Failure to Sanitize Alternate XSS Syntax
Weakness ID: 87 (Weakness Variant)Status: Draft
+ Description

Description Summary

The software fails to adequately filter user-controlled input for alternate script syntax.
+ Time of Introduction
  • Implementation
+ Applicable Platforms



+ Demonstrative Examples

Example 1

In the following example, an XSS sanitization routine checks for the lower-case "script" string but fails to account for alternate strings ("SCRIPT", for example).

(Bad Code)
Example Language: Java 
public String sanitize(String input, String mask) {
return input.replaceAll("script", mask);
+ Observed Examples
CVE-2002-0738XSS using "&={script}".
+ Potential Mitigations

Resolve all filtered input to absolute or canonical representations before processing.

Carefully check each input parameter against a rigorous positive specification (white list) defining the specific characters and format allowed. All input should be sanitized, not just parameters that the user is supposed to specify, but all data in the request, including tag attributes, 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.

This involves "HTML Entity Encoding" all non-alphanumeric characters from data that was received from the user and is now being written to the request.

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

Additionally, 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 Internet Explorer), this attribute prevents the user's session cookie from being accessed by client-side scripts, including scripts inserted due to a XSS attack.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness BaseWeakness Base79Failure to Preserve Web Page Structure ('Cross-site Scripting')
Development Concepts (primary)699
Research Concepts (primary)1000
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERAlternate XSS syntax
+ Related Attack Patterns
CAPEC-IDAttack Pattern Name
(CAPEC Version: 1.4)
199Cross-Site Scripting Using Alternate Syntax
+ Content History
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Sean EidemillerCigitalExternal
added/updated demonstrative examples
2008-07-01Eric DalciCigitalExternal
updated Time of Introduction
2008-09-08CWE Content TeamMITREInternal
updated Name, Relationships, Taxonomy Mappings
2009-07-27CWE Content TeamMITREInternal
updated Related Attack Patterns
Previous Entry Names
Change DatePrevious Entry Name
2008-09-09Alternate XSS Syntax