the underlying system or physical host that is being used to run the software under analysis.

Network N 0.7

The attacker must have privileges to access the network.

Enterprise Infrastructure E 1.0

The attacker must have privileges on a critical piece of enterprise infrastructure, such as a router, switch, DNS, domain controller, firewall, identity server, etc.

Default D 0.9

Median of the weights for Application, System, Network, and Enterprise Infrastructure.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.

This factor might not be applicable in an environment with high assurance requirements that wants strict enforcement of privilege separation, even between already-privileged users.

Quantified Q

This factor could be quantified with custom weights. Note that Quantified values are supported for completeness; however, since privilege layers are discrete entities, there might be limited circumstances in which a quantified model would be useful.

* A mnemonic for the main values in this factor is "SANE" (System, Application, Network, Enterprise Infrastructure).

2.4.3 Access Vector (AV)

The Access Vector identifies the channel through which an attacker must communicate to reach the code or functionality that contains the weakness. Note that these values are very similar to the ones used in CVSS, except CWSS distinguishes between physical access and local (shell/account) access.

While there is a close relationship between Access Vector and Required Privilege Layer, the two are distinct. For example, an attacker with "physical" access to a router might be able to affect the Network or Enterprise layer.

ValueCodeWeightDescription
Internet I 1.0

An attacker must have access to the Internet to reach the weakness.

Intranet R 0.8

An attacker must have access to an enterprise intranet that is shielded from direct access from the Internet, e.g. by using a firewall, but otherwise the intranet is accessible to most members of the enterprise.

Private Network V 0.8

An attacker must have access to a private network that is only accessible to a narrowly-defined set of trusted parties.

Adjacent Network A 0.7

An attacker must have access to a physical interface to the network, such as the broadcast or collision domain of the vulnerable software. Examples of local networks include local IP subnet, Bluetooth, IEEE 802.11, and local Ethernet segment.

Local L 0.5

The attacker must have an interactive, local (shell) account that interfaces directly with the underlying operating system.

Physical P 0.2

The attacker must have physical access to the system that the software runs on, or otherwise able to interact with the system via interfaces such as USB, CD, keyboard, mouse, etc.

Default D 0.75

Median of weights for relevant values.

Unknown U 0.5
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.
Quantified Q

This factor could be quantified with custom weights. Note that Quantified values are supported for completeness; however, since access vectors are discrete entities, there might be limited circumstances in which a quantified model would be useful.

2.4.4 Authentication Strength (AS)

The Authentication Strength covers the strength of the authentication routine that protects the code/functionality that contains the weakness.

When there are multiple authentication routine, or multiple code paths that can reach the same weakness, then the following guidance applies:

  • For each code path, analyze each authentication routine that exists along the code path, and choose the Value with the lowest weight (i.e., the strongest authentication routine along the code path). This is called the Code Path Value.
  • Collect all Code Path Values.
  • Select the Code Path Value that has the highest weight (i.e., contains the weakest routine).

This method evaluates each code path in terms of the code path's strongest authentication routine (since an attacker would have to bypass that control), then selects the weakest code path (i.e., the easiest route for the attacker to take).

ValueCodeWeightDescription
Strong S 0.7

The weakness requires strongest-available methods to tie the entity to a real-world identity, such as hardware-based tokens, and/or multi-factor authentication.

Moderate M 0.8

The weakness requires authentication using moderately strong methods, such as the use of certificates from untrusted authorities, knowledge-based authentication, or one-time passwords.

Weak W 0.9

The weakness requires a simple, weak authentication method that is easily compromised using spoofing, dictionary, or replay attacks, such as a static password.

None N 1.0

The weakness does not require any authentication at all.

Default D 0.85

Median of values for Strong, Moderate, Weak, and None.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.

This might not be applicable in an environment with high assurance requirements that seek to eliminate all weaknesses.

Quantified Q

This factor could be quantified with custom weights.

2.4.5 Level of Interaction (IN)

The Level of Interaction covers the actions that are required by the human victim(s) to enable a successful attack to take place.

ValueCodeWeightDescription
Automated A 1.0

No human interaction is required.

Typical/Limited T 0.9

The attacker must convince the user to perform an action that is common or regarded as "normal" within typical product operation. For example, clicking on a link in a web page, or previewing the body of an email, is common behavior.

Moderate M 0.8

The attacker must convince the user to perform an action that might appear suspicious to a cautious, knowledgeable user. For example: the user has to accept a warning that suggests the attacker's payload might contain dangerous content.

Opportunistic O 0.3

The attacker cannot directly control or influence the victim, and can only passively capitalize on mistakes or actions of others.

High H 0.1

A large amount of social engineering is required, possibly including ignorance or negligence on the part of the victim.

No interaction NI 0.0

There is no interaction possible, not even opportunistically; this typically would render the weakness as a "bug" instead of leading to a vulnerability. Since CWSS is for security, the weight is 0.

Default D 0.55

Median of values for Automated, Limited, Moderate, Opportunistic, High, and No interaction.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.

This might not be applicable in an environment with high assurance requirements, or an environment that has high concerns about insider attacks between people with an established trust relationship.

Quantified Q

This factor could be quantified with custom weights.

2.4.6 Deployment Scope (SC)

Deployment Scope identifies whether the weakness is present in all deployable instances of the software, or if it is limited to a subset of platforms and/or configurations. For example, a numeric calculation error might only be applicable for software that is running under a particular OS and a 64-bit architecture, or a path traversal issue might only affect operating systems for which "\" is treated as a directory separator.

ValueCode*WeightDescription
All A 1.0

Present in all platforms or configurations

Moderate M 0.9

Present in common platforms or configurations

Rare R 0.5

Only present in rare platforms or configurations

Potentially Reachable P 0.1

Potentially reachable**, but all code paths are currently safe, and/or the weakness is in dead code

Default D 0.7

The median of weights for RAMP values

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.
Quantified Q

This factor could be quantified with custom weights. The user may know what percentage of shipped (or supported) software contains this bug.

* A mnemonic for the main values in this factor is "RAMP" (Rare, All, Moderate, Potentially Reachable).

** "Potentially Reachable" has some overlap with "Locally True" in the Finding Confidence (FC) factor.

2.5 Environmental Metric Group

The Environmental metric group consists of the following factors:

  • Business Impact (BI)
  • Likelihood of Discovery (DI)
  • Likelihood of Exploit (EX)
  • External Control Effectiveness (EC)
  • Prevalence (P)

2.5.1 Business Impact (BI)

Business Impact describes the potential impact to the business or mission if the weakness can be successfully exploited.*

ValueCodeWeightDescription
Critical C 1.0

The business/mission could fail.

High H 0.9

The operations of the business/mission would be significantly affected.

Medium M 0.6

The business/mission would be affected, but without extensive damage to regular operations.

Low L 0.3

Minimal impact on the business/mission.

None N 0.0 No impact.
Default D 0.6

The median of weights for Critical, High, Medium, Low, and None.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.

