Inconsistent interpretation of HTTP requests in HAProxy - CVE-2026-33555

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Main Vulnerability Database Vulnerabilities #VU126147

Inconsistent interpretation of HTTP requests in HAProxy - CVE-2026-33555

Published: April 15, 2026


Vulnerability identifier: #VU126147
CSH Severity: Medium
CVSS v4: 5.1 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-33555
CWE-ID: CWE-444
Exploitation vector: Remote access
Exploit availability: No public exploit available

Vulnerability details

The vulnerability allows a remote attacker to perform HTTP request smuggling attacks.

The vulnerability exists due to improper validation of HTTP/3 requests. A remote attacker can send a specially crafted HTTP/3 request to the server and smuggle arbitrary HTTP headers.

Successful exploitation of vulnerability may allow an attacker to poison HTTP cache and perform phishing attacks.


Affected software

HAProxy
Debian Linux
SUSE Linux Enterprise High Availability Extension 15
SUSE Linux Enterprise Server 15
SUSE Linux Enterprise Server for SAP Applications 15
openSUSE Leap
openEuler
Fedora
haproxy-help
haproxy-debugsource
haproxy-debuginfo
haproxy
haproxy (Debian package)

How to mitigate CVE-2026-33555

Install updates from vendor's website.

HAProxy - addressed in versions 3.0.19, 3.1.17, 3.2.15, 3.3.6
haproxy-help - addressed in versions 2.6.6-19, 2.9.5-13, 3.0.11-2
haproxy-debugsource - addressed in versions 2.6.6-19, 2.9.5-13, 3.0.11-2
haproxy-debuginfo - addressed in versions 2.6.6-19, 2.9.5-13, 3.0.11-2
haproxy - addressed in versions 2.6.6-19, 2.9.5-13, 3.0.11-2
haproxy-debugsource - update to 2.8.11+git0.01c1056a4-150600.3.12.1
haproxy - update to 2.8.11+git0.01c1056a4-150600.3.12.1
haproxy-debuginfo - update to 2.8.11+git0.01c1056a4-150600.3.12.1
haproxy (Debian package) - update to 3.0.11-1+deb13u3
haproxy - addressed in versions 3.0.23-1.fc42, 3.0.23-1.fc43, 3.0.23-1.fc44

External References

Related Security Bulletins