CVE-2026-55203: Haproxy

Critical severity, CVSS 9.1. EPSS: 0.3% chance of exploitation in the next 30 days.

HAProxy through 3.4.0, fixed in commit 5985276, contains an integer overflow vulnerability in the fcgi_conn structure's drl field that allows buffer misparse as new FCGI record headers. When contentLength is 65535 and paddingLength is 1 or more, the drl field wraps to 0, causing incorrect record consumption and allowing malicious FastCGI backends to desynchronize the FCGI framing parser, potentially causing request routing errors, response smuggling, or memory safety issues.

Affected products

• Haproxy Haproxy: before 2.4.36 (fixed in 2.4.36); from 2.5, before 2.6.30 (fixed in 2.6.30); from 2.7, before 2.8.25 (fixed in 2.8.25); from 2.9, before 3.0.24 (fixed in 3.0.24); from 3.1, before 3.2.20 (fixed in 3.2.20); from 3.3, before 3.4.1 (fixed in 3.4.1)

Published 2026-06-18. Last modified 2026-10-08.