This factor might not be applicable in contexts in which the business impact is irrelevant, or if the impact is being assessed and considered in analytical processes that are outside of the CWSS score itself.

Quantified Q This factor could be quantified with custom weights. Some organizations might have specific measurements for the business value of the asset, for example, which could be integrated into this measurement.

* Since business concerns vary widely across organizations, CWSS 1.0 does not attempt to provide a more precise breakdown, e.g. in terms of financial, reputational, physical, legal, or other types of damage. This factor can be quantified to support any externally-defined models.

2.5.2 Likelihood of Discovery (DI)

Likelihood of Discovery* is the likelihood that an attacker can discover the weakness.

ValueCodeWeightDescription
High H 1.0

It is very likely that an attacker can discover the weakness quickly and with little effort using simple techniques, without access to source code or other artifacts that simplify weakness detection.

Medium M 0.6

An attacker might be able to discover the weakness, but would require certain skills to do so, possibly requiring source code access or reverse engineering knowledge. It may require some time investment to find the issue.

Low L 0.2

An attacker is unlikely to discover the weakness without highly specialized skills, access to source code (or its equivalent), and a large time investment.

Default D 0.6

The median of the High, Medium, and Low values.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.

This might not be applicable when the scorer assumes that all weaknesses will be discovered by an attacker.

Quantified Q

This factor could be quantified with custom weights.

* This factor was considered for removal in CWSS 1.0, since it can be difficult to measure and can be influenced by other factors such as Acquired Privilege, Technical Impact, and Prevalence. However, it has been preserved to reflect that some developers will use likelihood of discovery to help prioritize how quickly an issue should be fixed.

2.5.3 Likelihood of Exploit (EX)

Likelihood of Exploit is the likelihood that, if the weakness is discovered, an attacker with the required privileges/authentication/access would be able to successfully exploit it.

ValueCodeWeightDescription
High H 1.0

It is highly likely that an attacker would target this weakness successfully, with a reliable exploit that is easy to develop.

Medium M 0.6

An attacker would probably target this weakness successfully, but the chances of success might vary, or require multiple attempts to succeed.

Low L 0.2

An attacker probably would not target this weakness, or could have very limited chances of success.

None N 0.0

An attacker has no chance of success; i.e., the issue is a "bug" because there is no attacker role, and no benefit to the attacker.

Default D 0.6

Median of the High, Medium, and Low values. The "None" value is ignored with the expectation that few weakness findings would be scored using the value, and including it in the median calculation would reduce the weight to a non-intuitive level.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.

For example, the scorer might want to assume that attackers could exploit any weakness they can find, or be willing to invest significant resources to work around any possible barriers to exploit success.

Quantified Q

This factor could be quantified with custom weights.

Note that this factor is influenced by the Impact of a weakness, since attackers often target weaknesses that have the most severe impacts. Alternately, they may target weaknesses that are easy to trigger. It is also influenced by other factors such as the effectiveness of internal and external controls.

It might seem that the prevalence is also an influence, but prevalence is more closely related to Likelihood of Discovery.

2.5.4 External Control Effectiveness (EC)

External Control Effectiveness is the capability of controls or mitigations outside of the software that may render the weakness more difficult for an attacker to reach and/or trigger. For example, Address Space Layout Randomization (ASLR) and similar technologies reduce, but do not eliminate, the chances of success for a buffer overflow attack. However, ASLR is not directly instantiated within the software itself.

When there are multiple external controls, or multiple code paths that can reach the same weakness, then the following guidance applies:

  • For each code path, analyze each external control that exists along the code path, and choose the Value with the lowest weight (i.e., the strongest external control along the code path). This is called the Code Path Value.
  • Collect all Code Path Values.
  • Select the Code Path Value that has the highest weight (i.e., is the weakest control).

This method evaluates each code path in terms of the code path's strongest control (since an attacker would have to bypass that control), then selects the weakest code path (i.e., the easiest route for the attacker to take).

ValueCodeWeightDescription
None N 1.0 No controls exist.
Limited L 0.9

There are simplistic methods or accidental restrictions that might prevent a casual attacker from exploiting the issue.

Moderate M 0.7

The protection mechanism is commonly used but has known limitations that might be bypassed with some effort by a knowledgeable attacker.

Indirect (Defense-in-Depth) I 0.5

The control does not specifically protect against exploitation of the weakness, but it indirectly reduces the impact when a successful attack is launched, or otherwise makes it more difficult to construct a functional exploit.

For example, Address Space Layout Randomization (ASLR) and similar technologies reduce, but do not eliminate, the chances of success in a buffer overflow attack. Since the response is typically to exit the process, the result is still a denial of service.

Best-Available B 0.3

The control follows best current practices, although it may have some limitations that can be overcome by a skilled, determined attacker, possibly requiring the presence of other weaknesses. For example, Transport Layer Security (TLS) / SSL 3 are in operation throughout much of the Web, and stronger methods generally are not available due to compatibility issues.

Complete C 0.1

The control is completely effective against the weakness, i.e., there is no bug or vulnerability, and no adverse consequence of exploiting the issue. For example, a sandbox environment might restrict file access operations to a single working directory, which would protect against exploitation of path traversal.

A non-zero weight is used to reflect the possibility that the external control could be accidentally removed in the future, e.g. if the software's environment changes.

Default D 0.6

The median of Complete, Best-Available, Indirect, Moderate, Limited, and None.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.
Quantified Q This factor could be quantified with custom weights.

2.5.5 Prevalence (P)

The Prevalence* of a finding identifies how frequently this type of weakness appears in software.

This factor is intended for use in Generalized scoring of classes of weaknesses, such as the development of custom Top-N weakness lists. When scoring an individual weakness finding in an automated-scanning context, this factor is likely to use a "Not Applicable" value.

ValueCodeWeight**Description
Widespread W 1.0

The weakness is found in most or all software in the associated environment, and may occur multiple times within the same software package.

High H 0.9

The weakness is encountered very often, but it is not widespread.

Common C 0.8

The weakness is encountered periodically.

Limited L 0.7

The weakness is encountered rarely, or never.

Default D 0.85

The median of Limited, Common, High, and Widespread.

Unknown UK 0.5 There is not enough information to provide a value for this factor. Further analysis may be necessary. In the future, a different value might be chosen, which could affect the score.
Not Applicable NA 1.0 This factor is being intentionally ignored in the score calculation because it is not relevant to how the scorer prioritizes weaknesses.

When performing targeted scoring against specific weakness findings in an application, Prevalence is normally expected to be irrelevant, since the individual application and the analytical techniques determine how frequently the weakness occurs, and many aggregated scoring methods will generate larger scores if there are more weaknesses.

