Common Weakness Enumeration

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CWE-138: Improper Neutralization of Special Elements

Weakness ID: 138
Abstraction: Class
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The software receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could be interpreted as control elements or syntactic markers when they are sent to a downstream component.
+ Extended Description
Most languages and protocols have their own special elements such as characters and reserved words. These special elements can carry control implications. If software does not prevent external control or influence over the inclusion of such special elements, the control flow of the program may be altered from what was intended. For example, both Unix and Windows interpret the symbol < ("less than") as meaning "read input from a file".
+ 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 "Architectural Concepts" (CWE-1008)
MemberOfCategoryCategory1019Validate Inputs
+ 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 software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

ImplementationREALIZATION: This weakness is caused during implementation of an architectural security tactic.
+ 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.


(Language-Independent classes): (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.


Technical Impact: Execute Unauthorized Code or Commands; Alter Execution Logic; DoS: Crash, Exit, or Restart

+ Observed Examples
Read arbitrary files from mail client by providing a special MIME header that is internally used to store pathnames for attachments.
Setuid program does not cleanse special escape sequence before sending data to a mail program, causing the mail program to process those sequences.
Multi-channel issue. Terminal escape sequences not filtered from log files.
Multi-channel issue. Terminal escape sequences not filtered from log files.
+ Potential Mitigations

Phase: Implementation

Developers should anticipate that special elements (e.g. delimiters, symbols) will be injected into input vectors of their software system. One defense is to create a whitelist (e.g. a regular expression) that defines valid input according to the requirements specifications. Strictly filter any input that does not match against the whitelist. Properly encode your output, and quote any elements that have special meaning to the component with which you are communicating.

Phase: Implementation

Strategy: Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). A blacklist is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Phase: Implementation

Use and specify an appropriate output encoding to ensure that the special elements are well-defined. A normal byte sequence in one encoding could be a special element in another.

Phase: Implementation

Strategy: Input Validation

Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass whitelist validation schemes by introducing dangerous inputs after they have been checked.

Phase: Implementation

Strategy: Output Encoding

While it is risky to use dynamically-generated query strings, code, or commands that mix control and data together, sometimes it may be unavoidable. Properly quote arguments and escape any special characters within those arguments. The most conservative approach is to escape or filter all characters that do not pass an extremely strict whitelist (such as everything that is not alphanumeric or white space). If some special characters are still needed, such as white space, wrap each argument in quotes after the escaping/filtering step. Be careful of argument injection (CWE-88).
+ Weakness Ordinalities
(where the weakness exists independent of other weaknesses)
+ 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.
MemberOfCategoryCategory990SFP Secondary Cluster: Tainted Input to Command
+ Notes


This weakness can be related to interpretation conflicts or interaction errors in intermediaries (such as proxies or application firewalls) when the intermediary's model of an endpoint does not account for protocol-specific special elements.


See this entry's children for different types of special elements that have been observed at one point or another. However, it can be difficult to find suitable CVE examples. In an attempt to be complete, CWE includes some types that do not have any associated observed example.

Research Gap

This weakness is probably under-studied for proprietary or custom formats. It is likely that these issues are fairly common in applications that use their own custom format for configuration files, logs, meta-data, messaging, etc. They would only be found by accident or with a focused effort based on an understanding of the format.
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERSpecial Elements (Characters or Reserved Words)
PLOVERCustom Special Character Injection
Software Fault PatternsSFP24Tainted input to command
+ Content History
Submission DateSubmitterOrganizationSource
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigital
updated Description, Potential_Mitigations, Time_of_Introduction
2008-09-08CWE Content TeamMITRE
updated Description, Relationships, Other_Notes, Taxonomy_Mappings
2009-03-10CWE Content TeamMITRE
updated Description, Name
2009-07-27CWE Content TeamMITRE
updated Applicable_Platforms, Description, Observed_Examples, Other_Notes, Potential_Mitigations, Relationship_Notes, Relationships, Research_Gaps, Taxonomy_Mappings, Weakness_Ordinalities
2009-12-28CWE Content TeamMITRE
updated Relationships
2010-04-05CWE Content TeamMITRE
updated Description, Name
2010-12-13CWE Content TeamMITRE
updated Description
2011-03-29CWE Content TeamMITRE
updated Potential_Mitigations
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Common_Consequences, Relationships
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2017-01-19CWE Content TeamMITRE
updated Relationships
2017-05-03CWE Content TeamMITRE
updated Potential_Mitigations
2017-11-08CWE Content TeamMITRE
updated Modes_of_Introduction, Potential_Mitigations, Relationships
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Special Elements (Characters or Reserved Words)
2009-03-10Failure to Sanitize Special Elements
2010-04-05Improper Sanitization of Special Elements

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Page Last Updated: November 14, 2017