CVE-2026-55203
HighCVSS 7.5Exploitation Probability (EPSS)
Low risk24th percentile — higher than 24% of all known CVEs
Summary
HAProxy through version 3.4.0 contains an integer overflow vulnerability in the fcgi_conn structure's drl field. When contentLength is 65535 and paddingLength is 1 or more, the drl field wraps to 0, causing incorrect FCGI frame processing. Malicious FastCGI backends can exploit this to desynchronize the frame parser, leading to request routing errors, response smuggling, or memory safety issues.
Risk Assessment
An attacker can exploit this vulnerability to manipulate data flow between HAProxy and FastCGI backends, potentially leading to unauthorized data access, session hijacking, or response smuggling attacks.
Recommendation
Immediately update HAProxy to a version containing commit 5985276 or later. If updating is not possible, restrict trust to FastCGI backends and monitor traffic for anomalies in FCGI frames.
Original NVD description (English source)
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.