What’s Vulnerable › Vulnerabilities

CVE-2026-44435: Quicly: Remote Denial of Service via assertion failure when CRYPTO stream handshake data exceeds 32KB

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-16 · updated 2026-10-24
• Weakness: CWE-400

Description

Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit 937d0e9, 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, causing a Denial of Service. This issue has been fixed by commit 937d0e9.

Affected products

• h2o quicly: before 2026-05-29 — fixed in 2026-05-29

How to fix

Upgrade to a fixed version: h2o quicly 2026-05-29. Check the vendor advisory for mitigations.

References

• https://github.com/h2o/quicly/commit/937d0e9e7c669fc2bee4920632ab6aaac60e4d81
• https://github.com/h2o/quicly/security/advisories/GHSA-2cw9-5673-73gv

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