| Name | Cross-Site Scripting Using Alternate Syntax |
| Summary | The attacker uses alternate forms of keywords or commands that result in the same action as the primary form but which may not be caught by filters. For example, many keywords are processed in a case insensitive manner. If the site's web filtering algorithm does not convert all tags into a consistent case before the comparison with forbidden keywords it is possible to bypass filters (e.g., incomplete black lists) by using an alternate case structure. For example, the "script" tag using the alternate forms of "Script" or "ScRiPt" may bypass filters where "script" is the only form tested. Other variants using different syntax representations are also possible as well as using pollution meta-characters or entities that are eventually ignored by the rendering engine. The attack can result in the execution of otherwise prohibited functionality. |
| Prerequisites | Target client software must allow scripting such as JavaScript. |
| Solutions | Design: Use browser technologies that do not allow client side scripting.
Design: Utilize strict type, character, and encoding enforcement
Implementation: Ensure all content that is delivered to client is sanitized against an acceptable content specification.
Implementation: Ensure all content coming from the client is using the same encoding; if not, the server-side application must canonicalize the data before applying any filtering.
Implementation: Perform input validation for all remote content, including remote and user-generated content
Implementation: Perform output validation for all remote content.
Implementation: Disable scripting languages such as JavaScript in browser
Implementation: Patching software. There are many attack vectors for XSS on the client side and the server side. Many vulnerabilities are fixed in service packs for browser, web servers, and plug in technologies, staying current on patch release that deal with XSS countermeasures mitigates this. |
| Related Weaknesses |
| CWE ID | Description |
| CWE-20 | Improper Input Validation |
| CWE-71 | Apple '.DS_Store' |
| CWE-79 | Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') |
| CWE-85 | Doubled Character XSS Manipulations |
| CWE-86 | Improper Neutralization of Invalid Characters in Identifiers in Web Pages |
| CWE-87 | Improper Neutralization of Alternate XSS Syntax |
| CWE-692 | |
| CWE-697 | Insufficient Comparison |
| CWE-713 | |
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