What is CVE-2026-44435?

CVE-2026-44435 is a high-severity vulnerability in H2o Quicly, classified under Uncontrolled Resource Consumption. CVSS score: 7.5/10. Published 2026-07-16.

How severe is CVE-2026-44435?

High severity. CVSS v3 base score is 7.5 out of 10.

Resource exhaustion in H2o Quicly

CVE-2026-44435

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.

Vulnerability class: DoS (Denial of Service)

Published 2026-07-16 · last modified 2026-08-06.

CVSS v3 metric

CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

EPSS exploit prediction

EPSS: 0.005 (41.1th percentile), scored 2026-10-24.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 41st percentile — 41.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.005 · newest: 0.005 · change: 0.000

Affected products

H2o Quicly — versions < 937d0e9

Weakness classification (CWE)

References

github.com/h2o/quicly/security/advisories/GHSA-2cw9-5673-73gv (Vendor Advisory)
github.com/h2o/quicly/commit/937d0e9e7c669fc2bee4920632ab6aaac60e4d81 (Patch)

Frequently asked questions

What is CVE-2026-44435?: CVE-2026-44435 is a high-severity vulnerability in H2o Quicly, classified under Uncontrolled Resource Consumption. CVSS score: 7.5/10. Published 2026-07-16.
How severe is CVE-2026-44435?: High severity. CVSS v3 base score is 7.5 out of 10.