CVE-2026-44435: Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server

Q: How do I fix CVE-2026-44435?

quicly: upgrade to 2026-05-29. Exact commands for each OS are on this page.

Q: Is CVE-2026-44435 being exploited?

CVE-2026-44435 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-44435 affect?

CVE-2026-44435 affects quicly. Severity is high (CVSS 7.5). 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

Q: How urgent is CVE-2026-44435?

Priority 42/100. High severity; Remotely exploitable, no login, no user action

This month

HIGH 7.5REMOTEUPGRADE

CVE-2026-44435 is a high-severity flaw (CVSS 7.5) affecting quicly. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 59%)
CISA exploited list	No
Weakness	CWE-400, CWE-617
Published	2026-07-16
Last modified	2026-10-24

Publisher (CNA): GitHub, Inc. · Vendor: h2o

Last updated 2026-10-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-16 23:16 UTCPublished CVE record published
2026-10-24 20:10 UTCUpdated Record last modified

h2o quicly UPGRADE

Affected: < 2026-05-29

Fixed in: 2026-05-29

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy quicly || rpm -q quicly || quicly --version

2. UPGRADE to 2026-05-29 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade quicly

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade quicly

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-44435?

quicly: upgrade to 2026-05-29. Exact commands for each OS are on this page.

Is CVE-2026-44435 being exploited?

CVE-2026-44435 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-44435 affect?

CVE-2026-44435 affects quicly. Severity is high (CVSS 7.5). 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

How urgent is CVE-2026-44435?

Priority 42/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-44436 HIGH 7.5 UPGRADE Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server
CVE-2026-44433 MEDIUM 5.3 UPGRADE Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server
CVE-2026-44434 MEDIUM 5.3 UPGRADE Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server

References

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