Chunked Transfer Encoding Bypass in Undertow Web Server

Executive Summary

This vulnerability is in the ChunkReader component of the Undertow web server, which processes chunked transfer encoding in HTTP requests. A flaw in the security check for chunk size allows a specially crafted 15-digit chunk size to bypass size limits and overflow into internal state flags. This causes the server to terminate a request body early and treat remaining data as a separate, smuggled request.

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Detection Guidance

Detecting this vulnerability requires monitoring for HTTP request smuggling attempts. Check Undertow server logs for malformed chunked transfer encoding requests with 15-digit chunk sizes. Use network traffic analysis tools like tcpdump or Wireshark to inspect HTTP requests for unusual chunked encoding patterns. Enable Undertow debug logging to capture parsing errors in ChunkReader.

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Impact Analysis

Exploitation could allow an attacker to bypass security controls or poison web caches by embedding a second request within the first. It may also enable denial-of-service attacks or cause unintended server behavior. The attack requires precise TCP segmentation control but no privileges or user interaction.

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Compliance Impact

This vulnerability could potentially impact compliance with GDPR and HIPAA by enabling HTTP request smuggling, which may lead to unauthorized data access or manipulation. Attackers could bypass security controls to exfiltrate or alter sensitive data, violating confidentiality and integrity requirements under these regulations.

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Mitigation Strategies

Immediately update Undertow to the latest patched version if available. If no patch exists, disable chunked transfer encoding in your Undertow configuration. Implement network-level protections like WAF rules to block suspicious chunked requests. Monitor for exploitation attempts and isolate affected systems if suspicious activity is detected.

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