CVE-2026-44435 h2o CVE debrief

Who should care

Users of the H2O HTTP server with the Quicly QUIC protocol implementation should be aware of this vulnerability and take steps to ensure their systems are updated. This includes administrators, security teams, and operators who manage H2O HTTP server deployments. The vulnerability has a CVSS score of 7.5 and is considered HIGH severity, indicating that it could have significant operational impact if exploited.

Technical summary

The Quicly IETF QUIC protocol implementation has a Denial of Service vulnerability. An assertion failure is raised when the total number of valid handshake messages received over a CRYPTO stream of a single packet number space exceeds 32KB. The issue was fixed by commit 937d0e9. This vulnerability affects users of the H2O HTTP server with the Quicly QUIC protocol implementation. The vulnerability has a CVSS score of 7.5 and is considered HIGH severity.

Defensive priority

High

Recommended defensive actions

• Update Quicly to the latest version with commit 937d0e9 or later
• Monitor for unusual traffic patterns
• Implement compensating controls to limit exposure
• Review and verify affected scope and inventory checks
• Monitor for exception tracking and retest
• Track exceptions and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

Evidence is limited. Verify affected scope and inventory checks. Monitor for exception tracking and retest. The issue arises in Quicly, an IETF QUIC protocol implementation in the H2O HTTP server, where an assertion failure is raised when the total number of valid handshake messages received over a CRYPTO stream of a single packet number space exceeds 32KB. Limited evidence suggests that this issue was fixed by commit 937d0e9. Further verification is required to confirm affected systems and ensure updates are applied.

Official resources