CVE-2012-0007 in Anti-cross Site Scripting Library
Summary
by MITRE
The Microsoft Anti-Cross Site Scripting (AntiXSS) Library 3.x and 4.0 does not properly evaluate characters after the detection of a Cascading Style Sheets (CSS) escaped character, which allows remote attackers to conduct cross-site scripting (XSS) attacks via HTML input, aka "AntiXSS Library Bypass Vulnerability."
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Analysis
by VulDB Data Team • 09/01/2025
The CVE-2012-0007 vulnerability represents a critical security flaw in Microsoft's Anti-Cross Site Scripting (AntiXSS) Library versions 3.x and 4.0 that fundamentally undermines the library's ability to prevent cross-site scripting attacks. This vulnerability operates at the core of web application security by exploiting a specific parsing behavior in the library's CSS escape character handling mechanism. The flaw occurs when the library encounters CSS escaped characters within HTML input and fails to properly evaluate subsequent characters in the input stream, creating a pathway for malicious code execution.
The technical implementation of this vulnerability stems from the AntiXSS Library's insufficient validation of CSS escape sequences during the input sanitization process. When processing HTML input containing CSS escaped characters, the library's parsing logic incorrectly continues to evaluate the input stream beyond the initial escape sequence detection. This improper state management allows attackers to craft malicious payloads that bypass the library's intended protection mechanisms. The vulnerability specifically affects how the library handles character sequences that begin with CSS escape patterns, enabling attackers to inject script code that would normally be neutralized by the anti-xss filtering process.
The operational impact of this vulnerability extends beyond simple XSS attacks to potentially compromise entire web applications and user sessions. Attackers can leverage this bypass to execute arbitrary JavaScript code in the context of affected web applications, potentially leading to session hijacking, data theft, or privilege escalation. The vulnerability affects any web application that relies on Microsoft AntiXSS Library 3.x or 4.0 for input validation and sanitization, making it particularly dangerous in enterprise environments where this library is commonly deployed. The bypass occurs silently without alerting administrators or triggering typical security monitoring systems, as the malicious input appears to pass validation successfully.
Security professionals should understand that this vulnerability aligns with CWE-79 (Cross-site Scripting) and represents a specific implementation weakness in the input validation process. The ATT&CK framework categorizes this as a technique for 'Command and Control' and 'Persistence' through web-based attacks, as successful exploitation can establish persistent access to vulnerable applications. Organizations should immediately assess their dependency on the affected AntiXSS Library versions and implement immediate mitigations including upgrading to patched versions, implementing additional input validation layers, or deploying web application firewalls. The vulnerability demonstrates the critical importance of thorough testing of security libraries and the potential for seemingly benign parsing logic to create significant security weaknesses in web application defenses.