Quantified Q

This factor could be quantified with custom weights. Precise prevalence data may be available within limited use cases, provided the user is tracking weakness data at a low level of granularity. For example, a developer may be tracking weaknesses across a suite of products, or a code-auditing vendor could measure prevalence from the software analyzed across the entire customer base. In a previous version of CWSS, prevalence was calculated based on from raw voting data that was collected for the 2010 Top 25, which used discrete values (range 1 to 4) which were then adjusted to a 1-to-10 range.

* Note that this factor might be considered for removal in future versions. However, it is too closely tied to Generalized scoring methods and CWRAF to be removed within CWSS 1.0.

** Since software can be successfully attacked even in the presence of a single weakness, the selected weights do not provide significant distinction between each other.

(3) CWSS Score Formula

A CWSS 1.0 score can range between 0 and 100. It is calculated as follows:

BaseFindingSubscore * AttackSurfaceSubscore * EnvironmentSubscore

The BaseFindingSubscore supports values between 0 and 100. Both the AttackSurfaceSubscore and EnvironmentSubscore support values between 0 and 1.

3.1 Base Finding Subscore

The Base Finding subscore (BaseFindingSubscore) is calculated as follows:

Base = [ (10 * TechnicalImpact + 5*(AcquiredPrivilege + AcquiredPrivilegeLayer) + 5*FindingConfidence) * f(TechnicalImpact) * InternalControlEffectiveness ] * 4.0

f(TechnicalImpact) = 0 if TechnicalImpact = 0; otherwise f(TechnicalImpact) = 1.

The maximum potential BaseFindingSubscore is 100.

The definition of f(TechnicalImpact) has an equivalent in CVSS. It is used to ensure that if the Technical Impact is 0, that the other added factors do not inadvertently generate a non-zero score.

TechnicalImpact and the AcquiredPrivilege/AcquiredPrivilegeLayer combination are given equal weight, each accounting for 40% of the BaseFindingSubscore. (Each generate a sub-value with a maximum of 10). There is some adjustment for Finding Confidence, which accounts for 20% of the Base (maximum of 5). The InternalControlEffectiveness can adjust the score downward, perhaps to 0, depending on the strength of any internal controls that have been applied to the issue. After application of InternalControlEffectiveness, the possible range of results is between 0 and 25, so the 4.0 coefficient is used to adjust the BaseFindingSubscore to a range between 0 and 100.

3.2 Attack Surface Subscore

The AttackSurfaceSubscore is calculated as:

[ 20*(RequiredPrivilege + RequiredPrivilegeLayer + AccessVector) + 20*DeploymentScope + 15*LevelOfInteraction + 5*AuthenticationStrength ] / 100.0

The combination of required privileges / access makes up 60% of the Attack Surface subscore; deployment scope, another 20%; interaction, 15%; and authentication, 5%. The authentication requirements are not given much focus, under the assumption that strong proof of identity will not significantly deter an attacker from attempting to exploit the vulnerability.

This generates a range of values between 0 and 100, which are then divided by 100.

3.3 Environmental Subscore

The EnvironmentalSubscore is calculated as:

[ (10*BusinessImpact + 3*LikelihoodOfDiscovery + 4*LikelihoodOfExploit + 3*Prevalence) * f(BusinessImpact) * ExternalControlEffectiveness ] / 20.0

f(BusinessImpact) = 0 if BusinessImpact == 0; otherwise f(BusinessImpact) = 1

BusinessImpact accounts for 50% of the environmental score, and it can move the final score to 0. ExternalControlEffectiveness is always non-zero (to account for the risk that it can be inadvertently removed if the environment changes), but otherwise it can have major impact on the final score. The combination of LikelihoodOfDiscovery and LikelihoodOfExploit accounts for 35% of the score, and Prevalence at 15%.

3.4 Additional Features of the Formula

There is significant diversity in the kinds of scores that can be represented, although the use of multiplication of many different factors, combined with multiple weights with small values, means that the range of potential scores is somewhat skewed towards lower values.

Since "Not Applicable" values have a weight of 1, the formula always has a potential maximum score of 100.0. In extremely rare cases in which certain factors are treated as Not Applicable (e.g., Technical Impact, Business Impact, and Internal Control Effectiveness), then the minimum possible score could be non-zero.

When default values are used for a large number of factors for a single score, using the median weights as defined in CWSS 1.0, the scores will skew heavily to the low side. The median weight for a factor does not necessarily reflect the most likely value that could be used, so the selection of Default weights may be changed in future versions. Ideally, the formula would have a property in which the use of many default values produces a score that is relatively close to 50; the selection of non-default values would adjust the final score upward or downward, thereby increasing precision.

The use of "Unknown" values also generally produces scores that skew to the low side. This might be a useful feature, since scores will be higher if they have more specific information.

(4) CWSS Vectors, Scoring Examples, and Score Portability

Using the Codes as specified for each factor, a CWSS score can be stored in a compact, machine-parsable, human-readable format that provides the details for how the score was generated. This is very similar to how CVSS vectors are constructed.

Unlike CVSS, not all CWSS factors can be described symbolically with discrete values. Any factor can be quantified with continuous weights that override the originally-defined default discrete values, using the "Q" value. When calculated using CWRAF, the Impact factor is effectively an expression of 32 separate Technical Impacts and layers, many of which would not be applicable to a particular weakness. Treating each impact as a separate factor would roughly double the number of factors required to calculate a CWSS score. In addition, CWRAF's use of Business Value Context (BVC) to adjust scores for business-specific concerns also means that a CWSS score and its vector may appear to be inconsistent if they are "transported" to other domains or vignettes.

With this concern in mind, a CWSS 1.0 vector should explicitly list the weights for each factor, even though it increases the size of the vector representation.

The format of a single factor in a CWSS vector is:

FactorName:Value,Weight

For example, "P:NA,1.0" specifies a "Not Applicable" value for Prevalence with a weight of 1.0. A specifier of "AV:P,0.2" indicates the "Physical" value for Access Vector with a weight of 0.2.

Factors are separated by forward slash characters, such as:

AV:I,1.0/RP:L,0.9/AS:N,1.0

which lists values and weights for "AV" (Access Vector), "RP" (Required Privilege Level), and "AS" (Authentication Strength).

If a CWSS vector is provided that does not list the actual weights for a value, then an implementation should report a possible error or inconsistency, try to infer the CWSS version based on the vector's factors and values, re-calculate the CWSS score based on the inferred version, and compare this to the original score. If the scores are inconsistent, the implementation should report a possible error or inconsistency.

4.1 Example: Business-critical application

Consider a reported weakness in which an application is the primary source of income for a company, thus has critical business value. The application allows arbitrary Internet users to sign up for an account using only an email address. A user can then exploit the weakness to obtain administrator privileges for the application, but the attack cannot succeed until the administrator views a report of recent user activities - a common occurrence. The attacker cannot take complete control over the application, but can delete its users and data. Suppose further that there are no controls to prevent the weakness, but the fix for the issue is simple, and limited to a few lines of code.

