CVE-2026-44453: h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3

Q: How do I fix CVE-2026-44453?

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

Q: Is CVE-2026-44453 being exploited?

CVE-2026-44453 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-2026-44453 affect?

CVE-2026-44453 affects h2o. Severity is high (CVSS 7.5). h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 6b5370d, h2o is vulnerable to a Denial of Service attack when calling alloca under certain conditions. When serving static files, h2o builds the file path o

Q: How urgent is CVE-2026-44453?

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

This month

HIGH 7.5REMOTEUPGRADE

CVE-2026-44453 is a high-severity flaw (CVSS 7.5) 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 6b5370d, h2o is vulnerable to a Denial of Service attack when calling alloca under certain conditions. When serving static files, h2o builds the file path on stack, by calling alloca. The maximum size of the memory allocated using alloca can be as huge as ~600KB, which exceeds the default pthread stack size used by musl libc (128KB). If the amount of memory alloca

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 58%)
CISA exploited list	No
Weakness	CWE-770, CWE-789
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 13:25 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-2026-44453?

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

Is CVE-2026-44453 being exploited?

CVE-2026-44453 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-2026-44453 affect?

CVE-2026-44453 affects h2o. Severity is high (CVSS 7.5). h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 6b5370d, h2o is vulnerable to a Denial of Service attack when calling alloca under certain conditions. When serving static files, h2o builds the file path o

How urgent is CVE-2026-44453?

Priority 42/100. High severity; 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-2026-44452 MEDIUM 5.9 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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