ModSecurity Multipart Parser Filename Bypass Vulnerability

Executive Summary

ModSecurity versions 3.0.0 to 3.0.17 have a flaw in their multipart parser where the RFC 2231 filename* parameter is not prioritized over the filename parameter. Attackers can bypass WAF rules by providing a benign filename in filename= and a malicious filename in filename*=UTF-8''<malicious-name>. The backend processes the malicious filename, leading to potential security breaches.

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Detection Guidance

To detect this vulnerability, inspect ModSecurity logs for multipart requests with Content-Disposition headers containing both filename and filename* parameters. Check if WAF rules fail to block malicious filenames due to the bypass. Use tools like tcpdump or Wireshark to capture and analyze HTTP requests for suspicious multipart payloads.

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Impact Analysis

This vulnerability allows attackers to bypass file upload security checks, potentially uploading malicious files like web shells. If stored in a web-accessible directory, these files could enable remote code execution, leading to server compromise, data theft, or further network infiltration.

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Compliance Impact

This vulnerability could lead to unauthorized file uploads, violating data integrity and confidentiality requirements in GDPR and HIPAA. Compromised systems may expose sensitive data, resulting in non-compliance with these regulations and potential legal penalties.

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Mitigation Strategies

Upgrade ModSecurity to version 3.0.17 or later to apply the fix. Review and update WAF rules to explicitly handle filename* parameters. Monitor for bypass attempts and ensure backend systems validate filenames independently of the WAF.

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