This situation could be captured in the following CWSS vector:

(TI:H,0.9/AP:A,1.0/AL:A,1.0/IC:N,1.0/FC:T,1.0/

RP:L,0.9/RL:A,1.0/AV:I,1.0/AS:N,1.0/IN:T,0.9/SC:A,1.0/

BI:C,0.9/DI:H,1.0/EX:H,1.0/EC:N,1.0/P:NA,1.0)

The vector has been split into multiple lines for readability. Each line represents a metric group.

The factors and values are as follows:

FactorValue
Technical Impact High
Acquired Privilege Administrator
Acquired Privilege Layer Application
Internal Control Effectiveness None
Finding Confidence Proven True
Required Privilege Guest
Required Privilege Layer Application
Access Vector Internet
Authentication Strength None
Level of Interaction Typical/Limited
Deployment Scope All
Business Impact Critical
Likelihood of Discovery High
Likelihood of Exploit High
External Control Effectiveness None
Prevalence Not Applicable

The CWSS score for this vector is 92.6, derived as follows:

  • BaseSubscore:
    • [ (10 * TI + 5*(AP + AL) + 5*FC) * f(TI) * IC ] * 4.0
    • f(TI) = 1
    • = [ (10 * 0.9 + 5*(1.0 + 1.0) + 5*1.0) * 1 * 1.0 ] * 4.0
    • = [ (9.0 + 10.0 + 5.0) * 1.0 ] * 4.0
    • = 24.0 * 4.0
    • = 96.0
  • AttackSurfaceSubscore:
    • [ 20*(RP + RL + AV) + 20*SC + 15*IN + 5*AS ] / 100.0
    • = [ 20*(0.9 + 1.0 + 1.0) + 20*1.0 + 15*0.9 + 5*1.0 ] / 100.0
    • = [ 58.0 + 20.0 + 13.5 + 5.0 ] / 100.0
    • = 96.5 / 100.0
    • = 0.965
  • EnvironmentSubscore:
    • [ (10*BI + 3*DI + 4*EX + 3*P) * f(BI) * EC ] / 20.0
    • f(BI) = 1
    • = [ (10*1.0 + 3*1.0 + 4*1.0 + 3*1.0) * 1 * 1.0 ] / 20.0
    • = [ (10.0 + 3.0 + 4.0 + 3.0) * 1.0 ] / 20.0
    • = 20.0 / 20.0
    • = 1.0

The final score is:

96.0 * 0.965 * 1.0 = 92.64 == 92.6

4.2 Example: Wiki with limited business criticality

Consider this CWSS vector. Suppose the software is a wiki that is used for tracking social events for a mid-size business. Some of the most important characteristics are that there is medium technical impact to an application administrator from a regular user of the application, but the application is not business-critical, so the overall business impact is low. Also note that most of the environment factors are set to "Not Applicable."

(TI:M,0.6/AP:A,1.0/AL:A,1.0/IC:N,1.0/FC:T,1.0/

RP:RU,0.7/RL:A,1.0/AV:I,1.0/AS:W,0.9/IN:A,1.0/SC:NA,1.0/

BI:L,0.3/DI:NA,1.0/EX:NA,1.0/EC:N,1.0/RE:NA,1.0/P:NA,1.0)

The vector has been split into multiple lines for readability. Each line represents a metric group.

The factors and values are as follows:

FactorValue
Technical Impact Medium
Acquired Privilege Administrator
Acquired Privilege Layer Application
Internal Control Effectiveness None
Finding Confidence Proven True
Required Privilege Regular User
Required Privilege Layer Application
Access Vector Internet
Authentication Strength Weak
Level of Interaction Automated
Deployment Scope Not Applicable
Business Impact Low
Likelihood of Discovery Not Applicable
Likelihood of Exploit Not Applicable
External Control Effectiveness None
Prevalence Not Applicable

The CWSS score for this vector is 51.1, derived as follows:

  • BaseSubscore:
    • [ (10 * TI + 5*(AP + AL) + 5*FC) * f(TI) * IC ] * 4.0
    • f(TI) = 1
    • = [ (10 * 0.6 + 5*(1 + 1) + 5*1) * f(TI) * 1 ] * 4.0
    • = 84.0
  • AttackSurfaceSubscore:
    • [ 20*(RP + RL + AV) + 20*SC + 15*IN + 5*AS ] / 100.0
    • = [ 20*(0.7 + 1 + 1) + 20*1.0 + 15*1.0 + 5*0.9 ] / 100.0
    • = [ 54.0 + 20.0 + 15.0 + 4.5 ] / 100.0
    • = 93.5 / 100.0
    • = 0.94 (0.935)
  • EnvironmentSubscore:
    • [ (10*BI + 3*DI + 4*EX + 3*P) * f(BI) * EC ] / 20.0
    • f(BI) = 1
    • = [ (10*0.3 + 3*1.0 + 4*1.0 + 3*1.0) * f(BI) * 1 ] / 20.0
    • = [ (3.0 + 3.0 + 4.0 + 3.0) * 1.0 * 1.0 ] / 20.0
    • = [ 13.0 * 1.0 ] / 20.0
    • = 0.65

The final score is:

84.0 * 0.935 * 0.65 = 51.051 == 51.1

4.3 Other Approaches to CWSS Score Portability

Instead of recording each individual weight within a CWSS vector, several other methods could be adopted.

One possibility is to extend the CWSS vectors to record additional metadata that does not affect the score but reflects the version or other important information. The metadata portion would not necessarily need to capture weights, per se. For example, the CWSS version could be recorded by using a "factor" name such as "V" along with a value that represents the CWSS version, e.g. "V:1.1". This would add approximately 4 bytes to each CWSS vector. However, if the version is encoded within a vector, then the assigned weights would no longer need to be recorded (except for Quantified values), so the resulting vectors could be much shorter.

A different approach would be to attach metadata to a set of generated CWSS scores (such as the Technical Impact Scorecard if CWRAF is used), but it could be too easy for this metadata to become detached from the scores/vectors. Quantified factors would still need to be represented within a vector, since they could vary for each weakness finding.

Another approach is that when CWSS scores are transferred from one party to the other, then the receiving party could re-calculate the scores from the given CWSS vectors, then compare the re-calculated scores with the original scores. A difference in scores would suggest that different mechanisms are in use between the provider and receiverd, possibly a different CWSS version.

(5) Considerations for CWSS beyond 1.0

For future versions, the following should be considered.

5.1 Current Limitations of the Scoring Method

5.1.1 Score Weight and Distribution

The formula still needs to be refined to ensure that the range of potential scores is more evenly distibuted. There are probably unexpected interactions between factors that must be identified and resolved. CVSS scoring contains built-in adjustments that prevent many factors from affecting the score too much, while also giving some preference to impact over exploitability; similar built-in adjustments may need to be performed for CWSS.

