CVE-2026-33555

haproxy: HAProxy: Request smuggling via HTTP/3 parser desynchronization

Published Apr 13, 2026 CVSS 3.1 · 4 Medium EPSS < 1% KEV · Not listed
Description
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Debian DLA Debian DLA DLA-4842-1 haproxy security update
Debian DSA Debian DSA DSA-6291-1 haproxy security update
History
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'poc', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
Title haproxy: HAProxy: Request smuggling via HTTP/3 parser desynchronization
References
Metrics threat_severity

None

threat_severity

Moderate

Type Values Removed Values Added
Description 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.
First Time appeared Haproxy
Haproxy haproxy
Weaknesses CWE-130
CPEs cpe:2.3:a:haproxy:haproxy:*:*:*:*:*:*:*:*
Vendors & Products Haproxy
Haproxy haproxy
References
Metrics cvssV3_1

{'score': 4, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
mitre
Published
Updated
Oct 18, 2026, 8:07 PM
Reserved
Mar 22, 2026, 12:00 AM
Status
Modified
Published
Apr 13, 2026, 5:16 PM
Modified
Oct 18, 2026, 8:16 PM
Severity
Moderate
Public date
Apr 13, 2026, 12:00 AM