CVE-2026-90678
HighCVSS 7.5Exploitation Probability (EPSS)
Low risk43th percentile - higher than 43% of all known CVEs
Summary
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.
Risk Assessment
An attacker can smuggle a request past frontend rules such as a path-based deny and cause concurrent clients' requests, including their Authorization headers, to be consumed and lost. Exploitation is not deterministic but can be retried freely until it succeeds.
Recommendation
Upgrade HAProxy to a version later than 3.4.4 (or later than 3.5-dev5), or disable the HTTP/3/QUIC frontend if it is not needed. Until patched, consider limiting HTTP/1.1 backend connection reuse.
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-2026-55203High
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.
- 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)
An issue was discovered in HAProxy 3.3.0 through 3.4.4 and in 3.5-dev1 through 3.5-dev5. Exploitation requires an HTTP/3 frontend: HAProxy must be built with QUIC support and configured with a QUIC bind listener, and the affected traffic must reach a backend over HTTP/1.1 using chunked transfer coding on a reused connection. Under those conditions, when an HTTP/3 request carries no Content-Length header, the HTTP/3 multiplexer credits the length declared in a DATA frame header to the stream endpoint's known-input-payload estimate at the moment the frame header is decoded, before the payload has been received, and that declared length is emitted verbatim as the HTTP/1.1 chunk size. A remote unauthenticated client that declares more payload than it delivers and then ends the stream causes HAProxy to announce a chunk larger than the bytes it writes and to return the connection to the idle pool in a desynchronized state. The result is potential HTTP request smuggling on reused backend connections: an attacker can place a request past a frontend rule such as a path-based http-request deny, so that the smuggled request is never seen by HAProxy's HTTP analysis, and can cause concurrent clients' requests, including their request lines and Authorization headers, to be consumed as the attacker's request body and lost. Exploitation is not deterministic; it depends on a race with backend connection pooling, succeeding in a majority of but not all trials during testing, and can be retried freely. The mechanism was introduced in 3.3-dev10; releases 3.2.x and earlier are unaffected.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS