CVE-2026-44452: h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3

Q: How do I fix CVE-2026-44452?

h2o: upgrade to 2026-05-29. Exact commands for each OS are on this page.

Q: Is CVE-2026-44452 being exploited?

CVE-2026-44452 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-2026-44452 affect?

CVE-2026-44452 affects h2o. Severity is medium (CVSS 5.9). h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 8dc37cb, when h2o receives a ClientHello message over TLS or QUIC and it contains a zero-length SNI extension, the h2o server runs over the zero-length host

Q: How urgent is CVE-2026-44452?

Priority 34/100. Remotely exploitable, no login, no user action

Routine

MEDIUM 5.9REMOTEUPGRADE

CVE-2026-44452 is a medium-severity flaw (CVSS 5.9) affecting h2o. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 8dc37cb, when h2o receives a ClientHello message over TLS or QUIC and it contains a zero-length SNI extension, the h2o server runs over the zero-length hostname while trying to copy the hostname, assuming that it is NULL-terminated. This is a potential denial-of-service attack vector in sense that it might trigger segmentation violation. This issue has been fixed

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.9 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 65%)
CISA exploited list	No
Weakness	CWE-125, CWE-170
Published	2026-07-16
Last modified	2026-08-06

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

Last updated 2026-08-06 · 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
2026-08-06 13:56 UTCUpdated Record last modified

h2o h2o 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 h2o || rpm -q h2o || h2o --version

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

sudo apt update && sudo apt install --only-upgrade h2o

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade h2o

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-2026-44452?

h2o: upgrade to 2026-05-29. Exact commands for each OS are on this page.

Is CVE-2026-44452 being exploited?

CVE-2026-44452 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-2026-44452 affect?

CVE-2026-44452 affects h2o. Severity is medium (CVSS 5.9). h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 8dc37cb, when h2o receives a ClientHello message over TLS or QUIC and it contains a zero-length SNI extension, the h2o server runs over the zero-length host

How urgent is CVE-2026-44452?

Priority 34/100. Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-3960 CRITICAL 9.8 UPGRADE A critical remote code execution vulnerability exists in the unauthenticated REST API endpoint /99/ImportSQLTa
CVE-2026-44453 HIGH 7.5 UPGRADE h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3
CVE-2026-54340 HIGH 7.5 UPGRADE h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3

References

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