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

published 2026-07-16 3 mo ago modified 2026-10-24 15 d ago also EUVD-2026-45071

cvss: 7.5; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.5%; EPSS 0.5% chance of exploitation in 30 days, percentile 42%
public exploits: 0; none known

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.

Patch / fix

fixed in 1 links 1

h2o quicly 2026-05-29

github.com/h2o/quicly@937d0e9 commit

CISA SSVC

Exploitation: none
Automatable: yes
Technical impact: partial
Assessed: 3 mo ago (2026-07-17)

Exploit prediction

0.49% chance of exploitation in the next 30 days, higher than 42% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected products

h2o quicly

Classification

Weakness

CWE-617 Reachable Assertion
CWE-400 Uncontrolled Resource Consumption

References

github.com /h2o/quicly/commit/937d0e9e7c669fc2bee4920632ab6aaac60e4d81
patch
github.com /h2o/quicly/security/advisories/GHSA-2cw9-5673-73gv
vendor-advisory

Sources

CVE List CVE-2026-44435
NVD CVE-2026-44435

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary