CWE

Common Weakness Enumeration

A Community-Developed List of Software & Hardware Weakness Types

CWE Top 25 Most Dangerous Weaknesses
Home > CWE List > CWE- Individual Dictionary Definition (4.2)  
ID

CWE-184: Incomplete List of Disallowed Inputs

Weakness ID: 184
Abstraction: Base
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are not allowed by policy or otherwise require other action to neutralize before additional processing takes place, but the list is incomplete, leading to resultant weaknesses.
+ Extended Description
Developers often try to protect their products against malicious input by performing tests against inputs that are known to be bad, such as special characters that can invoke new commands. However, such lists often only account for the most well-known bad inputs. Attackers may be able to find other malicious inputs that were not expected by the developer, allowing them to bypass the intended protection mechanism.
+ Alternate Terms
Denylist / Deny List:
This is used by CWE and CAPEC instead of other commonly-used terms. Its counterpart is allowlist.
Blocklist / Block List:
This is often used by security tools such as firewalls, email or web gateways, proxies, etc.
Blacklist / Black List:
This term is frequently used, but usage has been declining as organizations have started to adopt other terms.
+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfClassClass - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.1023Incomplete Comparison with Missing Factors
ChildOfPillarPillar - a weakness that is the most abstract type of weakness and represents a theme for all class/base/variant weaknesses related to it. A Pillar is different from a Category as a Pillar is still technically a type of weakness that describes a mistake, while a Category represents a common characteristic used to group related things.693Protection Mechanism Failure
PeerOfVariantVariant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.86Improper Neutralization of Invalid Characters in Identifiers in Web Pages
PeerOfBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.625Permissive Regular Expression
CanPrecedeBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.78Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')
CanPrecedeBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CanPrecedeVariantVariant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.98Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')
CanPrecedeBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.434Unrestricted Upload of File with Dangerous Type
+ Relevant to the view "Software Development" (CWE-699)
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1215Data Validation Issues
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

PhaseNote
ImplementationDevelopers might begin to develop a list of bad inputs as a fast way to fix a particular weakness, instead of fixing the root cause. See [REF-141].
Architecture and DesignThe design might rely solely on detection of malicious inputs as a protection mechanism.
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Language-Independent (Undetermined Prevalence)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Access Control

Technical Impact: Bypass Protection Mechanism

+ Demonstrative Examples

Example 1

The following code attempts to stop XSS attacks by removing all occurences of "script" in an input string.

(bad code)
Example Language: Java 
public String removeScriptTags(String input, String mask) {
return input.replaceAll("script", mask);
}

Because the code only checks for the lower-case "script" string, it can be easily defeated with upper-case script tags.

+ Observed Examples
ReferenceDescription
product uses a denylist to identify potentially dangerous content, allowing attacker to bypass a warning
PHP remote file inclusion in web application that filters "http" and "https" URLs, but not "ftp".
Programming language does not filter certain shell metacharacters in Windows environment.
XSS filter doesn't filter null characters before looking for dangerous tags, which are ignored by web browsers. MIE and validate-before-cleanse.
Web-based mail product doesn't restrict dangerous extensions such as ASPX on a web server, even though others are prohibited.
Resultant XSS when only <script> and <style> are checked.
Privileged program does not clear sensitive environment variables that are used by bash. Overlaps multiple interpretation error.
SQL injection protection scheme does not quote the "\" special character.
Detection of risky filename extensions prevents users from automatically executing .EXE files, but .LNK is accepted, allowing resultant Windows symbolic link.
Product uses list of protected variables, but accidentally omits one dangerous variable, allowing external modification
Chain: product only removes SCRIPT tags (CWE-184), enabling XSS (CWE-79)
Chain: product only checks for use of "javascript:" tag (CWE-184), allowing XSS (CWE-79) using other tags
Chain: OS command injection (CWE-78) enabled by using an unexpected character that is not explicitly disallowed (CWE-184)
"\" not in list of disallowed values for web server, allowing path traversal attacks when the server is run on Windows and other OSes.
+ Potential Mitigations

Phase: Implementation

Strategy: Input Validation

Do not rely exclusively on detecting disallowed inputs. There are too many variants to encode a character, especially when different environments are used, so there is a high likelihood of missing some variants. Only use detection of disallowed inputs as a mechanism for detecting suspicious activity. Ensure that you are using other protection mechanisms that only identify "good" input - such as lists of allowed inputs - and ensure that you are properly encoding your outputs.
+ Weakness Ordinalities
OrdinalityDescription
Primary
(where the weakness exists independent of other weaknesses)
+ Detection Methods

Black Box

Exploitation of a vulnerability with commonly-used manipulations might fail, but minor variations might succeed.
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.990SFP Secondary Cluster: Tainted Input to Command
+ Notes

Relationship

Multiple interpretation errors can indirectly introduce inputs that should be disallowed. For example, a list of dangerous shell metacharacters might not include a metacharacter that only has meaning in one particular shell, not all of them; or a check for XSS manipulations might ignore an unusual construct that is supported by one web browser, but not others.

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERIncomplete Blacklist
+ References
[REF-140] Greg Hoglund and Gary McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. 2004-02-27. <https://www.amazon.com/Exploiting-Software-How-Break-Code/dp/0201786958>.
[REF-141] Steve Christey. "Blacklist defenses as a breeding ground for vulnerability variants". 2006-02-03. <http://seclists.org/fulldisclosure/2006/Feb/0040.html>.
[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 8, "Eliminating Metacharacters", Page 435. 1st Edition. Addison Wesley. 2006.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2006-07-19PLOVER
+ Modifications
Modification DateModifierOrganization
2008-07-01Sean EidemillerCigital
added/updated demonstrative examples
2008-07-01Eric DalciCigital
updated Potential_Mitigations, Time_of_Introduction
2008-09-08CWE Content TeamMITRE
updated Detection_Factors, Relationships, Other_Notes, Relationship_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2008-11-24CWE Content TeamMITRE
updated Observed_Examples
2009-05-27CWE Content TeamMITRE
updated Description, Other_Notes, Relationship_Notes, Time_of_Introduction
2010-02-16CWE Content TeamMITRE
updated Relationships
2010-04-05CWE Content TeamMITRE
updated Related_Attack_Patterns
2010-06-21CWE Content TeamMITRE
updated Demonstrative_Examples
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated References, Related_Attack_Patterns, Relationships
2013-02-21CWE Content TeamMITRE
updated Potential_Mitigations
2014-07-30CWE Content TeamMITRE
updated Demonstrative_Examples, Relationships
2015-12-07CWE Content TeamMITRE
updated Relationships
2017-05-03CWE Content TeamMITRE
updated Potential_Mitigations, Related_Attack_Patterns
2017-11-08CWE Content TeamMITRE
updated Applicable_Platforms, References
2018-03-27CWE Content TeamMITRE
updated Observed_Examples, Relationships
2019-01-03CWE Content TeamMITRE
updated Related_Attack_Patterns
2019-06-20CWE Content TeamMITRE
updated Related_Attack_Patterns, Relationships
2020-02-24CWE Content TeamMITRE
updated Alternate_Terms, Description, Detection_Factors, Modes_of_Introduction, Name, Observed_Examples, Potential_Mitigations, Relationship_Notes, Relationships
2020-06-25CWE Content TeamMITRE
updated Alternate_Terms, Observed_Examples
+ Previous Entry Names
Change DatePrevious Entry Name
2020-02-24Incomplete Blacklist
More information is available — Please select a different filter.
Page Last Updated: June 25, 2020