CAPEC-199

Detailed Abstraction Level
Meta — Very abstract, high-level category
Standard — Specific enough to understand
Detailed — Tied to specific technique Draft MITRE CAPEC Status
Stable — Fully reviewed and complete
Draft — Under development
Incomplete — Partially defined
Deprecated — No longer recommended
Obsolete — Replaced by another CAPEC Likelihood: High Severity: High
XSS Using Alternate Syntax

Description

An adversary 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.

Mitigations

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.

Skills Required

[Low] To inject the malicious payload in a web page

[High] To bypass non trivial filters in the application