5.1.2 Raising the Priority of Design or Architectural Flaws

CWSS 1.0 does not provide any clear mechanism to give higher priority to design or architecture flaws versus implementation vulnerabilities. This can be an important distinction, because compromise of a design flaw could potentially compromise the entire software package, but a single CWSS score for such a design flaw could become "buried" if there are many different implementation bugs. For example, a single report of the lack of an input validation framework might be expected to carry more weight than multiple individual XSS and SQL injection bugs. This issue can be exacerbated if aggregated scores are used.

It might be possible to use the Business Impact factor, but this factor might already be in use for its originally-intended purpose.

A "Weakness Scope" factor could be used to cover the following scenario. Within CWSS 1.0, design and architecture flaws receive the same relative priority as implementation issues, even though they may lead to a complete compromise of the software. It may be reasonable to use a separate factor in order to give design/architecture flaws a larger weight, e.g. so that the lack of an input validation framework (one "finding") can be given higher priority than hundreds of individual findings for XSS or SQL injection.*

5.1.3 Compound Elements (Chains, Etc.)

There are also some challenges for scoring findings that combine multiple weaknesses. By their nature, compound elements such as chains and composites involve interactions between multiple reported weaknesses. With some detection techniques such as automated code scanning, multiple CWE entries might be reported as separate findings for a single chain or composite. If individual scores within each link of the chain are counted separately, this could wind up artificially inflating any aggregate scores. However, sometimes the compound element is more than the sum of its parts, and the combination of multiple weaknesses has a higher impact than the maximum impact of any individual weakness. This is not well-handled in the current CWSS scheme; however, it should be noted that CVSSv3 is likely to include guidance for scoring chains as their own independent entities, so future versions of CWSS might follow suit.

5.1.4 Other Types of Software Assessments

It is anticipated that CWSS may be considered for use in other types of software assessments, such as safety, reliability, and quality. Weaknesses or other issues related to code quality might receive higher prioritization within a CWRAF vignette-oriented scheme, since safety, compliance, or maintainability might be important. This usage is not explicitly supported with CWSS 1.0. However, such quality-related issues could be scored in which the Required Privilege is the same as Acquired Privilege, and the Required Privilege Layer is the same as the Acquired Privilege Layer; the Business Impact could also be used.

5.2 Community Review and Validation of Factors

Pending community review, future versions of CWSS might modify, remove, or add factors to the methodology.

As long as there are enough stakeholders or use-cases for whom a factor is important, then it is a strong argument for keeping the factor within CWSS, even if it is not essential for everyone. Others could use the "Not Applicable" value when the factor is not relevant to their own environment. However, the more factors in the metric, the more complexity.

5.3 Impact of CWSS and CWE Version Changes to Scoring

The values for the factors involved in scoring could change frequently based on industry trends. For example, the likelihood of discovery of a particular weakness may change - rising if detection techniques improve (or if there is a shift in focus because of increases in attacks), or falling if enough developers have methods for avoiding the weakness, and/or if automatic protection mechanisms reach wide-scale adoption.

In the future, default values for some factors might be directly obtained from CWE data. However, new CWE versions are released on a regular basis, approximately 4 or 5 times a year. If a CWE entry is modified in a way that affects CWSS-related factors, then the resulting CWSS score for a weakness might differ depending on which version of CWE is used. Theoretically, however, this could be automatically detected by observing inconsistencies in the weights used for "Default" values in CWSS vectors.

Because of these underlying changes, there is a significant risk that CWSS scores will not be comparable across organizations or assessments if they were calculated using different versions of CWSS, vignettes, or CWE.

In anticipation of such changes, CWSS design should consider including CWE and/or CWSS version numbers in the CWSS vector representation (or associated metadata).

Finally, these changes should not occur too frequently, since each change could cause CWSS scores to change unpredictably, causing confusion and possibly interfering with strategic efforts to fix weaknesses whose importance has suddenly been reduced.* Although this may be inevitable for CWSS as a natural result of growth, the community should attempt to prevent this from happening where possible.

While scores may change as CWSS and CWE evolve, there is not necessarily a requirement for an organization to re-score whenever a new version is released, especially if the organization is using CWSS for internal purposes only. The variability of scores is largely a problem for sharing scores between organizations. For example, a software developer may have its own internally-defined vignettes and BVC, so the developer may not have a need (or willingness) to share CWSS scores outside the organization.

*CVSS encountered these problems when changing from version 1 to version 2, and there were significant labor costs to re-score vulnerabilities, which numbered in the tens of thousands. As a result of this, there has been significant reluctance by the CVSS SIG to make any substantive changes beyond version 2.
(6) Future Activities

After the release of CWSS 1.0, the schedule for future development is uncertain. However, future plans might include:

  • Continue to obtain stakeholder validation or feedback for the existing factors, values, and weights. Now that CWSS is seeing some real-world use, these stakeholders will have valuable input to improve the metric.
  • Continue to modify the scoring method and formula so that there is less bias towards low scores, and incomplete or non-applicable data does not adversely affect the scores.
  • Refine and evaluate aggregated scoring techniques.
  • Define a data exchange representation for CWSS scores and vectors, e.g. XML/XSD.
  • The values for Authentication Strength (AS) might need to be defined more clearly. It might be reasonable to adopt approaches such as the four levels outlined in NIST Special Publication 800-63 ("Electronic Authentication Guideline") and OMB Memo 04-04. However, since the strength of an authentication mechanism may degrade over time due to advances in attack techniques or computing power, it might be useful to select values that are effectively "future-proof." (On the other hand, this might make it difficult to compare CWSS scores if they were assigned at different times.)
  • It is not clear whether the SANE model of privilege layers is sufficiently expressive or useful. Once CVSSv3 is released and independently validated, CVSS' model should be revisited.

(7) Community Participation in CWSS

Currently, members of the software assurance community can participate in the development of CWSS in the following ways:

  • Provide feedback on this document.
  • Review the factors that are currently defined; suggest modifications to the current factors, and any additional factors that would be useful.
  • Evaluate the scoring formula and the relative importance of factors within that formula.
  • Define specific use cases for CWSS.
Appendix A: CVSS Comparison

A.1 CVSS Overview

The Common Vulnerability Scoring System (CVSS) is commonly used when ranking vulnerabilities as they appear in deployed software. CVSS provides a common framework for consistently scoring vulnerabilities.

Conceptually, CVSS and CWSS are very similar. There are some important strengths and limitations with CVSS, however.

One of CVSS' strengths lies in its simplicity. CVSS divides the overall score into 14 separate characteristics within three metric groups: Base, Temporal, and Environmental. Each characteristic is decomposed into two or more distinct values. For example, the Access Vector reflects the location from which an attacker must exploit a vulnerability, with possible values of Local (authenticated to the local system), Remote (across the network), or Network Adjacent (on the same physical or logical network). Typically, in addition to the CVSS score, a vector is provided that identifies the selected values for each characteristic.

