CVE-2024-45403: h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3

Routine

LOW 3.7REMOTEUPGRADE

CVE-2024-45403 is a low-severity flaw (CVSS 3.7) 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. When h2o is configured as a reverse proxy and HTTP/3 requests are cancelled by the client, h2o might crash due to an assertion failure. The crash can be exploited by an attacker to mount a Denial-of-Service attack. By default, the h2o standalone server automatically restarts, minimizing the impact. However, HTTP requests that were served concurrently will still be disrupted. The

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	3.7 (Low)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.7% (top 49%)
CISA exploited list	No
Weakness	CWE-617
Published	2024-10-11
Last modified	2026-06-17

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

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2024-10-11 15:15 UTCPublished CVE record published
2026-06-17 07:54 UTCUpdated Record last modified

dena h2o UPGRADE

Affected: >= 2024-06-18 and < 2024-09-04

Fixed in: 2024-09-04

Safe to downgrade to: a release before 2024-06-18

1. Check installed version

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

2. UPGRADE to 2024-09-04 (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 2024-06-18 (Debian / Ubuntu)

apt-cache madison h2o sudo apt install h2o=<VERSION_BEFORE_2024-06-18> 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_2024-06-18> 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-2024-45403?

h2o: upgrade to 2024-09-04. Exact commands for each OS are on this page.

Is CVE-2024-45403 being exploited?

CVE-2024-45403 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-2024-45403 affect?

CVE-2024-45403 affects h2o. Severity is low (CVSS 3.7). h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. When h2o is configured as a reverse proxy and HTTP/3 requests are cancelled by the client, h2o might crash due to an assertion failure. The crash can be exploited by an att

How urgent is CVE-2024-45403?

Priority 31/100. Remotely exploitable, no login, no user action

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References

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