CVE-2026-55204: HAProxy through 3.4.0, fixed in commit 9a6d1fe, contains a null pointer dereference vulnerability in hpack_dh

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HIGH 7.5REMOTEUPGRADE

CVE-2026-55204 is a high-severity flaw (CVSS 7.5) affecting haproxy. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

HAProxy through 3.4.0, fixed in commit 9a6d1fe, contains a null pointer dereference vulnerability in hpack_dht_insert() within src/hpack-tbl.c that fails to validate the return value of hpack_dht_defrag() when the memory pool is exhausted. An attacker can trigger HPACK dynamic table insertions under memory pressure to dereference a NULL pointer and crash HAProxy worker processes, causing denial of service.

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-476
Published	2026-06-18
Last modified	2026-10-26

Publisher (CNA): VulnCheck · Vendor: haproxy

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-18 17:16 UTCPublished CVE record published
2026-10-26 11:10 UTCUpdated Record last modified

haproxy haproxy UPGRADE

Affected: < 2.4.36 · >= 2.5 and < 2.6.30 · >= 2.7 and < 2.8.25 · >= 2.9 and < 3.0.24 · >= 3.1 and < 3.2.20 · >= 3.3 and < 3.4.1

Fixed in: 2.4.36, 2.6.30, 2.8.25, 3.0.24, 3.2.20, 3.4.1

Safe to downgrade to: a release before 2.5

1. Check installed version

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

2. UPGRADE to 2.4.36 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade haproxy

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 2.5 (Debian / Ubuntu)

apt-cache madison haproxy sudo apt install haproxy=<VERSION_BEFORE_2.5> sudo apt-mark hold haproxy

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

dnf --showduplicates list haproxy sudo dnf downgrade haproxy-<VERSION_BEFORE_2.5> sudo dnf versionlock add haproxy

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-2026-55204?

haproxy: upgrade to 2.4.36, 2.6.30, 2.8.25, 3.0.24, 3.2.20, 3.4.1. Exact commands for each OS are on this page.

Is CVE-2026-55204 being exploited?

CVE-2026-55204 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-2026-55204 affect?

CVE-2026-55204 affects haproxy. Severity is high (CVSS 7.5). HAProxy through 3.4.0, fixed in commit 9a6d1fe, contains a null pointer dereference vulnerability in hpack_dht_insert() within src/hpack-tbl.c that fails to validate the return value of hpack_dht_defrag() when the memory pool is exhausted.

How urgent is CVE-2026-55204?

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

Software pages

HAProxy vulnerabilities

Related CVEs

CVE-2026-55203 HIGH 7.5 UPGRADE HAProxy through 3.4.0, fixed in commit 5985276, contains an integer overflow vulnerability in the fcgi_conn st

References

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