CVE-2026-26080

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.

A vulnerability of the HTTP request smuggling type was discovered in HAProxy versions 3.3.0 through 3.4.4 and 3.5-dev1 through 3.5-dev5. It requires an HTTP/3 frontend (built with QUIC support and a QUIC bind listener) and a backend reached over HTTP/1.1 using chunked transfer coding on a reused connection. When an HTTP/3 request carries no Content-Length header, the declared length from a DATA frame header is credited before the payload is received and emitted verbatim as the HTTP/1.1 chunk size, desynchronizing the connection.

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.

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.

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.

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.

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.