With the associated documentation, CVSS scoring is fairly repeatable, i.e., different analysts will typically generate the same score for a vulnerability. However, different scores can be generated when information is incomplete, and significant variation is possible if an analyst does not closely follow documentation. While the simplified Confidentiality/Integrity/Availability model does not provide the depth and flexibility desired by some security experts, CVSS does provide the consistency that is useful for non-expert system and network administrators for prioritizing vulnerabilities.

CVSS has been widely adopted, especially the use of base scores from the Base metric group. Some organizations use the Temporal and Environmental portions of CVSS, but this is relatively rare, so these metric groups may not have been sufficiently vetted in the real world.

A.2 Usage Scenarios for CWSS versus CVSS

The usage scenarios for CVSS differ from CWSS in the following ways:
  • CVSS assumes that a vulnerability has already been discovered and verified; CWSS can be applied earlier in the process, before any vulnerabilities have been proven. CVSS is not scalable to the security assessment of a single software package. A detailed assessment, such as an automated code scan, may report thousands of weakness findings. Because of the high volume, these findings often need to be scored and prioritized before they can be more closely examined to determine if they lead to vulnerabilities. CWSS explicitly supports "unknown" values when there is incomplete information.
  • CVSS scoring does not account for incomplete information, but CWSS scoring has built-in support for incomplete information. Within CWSS, scoring may be necessary before the weakness is even known to contribute to a vulnerability. For example, the scoring entity - whether a human or machine - might not know the expected operating environment or authentication requirements during initial CWSS scoring. In CVSS, incomplete information is sometimes a problem because many vulnerability reports do not contain all the relevant details needed for scoring. When information is missing, a conservative approach is to select values that will generate the largest CVSS score. This approach is used by the National Vulnerability Database and others, but it can artificially inflate scores. Conservative scoring is viable, as long as most vulnerability reports contain sufficient information. Within a weakness-scoring context, a high percentage of weakness findings will be missing a critical piece of information, since some detection techniques will not be able to reliably determine if a weakness can be exploited by an attacker without further analysis. As a result, CWSS provides a way to explicitly record when information is unavailable.
  • CVSSv2 scoring has a large bias towards the impact on the physical system; CWSS has a small bias in favor of the application containing the weakness. In some contexts, users may strongly prefer to score issues based on their impact to business-critical data or functionality, which might have limited implications for the impact to the overall physical system. For example, the maximum possible score for CVSS is often 7.0 for Oracle products, since these products typically run with limited privileges. CVSSv3 will remove this bias toward the system, but its model still differs from the model that CWSS uses.

Early CWSS development sought to preserve the strengths of CVSS while also attempting to avoid some of the associated limitations.

A.3 Comparison of CVSSv2 Factors with CWSS Factors

Note that in CVSS, the Access Complexity (AC) value combines multiple characteristics that are split into distinct factors within CWSS, such as Required Privilege Level and Level of Interaction.

CVSSCWSSNotes
Confidentiality Impact (C), Integrity Impact (I), Availability Impact (A), Security Requirements (CR, IR, AR), Collateral Damage Potential (CDP) Technical Impact CWSS attempts to use a more fine-grained "Technical Impact" model than confidentiality, integrity, and availability. Business Value Context adjustments effectively encode the security requirements from the Environmental portion of CVSS. The CDP is indirectly covered within the BVC's linkage between business concerns and technical impacts.
Access Complexity (AC), Target Distribution (TD) Deployment Scope Deployment Scope is indirectly covered by CVSS' Access Complexity, which combines multiple distinct factors into a single item. It also has an indirect association with Target Distribution (TD).
Access Vector (AV) Access Vector The values are similar, but CWSS distinguishes between physical access and local (shell/account) access.
Access Complexity (AC) Required Privilege Level Required Privilege Level is indirectly covered by CVSS' Access Complexity, which combines multiple distinct factors into a single item.
N/A Authentication Strength This is not directly specified within CVSS, but scorers might consider the authentication strength when evaluating Access Complexity (AC).
Authentication (Au) Authentication Instances
N/A Likelihood of Discovery Within many CVSS use-cases, the vulnerability has already been discovered and disclosed by another party when CVSS scoring takes place. So there is no need to track the likelihood of discovery, as the likelihood is (effectively) 1.0. However, within some CWSS use-cases, the issue is only known to the developer at the time of scoring, and the developer may choose to increase the priority of issues that are most likely to be discovered.
N/A Likelihood of Exploit This is not covered in CVSS.
Access Complexity (AC) Interaction Requirements
Access Complexity (AC), Remediation Level (RL) Internal Control Effectiveness (IC) The presence (or absence) of controls/mitigations may affect the CVSS Access Complexity.
Access Complexity (AC) External Control Effectiveness (EC) The presence (or absence) of controls/mitigations may affect the CVSS Access Complexity. However, a single CVE vulnerability could have different CVSS scores based on vendor-specific configurations.
Report Confidence (RC) Finding Confidence
N/A Remediation Effort (RE)
Exploitability (E) N/A
Target Distribution (TD) N/A There is no direct connection in CWSS 1.0 for target distribution; there is no consideration of how many installations may be using the software.

A.4 Other Differences between CVSS and CWSS

CWSS scores and CVSS scores are not necessarily comparable. Even if CWSS scores (with a maximum of 100) are "normalized" to a CVSS range by dividing by 10 (which would produce CVSS-equivalent scores within the range of 0 to 10), this does not mean that a CWSS score of 7 is equivalent to a CVSS 7. This might be a desirable feature by some consumers, but since CWSS is often measuring completely different characteristics than CVSS does, scoring equivalence might not be feasible. In addition, in practice, CVSS scores do not follow a regular distribution, generally with a skew towards high scores; it is possible that CWSS might have better distribution.

Some organizations attempted to modify CVSS in order to address some of the unique requirements of software security analysis; these modifications are mentioned in Appendix B.

The metric groups in CWSS are different than those in CVSS, by design. Some reviewers of early CWSS versions suggested that CWSS adopt the same set of metric groups that are used by CVSS - Base, Temporal, and Environmental. However, since CWSS scores can be calculated in early, low-information scenarios, many factors are "temporal" in nature, regardless of which group they are in; also, these scores are likely to change as further analysis yields more information about the weakness. CWSS supports the use of values such as "Unknown" or "Default", which can be filled in at a later time.

One aspect of CVSS that is not explicitly modeled in CWSS is the notion of "partial" impacts. However, the acquired privileges, privilege layer, technical impact, and business impact are roughly equivalent, with more expressive power.

Appendix B: Other Scoring Methods

B.1 2008 CWSS Kickoff Meeting

In October 2008, a single-day kickoff meeting for CWSS was held. Several participants described their scoring approaches:

Veracode reported an adaptation of CVSS to evaluate detected weaknesses/vulnerabilities. Each issue is given weights for Confidentiality, Integrity, and Availability, based on its associated CWE entry. The weighting considers the average likely severity to occur. For example, a buffer overflow could allow an attacker to cause a crash, but it is not always exploitable for code execution.

