CVE-2026-55204

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.

HAProxy 3 before 3.4.6 allows a remote unauthenticated attacker to cause a denial of service (persistent worker-thread CPU exhaustion) via an HTTP/1.1 chunked request. The parser repeatedly re-processes the same bytes without making progress, and the thread stays busy at 100% CPU even after the client disconnects.

HAProxy 3.2 before 3.4.6 allows a remote unauthenticated attacker to cause a denial of service (worker-thread CPU exhaustion) via a truncated HTTP/2 PING frame. h2c_handle_ping does not check for a short read, causing the H2 multiplexer tasklet to be repeatedly woken without progress and high CPU usage.

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 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.

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.

Inefficient algorithm complexity in mjson in HAProxy allows remote attackers to cause a denial of service via specially crafted JSON requests.

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.