Common Weakness Enumeration

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CWE-81: Improper Neutralization of Script in an Error Message Web Page

Improper Neutralization of Script in an Error Message Web Page
Weakness ID: 81 (Weakness Variant)Status: Incomplete
+ Description

Description Summary

The software receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special characters that could be interpreted as web-scripting elements when they are sent to an error page.

Extended Description

Error pages may include customized 403 Forbidden or 404 Not Found pages.

When an attacker can trigger an error that contains unneutralized input, then cross-site scripting attacks may be possible.

+ Time of Introduction
  • Implementation
  • Operation
+ Applicable Platforms



+ Common Consequences

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

+ Observed Examples
XSS in default error page from Host: header.
XSS in error message.
XSS in error page from targeted parameter.
+ Potential Mitigations

Phase: Implementation

Do not write user-controlled input to error pages.

Phase: Implementation

Carefully check each input parameter against a rigorous positive specification (white list) defining the specific characters and format allowed. All input should be neutralized, not just parameters that the user is supposed to specify, but all data in the request, including hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS vulnerabilities is to validate only fields that are expected to be redisplayed by the site. We often encounter data from the request that is reflected by the application server or the application that the development team did not anticipate. Also, a field that is not currently reflected may be used by a future developer. Therefore, validating ALL parts of the HTTP request is recommended.

Phase: Implementation

Strategy: Output Encoding

Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component.

The problem of inconsistent output encodings often arises in web pages. If an encoding is not specified in an HTTP header, web browsers often guess about which encoding is being used. This can open up the browser to subtle XSS attacks.

Phase: Implementation

With Struts, write all data from form beans with the bean's filter attribute set to true.

Phase: Implementation

Strategy: Identify and Reduce Attack Surface

To help mitigate XSS attacks against the user's session cookie, set the session cookie to be HttpOnly. In browsers that support the HttpOnly feature (such as more recent versions of Internet Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible to malicious client-side scripts that use document.cookie. This is not a complete solution, since HttpOnly is not supported by all browsers. More importantly, XMLHTTPRequest and other powerful browser technologies provide read access to HTTP headers, including the Set-Cookie header in which the HttpOnly flag is set.

Effectiveness: Defense in Depth

+ Weakness Ordinalities
(where the weakness is typically related to the presence of some other weaknesses)
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness BaseWeakness Base79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory896SFP Cluster: Tainted Input
Software Fault Pattern (SFP) Clusters (primary)888
CanAlsoBeWeakness BaseWeakness Base209Information Exposure Through an Error Message
Research Concepts1000
CanAlsoBeWeakness ClassWeakness Class390Detection of Error Condition Without Action
Research Concepts1000
+ Causal Nature


+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERXSS in error pages
+ References
[REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 11: Failure to Handle Errors Correctly." Page 183. McGraw-Hill. 2010.
+ Content History
Submission DateSubmitterOrganizationSource
Externally Mined
Modification DateModifierOrganizationSource
updated Time_of_Introduction
updated Description, Relationships, Taxonomy_Mappings, Weakness_Ordinalities
updated Description
updated Description, Name
updated Related_Attack_Patterns
updated Description, Name, Potential_Mitigations
updated Potential_Mitigations
updated Common_Consequences
updated References, Relationships
updated Potential_Mitigations
updated Potential_Mitigations
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11XSS in Error Pages
2009-05-27Failure to Sanitize Directives in an Error Message Web Page
2010-06-21Improper Sanitization of Script in an Error Message Web Page
Page Last Updated: June 23, 2014