h3 Server DoS via Malicious Chunked Cookie Parsing

Executive Summary

This is a Denial of Service (DoS) vulnerability in the h3 library affecting versions before 2.0.1-rc.18. It occurs in the setChunkedCookie() and deleteChunkedCookie() functions where user-controlled cookie values are parsed without validating the chunk count. Attackers can send a crafted cookie with an extremely large chunk count, triggering an O(n²) cleanup loop that hangs the server process.

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Detection Guidance

Monitor for unusually high CPU usage or unresponsive servers after receiving requests with cookies containing __chunked__ followed by a large number. Check server logs for requests with crafted cookie headers like 'Cookie: h3=__chunked__999999'.

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Impact Analysis

An attacker can exploit this to cause a complete DoS by sending a single unauthenticated request with a malicious cookie. The server will hang indefinitely, unable to process other requests. This leads to downtime, service unavailability, and potential loss of functionality for applications using affected h3 versions.

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Compliance Impact

This vulnerability causes a Denial of Service (DoS) by hanging the server process, which can lead to prolonged unavailability of services. For GDPR, this may violate Article 32 requirements for availability and resilience of processing systems. For HIPAA, it could disrupt access to protected health information, potentially violating Security Rule requirements for access controls and availability.

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Mitigation Strategies

Update the h3 library to version 2.0.1-rc.18 or later. If immediate update is not possible, implement input validation to limit the chunk count in cookies to a reasonable maximum value (e.g., 100).

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