CVE-2026-55203
HighCVSS 7.5Exploitation Probability (EPSS)
Low risk24th percentile - higher than 24% of all known CVEs
Summary
HAProxy up to 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 buffer parsing as new FCGI record headers. This allows malicious FastCGI backends to desynchronize the FCGI framing parser.
Risk Assessment
The risk includes request routing errors, response smuggling, and potential memory safety issues, which could lead to data integrity breaches and service availability problems.
Recommendation
It is recommended to immediately update HAProxy to a version containing commit 5985276 or later, which fixes this vulnerability.
Other vulnerabilities in HAProxy
See all- CVE-2016-5360High
HAproxy 1.6.x before 1.6.6, when a deny comes from a reqdeny rule, allows remote attackers to cause a denial of service (uninitialized memory access and crash) or possibly have unspecified other impact via unknown vectors.
- CVE-2026-26081Medium
HAProxy Community Edition versions 3.0 through 3.3 before 3.3.3 lacks a length check for the NEW_TOKEN format. HAProxy Enterprise and ALOHA are also affected.
- CVE-2026-26080Low
HAProxy Community Edition 3.2.x through 3.3.x before 3.3.3 can enter a loop or crash because varint is mishandled. HAProxy Enterprise and ALOHA are also affected.
- CVE-2026-55204High
HAProxy up to version 3.4.0 contains a null pointer dereference vulnerability in the hpack_dht_insert() function in src/hpack-tbl.c. The flaw occurs when hpack_dht_defrag() returns null due to memory pool exhaustion and its return value is not validated. An attacker can exploit this by triggering HPACK dynamic table insertions under memory pressure, causing HAProxy worker processes to crash and resulting in denial of service.
- CVE-2026-33555Medium
In HAProxy before version 3.3.6, a vulnerability was found in the HTTP/3 parser. It does not verify that the received body length matches a previously declared Content-Length header when the stream is closed via a frame with an empty payload. The issue exists since version 2.6.
- CVE-2023-40225High
HAProxy versions from 2.0.32 to 2.6.14 and from 2.7.0 to 2.8.1 forward empty Content-Length headers, violating RFC 9110 section 8.6. In rare cases, an HTTP/1 server behind HAProxy may interpret the payload as an extra request.
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.