High severity · published Jul 16, 2026
CVE-2026-44435
Quicly: Remote Denial of Service via assertion failure when CRYPTO stream handshake data exceeds 32KB
7.5
CVSS
0.5%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
None
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.5% · higher than 42% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
QU
quicly · h2o
< 937d0e9 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-44435
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-44435
Patch
https://github.com/h2o/quicly/commit/937d0e9e7c669fc2bee4920632ab6aaac60e4d81
Vendor Advisory
https://github.com/h2o/quicly/security/advisories/GHSA-2cw9-5673-73gv
More from quicly
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CVE-2026-44436
Quicly is vulnerable to connection state corruption
7.5
CVE-2026-44433
Quicly is vulnerable to memory exhaustion
7.5
CVE-2025-61684
Quicly has assertion failures
5.3
CVE-2026-44434
Quicly is vulnerable to stateless reset injection