For aggregated scores, Veracode has several "VERAFIED Security Marks" that are based on a calculated Security Quality Score (SQS), which ranges from 0 to 100. The "VerAfied" security mark is used to indicate software that Veracode has assessed to be free of "very high," "high," or "medium" severity vulnerabilities, and free of automatically-detectable vulnerabilities from the CWE/SANS Top 25 or OWASP Top Ten. Two "High Assurance" variations of the mark include a manual assessment step that covers the remainder of the CWE/SANS Top 25 or OWASP Top Ten that could not be identified by automatic detection.

The Veracode Rating System uses a three-letter rating system (with grades of "A", "B", "C", "D", and "F"). The first letter is used for the results from binary analysis, the second for automated dynamic analysis, and the third for human testing.

Cigital described a feasibility study of CVSSv2 with respect to weaknesses. Some attributes such as "Target Distribution" did not fit well. Other attributes were extended to add more granularity. A polynomial scoring method was recommended. It was also regarded as important to model the distinction between the likelihood and the impact.

Cenzic provided details of the Hailstorm Application Risk Metric (HARM). It is a quantitative score that is utilized by black box analysis of web applications. The goal of the metric was to provide a scalable approach to focus remediation efforts. The metric was split into 4 impact areas relevant to web application security: the browser, the session, the web application, and the server. The benefit to this approach was that it was easily consumable.

CERT/SEI presented its approach to scoring the C Secure Coding Rules. The FMECA metric, an ISO standard, was used. It characterizes items in terms of Severity, Likelihood (of leading to a vulnerability), and Remediation Cost.

B.2 2010 SANS/CWE Top 25

The 2010 SANS/CWE Top 25 Most Dangerous Software Errors list attempted to perform quantitative prioritization of CWE entries using a combination of Prevalence and Importance, which became the basis of CWSS 0.1 later in the year. A survey approach was taken in which respondents performed their own individual evaluation of Prevalence and Importance for 41 candidate weaknesses, from which the final scores were determined. To reflect the diverse opinions and use cases of the respondents for the general Top 25 list, the Importance factor was used instead of Impact. In an attempt to force consensus, respondents were restricted to 4 selections of the highest value for Importance ("Critical") and Prevalence ("Widespread"), although this forced choice was not popular; it will probably be abandoned in future versions of the Top 25. Many respondents used high-level rollup data, or a rough consensus of opinion with the organization, sometimes covering multiple teams or functions. Very few respondents had real-world data at the low level of granularity used by the Top 25 (typically the "Base" level of abstraction for CWE). An evaluation by PlexLogic later found that the two variables were not entirely independent. This discovery makes some sense, because the vulnerability research community tends to focus on vulnerabilities/weaknesses with the highest impact. When reliable attack techniques are devised for a particular weakness/vulnerability, it becomes easier for more researchers to find them, which can lead to widespread exploitation. Consequently, this raises the relative Importance of a weakness.

The 2010 Top 25 was structured in a way to support multiple points of view that could reflect different prioritizations of the weaknesses. The creation of separate focus profiles stemmed from some critiques of the original 2009 Top 25, in which a generalized Top 25 list would not necessarily be useful to all audiences, and that a customized prioritization would be ideal. Eight focus profiles were provided with the 2010 Top 25. For example, the Educational Emphasis focus profile evaluated weaknesses that are regarded as important from an educational perspective within a school or university context. It emphasized the CWE entries that graduating students should know, including weaknesses that were historically important or increased the breadth of coverage. A separate focus profile ranked weaknesses based solely on their evaluated Importance, which would be useful to software customers who want the most serious issues removed, without consideration for how frequently they occur or how resource-intensive it is to fix. These ranking-oriented focus profiles made the Top 25 more useful to certain audiences, and their construction and management have served as a useful predecessor to CWSS and vignettes.

While the 2009 Top 25 did not rank items, several factors were presented that were thought to be relevant to an audience: attack frequency, impact or consequences, prevalence, and ease of detection. Other considerations included remediation cost, amount of public knowledge, and the likelihood that the weakness discovery would increase in the future.

B.3 2010 OWASP Top Ten

In contrast to previous versions, the 2010 OWASP Top Ten shifted focus from weaknesses/vulnerabilities to risks, which typically caused each OWASP Top Ten entry to cover multiple related weakness types that posed the same risk. Factors for prioritization included Ease of Exploit, Prevalence, Detectability, and Technical Impact. Input from contributors was solicited to determine the values for these factors, but the final decision for each factor was made by the Top Ten editorial staff based on trend information from several real-world data sources. A metric was developed that used these factors to prioritize the final Top Ten list.

B.4 Other Models

Microsoft's STRIDE model characterizes issues in terms of Spoofing Identity, Tampering with Data, Repudiation, Information Disclosure, Denial of Service, and Elevation of Privilege. The DREAD scheme evaluates issues based on Damage Potential, Reproducibility of the issue, Exploitability, Affected Users, and Discoverability. Many of these attributes have equivalent factors in CWSS.

Appendix C: Design Considerations

For CWSS to be most effective to its stakeholders, several aspects of the problem area were considered when designing the framework and metrics.

  • Flexibility: CWSS should be automatable and flexible wherever possible, but support human input as well.
  • Selected Automation: It is assumed that portions of CWSS scores can be automatically generated. For example, some factors may be dependent on the type of weakness being scored; potentially, the resulting subscores could be derived from CWE data. As another example, a web script might only be accessible by an administrator, so all weaknesses may be interpreted in light of this required privilege.
  • Scalability: Some usage scenarios may require the scoring of thousands of weaknesses, such as defect reports from an automated code scanning tool. When such a high volume is encountered, there are too many issues to analyze manually. As a result, automated scoring must be supported.
  • Stakeholder Analysis: The potential CWSS stakeholders, their needs, and associated use cases should be analyzed to understand their individual requirements. This might require support for multiple scoring techniques or methods.
  • Usability vs. Completeness: Associated metrics must balance usability with completeness, i.e., they cannot be too complex.
  • Prioritization Flexibility: Environmental conditions and business/mission priorities should impact how scores are generated and interpreted.

Appendix D: Generalized Scoring Approaches

While CWSS 1.0 is focused on targeted scoring, it could be further adapted for scoring weaknesses in a general fashion, e.g. to develop a relative prioritization of issues such as buffer overflows, XSS, and SQL injection, independent of any specific software package.

A generalized scoring approach could account for:

  • Prevalence: how often does this appear at least once within a software package?
  • Frequency: in a software package in which this weakness occurs, how often does it occur? (perhaps summarized as "diffusion")
  • Likelihood of Discovery
  • Likelihood of Exploit
  • Technical Impact

In the earlier CWSS 0.1, the formula was:

Prevalence x Importance

This formula was a characterization of the metric used for the 2010 CWE/SANS Top 25. Importance was derived from the vignette-specific subscores for Technical Impacts of the CWE entry. Prevalence could be obtained from general information (derived from CWE content, or from other sources), with the possibility of vignette-specific specifications of prevalence. For example, XSS or SQL injection might occur more frequently in a web-based retail context than in embedded software.

