CVE-2026-33555: An issue was discovered in HAProxy before 3.3.6

This week

MEDIUM 4.0REMOTEUPGRADE

CVE-2026-33555 is a medium-severity flaw (CVSS 4.0) affecting haproxy. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

An issue was discovered in HAProxy before 3.3.6. The HTTP/3 parser does not check that the received body length matches a previously announced content-length when the stream is closed via a frame with an empty payload. This can cause desynchronization issues with the backend server and could be used for request smuggling. The earliest affected version is 2.6.

Why this priority

Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	4.0 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 77%)
CISA exploited list	No
Weakness	CWE-130
Published	2026-04-13
Last modified	2026-10-18

Publisher (CNA): MITRE · Vendor: haproxy

Last updated 2026-10-18 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-13 17:16 UTCPublished CVE record published
2026-10-18 20:16 UTCUpdated Record last modified

haproxy haproxy UPGRADE

Affected: >= 2.6.0 and < 3.3.6

Fixed in: 3.3.6

Safe to downgrade to: a release before 2.6.0

1. Check installed version

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

2. UPGRADE to 3.3.6 (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 2.6.0 (Debian / Ubuntu)

apt-cache madison haproxy sudo apt install haproxy=<VERSION_BEFORE_2.6.0> 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_2.6.0> 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-2026-33555?

haproxy: upgrade to 3.3.6. Exact commands for each OS are on this page.

Is CVE-2026-33555 being exploited?

CVE-2026-33555 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-2026-33555 affect?

CVE-2026-33555 affects haproxy. Severity is medium (CVSS 4.0). An issue was discovered in HAProxy before 3.3.6. The HTTP/3 parser does not check that the received body length matches a previously announced content-length when the stream is closed via a frame with an empty payload. This can cause desync

How urgent is CVE-2026-33555?

Priority 30/100. Public exploit reference; Remotely exploitable, no login, no user action

Software pages

HAProxy vulnerabilities

Related CVEs

CVE-2026-55204 HIGH 7.5 UPGRADE HAProxy through 3.4.0, fixed in commit 9a6d1fe, contains a null pointer dereference vulnerability in hpack_dh
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
CVE-2026-26080 LOW 3.7 UPGRADE UPGRADE HAProxy Community Edition 3.2.x through 3.3.x before 3.3.3 can enter a loop or crash because varint is mishand
CVE-2026-26081 MEDIUM 4.8 UPGRADE UPGRADE HAProxy Community Edition 3.0 through 3.3 before 3.3.3 lacks a length check for the NEW_TOKEN format

References

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