CSS Proxy Response Validation Flaw in Roundcube Webmail

Executive Summary

This vulnerability affects Roundcube Webmail versions before 1.6.18 and 1.7.x before 1.7.3. It involves unvalidated responses from the CSS proxy, which could lead to information disclosure or cross-site scripting (XSS) attacks through MIME sniffing.

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Detection Guidance

Detection involves checking Roundcube Webmail version and inspecting proxy responses for unvalidated CSS content. Verify if your version is below 1.6.18 or 1.7.3. Monitor network traffic for suspicious MIME types or scripts in CSS responses.

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Impact Analysis

An attacker could exploit this to steal sensitive information or execute malicious scripts in your browser when accessing Roundcube Webmail. This may result in unauthorized data access, session hijacking, or other security breaches.

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Compliance Impact

This vulnerability could lead to data breaches, violating GDPR's data protection requirements or HIPAA's safeguards for protected health information. Non-compliance may result in legal penalties, reputational damage, and loss of trust.

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Mitigation Strategies

Upgrade Roundcube Webmail to version 1.6.18 or 1.7.3 or later immediately. Disable CSS proxy functionality if not required. Implement input validation for CSS responses and enforce strict MIME type checks.

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