CVE-2026-54340: h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3

This month

HIGH 7.5REMOTEUPGRADE

CVE-2026-54340 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 9265bdd, there is an HTTP/2 state amplification issue that combines HPACK decompression amplification with Slowloris-style stream stalling. Amplified decoded header state can be retained by stalled HTTP/2 streams, and depending on the configuration, additional limits are needed to bound decoded header state and prevent attack. This issue has been fixed by commit 9

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-400, CWE-770
Published	2026-07-17
Last modified	2026-08-05

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

Last updated 2026-08-05 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-17 00:16 UTCPublished CVE record published
2026-08-05 19:47 UTCUpdated Record last modified

h2o h2o UPGRADE

Affected: >= 2026-05-29 and < 2026-06-04

Fixed in: 2026-06-04

Safe to downgrade to: a release before 2026-05-29

1. Check installed version

apt-cache policy h2o || rpm -q h2o || h2o --version

2. UPGRADE to 2026-06-04 (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.

3. If no update is available yet, DOWNGRADE to a release before 2026-05-29 (Debian / Ubuntu)

apt-cache madison h2o sudo apt install h2o=<VERSION_BEFORE_2026-05-29> sudo apt-mark hold h2o

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list h2o sudo dnf downgrade h2o-<VERSION_BEFORE_2026-05-29> sudo dnf versionlock add h2o

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

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-2026-54340?

h2o: upgrade to 2026-06-04. Exact commands for each OS are on this page.

Is CVE-2026-54340 being exploited?

CVE-2026-54340 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-2026-54340 affect?

CVE-2026-54340 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 9265bdd, there is an HTTP/2 state amplification issue that combines HPACK decompression amplification with Slowloris-style stream stalling. Amplified decode

How urgent is CVE-2026-54340?

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-2026-44453 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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