Last updated 25 August 2025

A flaw was found in HAProxy's headers processing that causes HAProxy to drop important headers fields such as Connection, Content-length, Transfer-Encoding, and Host after having partially processed them. A maliciously crafted HTTP request could be used in an HTTP request smuggling attack to bypass filtering and detection by HAProxy.

HAProxy - Integer Overflow in FCGI Demux Record Length Field

A flaw was found in the way haproxy processed certain HTTP/2 request packets. An attacker could send crafted HTTP/2 request packets which cause memory corruption, leading to a crash or potential remote arbitrary code execution with the permissions of the user running haproxy.

HAProxy - NULL Pointer Dereference in hpackdhtinsert Function

HAProxy before 2.8.2 accepts # as part of the URI component, which might allow remote attackers to obtain sensitive information or have unspecified other impact upon misinterpretation of a pathend rule, such as routing index.html#.png to a static server.

HAProxy 2.9.x before 2.9.10, 3.0.x before 3.0.4, and 3.1.x through 3.1-dev6 allows a remote denial of service for HTTP/2 zero-copy forwarding (h2send loop) under a certain set of conditions, as exploited in the wild in 2024.

A flaw was found in haproxy. A stemming from an inefficient algorithmic complexity issue within its bundled mjson parsing library. This vulnerability is triggered when haproxy is configured to analyze JSON content, such as with the jsonquery or jwtpayloadquery function

A flaw was found in haproxy. Anybody who can add a "set-cookie2 X=Y" header into the return path from their server/backend will kill haproxy in 2.2 (and onwards), resulting in a denial of service.

A flaw was discovered in the HPACK decoder of haproxy before 1.8.14, that is used for HTTP/2. An out-of-bounds read access in hpackvalididx() resulted in a remote crash and denial of service. According to the report, there is no risk of escalation or code execution.

An issue was discovered in HAProxy 2.2 before 2.2.16, 2.3 before 2.3.13, and 2.4 before 2.4.3. It does not ensure that the scheme and path portions of a URI have the expected characters. For example, the authority field (as observed on a target HTTP/2 server) might differ from what the routing rules were intended to achieve.

An issue was discovered in HAProxy 2.2 before 2.2.16, 2.3 before 2.3.13, and 2.4 before 2.4.3. It can lead to a situation with an attacker-controlled HTTP Host header, because a mismatch between Host and authority is mishandled.

An issue was discovered in HAProxy before 1.8.8. The incoming H2 frame length was checked against the maxframesize setting instead of being checked against the bufsize. The maxframesize only applies to outgoing traffic and not to incoming, so if a large enough frame size is advertised in the SETTINGS frame, a wrapped frame will be defragmented into a temporary allocated buffer where the second fragment may overflow the heap by up to 16 kB. It is very unlikely that this can be exploited for code execution given that buffers are very short lived and their addresses not realistically predictable in production, but the likelihood of an immediate crash is absolutely certain.

An out-of-bounds read issue was discovered in the HTTP/2 protocol decoder in HAProxy 1.8.x and 1.9.x through 1.9.0 which can result in a crash. The processing of the PRIORITY flag in a HEADERS frame requires 5 extra bytes, and while these bytes are skipped, the total frame length was not re-checked to make sure they were present in the frame.

An issue was discovered in dns.c in HAProxy through 1.8.14. In the case of a compressed pointer, a crafted packet can trigger infinite recursion by making the pointer point to itself, or create a long chain of valid pointers resulting in stack exhaustion.

A flaw was found in HAProxy before 2.0.6. In legacy mode, messages featuring a transfer-encoding header missing the "chunked" value were not being correctly rejected. The impact was limited but if combined with the "http-reuse always" setting, it could be used to help construct an HTTP request smuggling attack against a vulnerable component employing a lenient parser that would ignore the content-length header as soon as it saw a transfer-encoding one (even if not entirely valid according to the specification).

An out-of-bounds read in dnsvalidatednsresponse in dns.c was discovered in HAProxy through 1.8.14. Due to a missing check when validating DNS responses, remote attackers might be able read the 16 bytes corresponding to an AAAA record from the non-initialized part of the buffer, possibly accessing anything that was left on the stack, or even past the end of the 8193-byte buffer, depending on the value of acceptedpayloadsize.

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.

An integer overflow exists in HAProxy 2.0 through 2.5 in htxaddheader that can be exploited to perform an HTTP request smuggling attack, allowing an attacker to bypass all configured http-request HAProxy ACLs and possibly other ACLs.

HAProxy through 2.0.2 allows attackers to cause a denial of service (hapanic) via vectors related to htxmanageclientsidecookies in protohtx.c.

An information leak vulnerability was discovered in HAProxy 2.1, 2.2 before 2.2.27, 2.3, 2.4 before 2.4.21, 2.5 before 2.5.11, 2.6 before 2.6.8, 2.7 before 2.7.1. There are 5 bytes left uninitialized in the connection buffer when encoding the FCGIBEGINREQUEST record. Sensitive data may be disclosed to configured FastCGI backends in an unexpected way.

headerv2.go in mastercactapus proxyprotocol before 0.0.2, as used in the mastercactapus caddy-proxyprotocol plugin through 0.0.2 for Caddy, allows remote attackers to cause a denial of service (webserver panic and daemon crash) via a crafted HAProxy PROXY v2 request with truncated source/destination address data.

HTTP request/response smuggling vulnerability in HAProxy version 2.7.0, and 2.6.1 to 2.6.7 allows a remote attacker to alter a legitimate user's request. As a result, the attacker may obtain sensitive information or cause a denial-of-service (DoS) condition.

HAProxy through 2.0.32, 2.1.x and 2.2.x through 2.2.30, 2.3.x and 2.4.x through 2.4.23, 2.5.x and 2.6.x before 2.6.15, 2.7.x before 2.7.10, and 2.8.x before 2.8.2 forwards empty Content-Length headers, violating RFC 9110 section 8.6. In uncommon cases, an HTTP/1 server behind HAProxy may interpret the payload as an extra request.

A flaw was found in haproxy. A stemming from an inefficient algorithmic complexity issue within its bundled mjson parsing library. This vulnerability is triggered when haproxy is configured to analyze JSON content, such as with the jsonquery or jwtpayloadquery function

HAProxy through 3.4.0, fixed in commit 5985276, contains an integer overflow vulnerability in the fcgiconn 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.

HAProxy through 3.4.0, fixed in commit 9a6d1fe, contains a null pointer dereference vulnerability in hpackdhtinsert() within src/hpack-tbl.c that fails to validate the return value of hpackdhtdefrag() when the memory pool is exhausted. An attacker can trigger HPACK dynamic table insertions under memory pressure to dereference a NULL pointer and crash HAProxy worker processes, causing denial of service.

An uncontrolled resource consumption vulnerability was discovered in HAProxy which could crash the service. This issue could allow an authenticated remote attacker to run a specially crafted malicious server in an OpenShift cluster. The biggest impact is to availability.

HAProxy Kubernetes Ingress Controller before 3.1.13, when the config-snippets feature flag is used, accepts config snippets from users with create/update permissions. This can result in obtaining an ingress token secret as a response. The fixed versions of HAProxy Enterprise Kubernetes Ingress Controller are 3.0.16-ee1, 1.11.13-ee1, and 1.9.15-ee1.

Incorrect caching of responses to requests including an Authorization header in HAProxy 1.8.0 through 1.8.9 (if cache enabled) allows attackers to achieve information disclosure via an unauthenticated remote request, related to the protohttp.c checkrequestforcacheability function.