CVE-2026-55203: HAProxy - Integer Overflow in FCGI Demux Record Length Field

Severity: critical (CVSS 9.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-18 · updated 2026-10-26
Weakness: CWE-190

Description

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: before 2.4.36; from 2.5 before 2.6.30; from 2.7 before 2.8.25; from 2.9 before 3.0.24; from 3.1 before 3.2.20; from 3.3 before 3.4.1 — fixed in 2.4.36, 2.6.30, 2.8.25, 3.0.24, 3.2.20, 3.4.1

How to fix

Upgrade to a fixed version: HAProxy 2.4.36, 2.6.30, 2.8.25, 3.0.24. Check the vendor advisory for mitigations.

References

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