D.1 Prevalence Assessment

In the earlier CWSS version 0.1, prevalence scores for the 2010 Top 25 were obtained by re-using raw voting data from the 2010 Top 25 participants. The original 1-4 scale (with discrete values) was extended to use values between 1 and 10. When using real-world prevalence data, this artificial normalization might not be necessary.

The following table summarizes the prevalence scores for some of the Top 25 entries. Notice the high prevalence value for XSS; this reflects the fact that nearly all of the voting members scored XSS as "Widespread." Complete details are available on a separate page.

Top 25 RankCWENamePrevalence (1-10)
[1] CWE-79 XSS 9.46
[2] CWE-89 SQL Injection 7.43
[3] CWE-120 Classic Buffer Overflow 6.04
[4] CWE-352 Cross-site Request Forgery 7.75
[16] CWE-209 Information Exposure Through an Error Message 7.11

Appendix E: Aggregated Scoring Methods: Measuring Weakness Surface

For years, software consumers have wanted clear guidance on how secure a software package is, but the only available methods have been proprietary, crude, or indirect, such as:

  • Crude methods, such as counting the number and/or severity of publicly reported vulnerabilities
  • Proprietary methods developed by consultants or tool developers
  • Indirect methods, such as the attack surface metric, which likely has a strong association with overall software security, although this has not necessarily been empirically proven.

A software package could be evaluated in light of the number and importance of weaknesses that have been detected, whether from automated or manual techniques. The results from these individual weaknesses could be aggregated to develop a single score, tentatively called the "Weakness Surface." This could move the software assurance community one step closer to consumer-friendly software security indicators such as the Software Facts Label concept, as originally proposed by Jeff Williams (Aspect Security) and Paul Black (NIST).

When there is a set of targeted weaknesses for a single software package, there are several possible aggregated scoring methods, including but not necessarily limited to:

  • Compute the sum of all individual weakness scores
  • Choose the highest of all individual weakness scores
  • Select a subset of individual weakness scores exceeding a stated minimum, and add the scores together
  • Compute the sum of all individual weakness scores, then normalize these scores according to KLOC or other metrics that reflect code size, i.e., "defect density."
  • Normalize the results on a per-executable basis.
  • Normalize the results to a point scale between 0 (no assurance) and 100 (high assurance).

Early CWSS implementations have typically aggregated based on either the sum of all scores, or by choosing the highest of all scores.

Some methods from the 2008 CWSS kickoff workshop may be adaptable or applicable; see Appendix B. In addition, some SCAP users have begun creating aggregated metrics for a host system by aggregating CVSS scores for the individual CVE vulnerabilities that are detected on the host. These users may have some useful guidance for the CWSS approach to aggregate scoring.

Change Log

DateDocument VersionNotes
September 5, 2014 1.0.1

Changed 4.2 example ("Wiki with limited business criticality") vector weights to use 1.0 instead of 1, for consistenc with the 4.1 example. Credit: an external contributor.

Changed 4.2 example ("Wiki with limited business criticality") vector values "AS:L" to "AS:W" and "AV:N" to "AV:I". The example was using values that were valid for 0.8 but changed (i.e., made obsolete) in 1.0. Credit: an external contributor.

Changed 4.2 example ("Wiki with limited business criticality") to include table listing vector values.

Added description for Default value in Prevalence (P) table.

July 17, 2014 1.0

Removed Authentication Instances (AI). The CVSSv3 team found that in this factor, the Multiple value was "rarely, if ever, used" within several real-world implementations of CVSSv2, and the CVSS SIG will remove this factor in CVSSv3. In addition, within CVSSv2, there was sometimes user confusion between this factor and the Access Vector's "Local" value that could cause inconsistent values to be chosen.

Removed Remediation Effort (RE). This factor does not directly contribute to the inherent risk or severity of a weakness, so it is outside the scope of CWSS scoring. Remediation effort could be better handled using separate processes within the context of a remediation plan. For example, bug databases and external maintenance/release processes might be more appropriate for recording this information.

Changed scoring formula to handle removal of AuthenticationInstances and RemediationEffort. Within the Attack Surface Subscore, the multiplier for LevelOfInteraction was adjusted from 10 to 15. Within the Environmental Subscore, the multiplier for LikelihoodOfExploit was increased from 3 to 4, which reflects current trends in vulnerability management that place greater emphasis on likelihood of exploit than raw CVSS scores.

Modified introduction.

Clarified distinction between CWSS and CVSS.

Created new images for metric groups and scoring, omitting factors that were removed for 1.0.

Improve display of tables, formulas, etc.

Give numbers to sections.

Acquired Privilege (AP) - Default weight changed from 1.0 to 0.5 (typo).

Changed "Unk" code to "UK" everywhere. Listed "Unknown" description in each factor.

Clarified and fixed values / definitions for AP, AL, RP, and RL.

Level of Interaction (IN) - changed codes to use only 1 to 2 uppercase letters.

Deployment Scope (SC) - changed codes to use only 1 to 2 uppercase letters.

June 27, 2011 0.8

Bumped up version number to synchronize with CWRAF.

June 23, 2011 0.6

Major changes to the formula to better reflect relative priorities of the factors.

Modified Access Vector (AV) to include Internet, Intranet, and Private Network values.

Renamed Remediation Cost (RC) to Remediation Effort (RE), and changed the available values.

Changed External Control Effectiveness (EC) weight for "Complete" to 0.1, to reflect the possibility of accidental removal of the control if the environment changes.

Modified values for Authentication Strength (AS) and added notes for potential enhancements.

Modified weights for Prevalence (P) so the range of variation is more narrow.

All weights for Unknown values were changed to 0.5 so that lack of information does not over-emphasize scores; additional information can move scores up or down, accordingly.

Changed Defense-in-Depth/"D" value to Indirect/"I" for internal and external control effectiveness to avoid conflict with the Default/"D" value.

Removed most references to vignettes, technical impact scorecards, business value context, etc. - now covered in CWRAF.

Skipped version 0.5 to reflect maturity and for alignment with other efforts.

April 26, 2011 0.4

Removed content that became part of Common Weakness Risk Analysis Framework (CWRAF).

Reorganized metric groups.

Defined new factors - Business Impact, Acquired Privilege, Acquired Privilege Layer.

Defined a new formula.

Added "Default" values to each factor.

March 7, 2011 0.3 Created overview images and shortened the introduction. Defined technology groups, added more business domains, added more vignettes. Annotated each factor that could be quantified. Updated stakeholders section. Integrated CVSS descriptions into a single section. Added more details on the scoring method, including CWSS vectors.
February 11, 2011 0.2 Added business domains, archetypes, and Business Value Context; identified detailed factors; emphasized use of CWSS for targeted scoring; reorganized sections; made other modifications based on community feedback.
December 2, 2010 0.1 Initial version for review by limited audience

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