XSS Filter Bypass in Joomla Core via HTML5 Entity Decode Mismatch

Executive Summary

This is a Cross-Site Scripting (XSS) vulnerability in Joomla! Core versions 1.5.0-5.4.8 and 6.0.0-6.1.3. The issue occurs in the InputFilter component where HTML5 entity decoding mismatches allow bypassing XSS protections. Specifically, the checkAttribute method normalizes attribute values before checking for malicious 'javascript:' schemes but fails to decode HTML5 entities first, enabling XSS attacks.

Useful 0 Not Useful 0

Detection Guidance

This vulnerability involves an XSS filter bypass in Joomla's InputFilter due to HTML5 entity decode mismatch. Detection requires checking Joomla versions 1.5.0-5.4.8 or 6.0.0-6.1.3 for vulnerable InputFilter.php files. Use commands like 'find /var/www -name InputFilter.php' to locate files and 'grep -r "checkAttribute" /path/to/joomla' to inspect code for the vulnerable method.

Useful 0 Not Useful 0

Impact Analysis

An attacker could exploit this to inject malicious scripts into web pages viewed by users. This may lead to theft of sensitive data like cookies or session tokens, unauthorized actions on behalf of users, or defacement of websites. The impact depends on user privileges and site functionality.

Useful 0 Not Useful 0

Compliance Impact

This XSS vulnerability could lead to unauthorized data access or modification, violating GDPR's integrity and confidentiality principles or HIPAA's safeguards for protected health information. Organizations may face compliance violations, fines, or reputational damage if exploited.

Useful 0 Not Useful 0

Mitigation Strategies

Immediately update Joomla to a patched version beyond 5.4.8 or 6.1.3. If updating is not possible, apply the Joomla security patch for [20260915] manually. Disable or restrict user input that could trigger XSS vectors until patched.

Useful 0 Not Useful 0