Common Weakness Enumeration

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CWE-838: Inappropriate Encoding for Output Context

Weakness ID: 838
Abstraction: Base
Status: Incomplete
Presentation Filter:
+ Description

Description Summary

The software uses or specifies an encoding when generating output to a downstream component, but the specified encoding is not the same as the encoding that is expected by the downstream component.

Extended Description

This weakness can cause the downstream component to use a decoding method that produces different data than what the software intended to send. When the wrong encoding is used - even if closely related - the downstream component could decode the data incorrectly. This can have security consequences when the provided boundaries between control and data are inadvertently broken, because the resulting data could introduce control characters or special elements that were not sent by the software. The resulting data could then be used to bypass protection mechanisms such as input validation, and enable injection attacks.

While using output encoding is essential for ensuring that communications between components are accurate, the use of the wrong encoding - even if closely related - could cause the downstream component to misinterpret the output.

For example, HTML entity encoding is used for elements in the HTML body of a web page. However, a programmer might use entity encoding when generating output for that is used within an attribute of an HTML tag, which could contain functional Javascript that is not affected by the HTML encoding.

While web applications have received the most attention for this problem, this weakness could potentially apply to any type of software that uses a communications stream that could support multiple encodings.

+ Applicable Platforms



+ Common Consequences

Technical Impact: Modify application data; Execute unauthorized code or commands

An attacker could modify the structure of the message or data being sent to the downstream component, possibly injecting commands.

+ Demonstrative Examples

Example 1

This code dynamically builds an HTML page using POST data:

(Bad Code)
Example Language: PHP 
$username = $_POST['username'];
$picSource = $_POST['picsource'];
$picAltText = $_POST['picalttext'];
echo "<title>Welcome, " . htmlentities($username) ."</title>";
echo "<img src='". htmlentities($picSource) ." ' alt='". htmlentities($picAltText) . '" />';

The programmer attempts to avoid XSS exploits (CWE-79) by encoding the POST values so they will not be interpreted as valid HTML. However, the htmlentities() encoding is not appropriate when the data are used as HTML attributes, allowing more attributes to be injected.

For example, an attacker can set picAltText to:

"altTextHere' onload='alert(document.cookie)"

This will result in the generated HTML image tag:

Example Language: HTML 
<img src='pic.jpg' alt='altTextHere' onload='alert(document.cookie)' />

The attacker can inject arbitrary javascript into the tag due to this incorrect encoding.

+ Observed Examples
Server does not properly handle requests that do not contain UTF-8 data; browser assumes UTF-8, allowing XSS.
+ Potential Mitigations

Phase: Implementation

Strategy: Output Encoding

Use context-aware encoding. That is, understand which encoding is being used by the downstream component, and ensure that this encoding is used. If an encoding can be specified, do so, instead of assuming that the default encoding is the same as the default being assumed by the downstream component.

Phase: Architecture and Design

Strategy: Output Encoding

Where possible, use communications protocols or data formats that provide strict boundaries between control and data. If this is not feasible, ensure that the protocols or formats allow the communicating components to explicitly state which encoding/decoding method is being used. Some template frameworks provide built-in support.

Phase: Architecture and Design

Strategy: Libraries or Frameworks

Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

For example, consider using the ESAPI Encoding control [R.838.7] or a similar tool, library, or framework. These will help the programmer encode outputs in a manner less prone to error.

Note that some template mechanisms provide built-in support for the appropriate encoding.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class116Improper Encoding or Escaping of Output
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory845CERT Java Secure Coding Section 00 - Input Validation and Data Sanitization (IDS)
Weaknesses Addressed by the CERT Java Secure Coding Standard (primary)844
ChildOfCategoryCategory8672011 Top 25 - Weaknesses On the Cusp
Weaknesses in the 2011 CWE/SANS Top 25 Most Dangerous Software Errors (primary)900
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CERT Java Secure CodingIDS12-JPerform lossless conversion of String data between differing character encodings
CERT Java Secure CodingIDS13-JUse compatible encodings on both sides of file or network IO
+ References
[R.838.1] Jim Manico. "Injection-safe templating languages". 2010-06-30. <>.
[R.838.2] Dinis Cruz. "Can we please stop saying that XSS is boring and easy to fix!". 2010-09-25. <>.
[R.838.3] Ivan Ristic. "Canoe: XSS prevention via context-aware output encoding". 2010-09-24. <>.
[R.838.4] Jim Manico. "What is the Future of Automated XSS Defense Tools?". 2011-03-08. <>.
[R.838.5] [REF-15] Jeremiah Grossman, Robert "RSnake" Hansen, Petko "pdp" D. Petkov, Anton Rager and Seth Fogie. "XSS Attacks". Preventing XSS Attacks. Syngress. 2007.
[R.838.6] [REF-20] OWASP. "DOM based XSS Prevention Cheat Sheet". <>.
[R.838.7] [REF-21] OWASP. "OWASP Enterprise Security API (ESAPI) Project". <>.
+ Content History
Submission DateSubmitterOrganizationSource
2011-03-24MITREInternal CWE Team
Modification DateModifierOrganizationSource
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences, Relationships, Taxonomy_Mappings
2011-06-27CWE Content TeamMITREInternal
updated Demonstrative_Examples, Related_Attack_Patterns, Relationships
2012-05-11CWE Content TeamMITREInternal
updated Potential_Mitigations, References, Relationships, Taxonomy_Mappings

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Page Last Updated: January 18, 2017