HAProxy up to 3.3.14/3.4.4/3.5-dev5 HTTP/3 Multiplexer request smuggling

Summaryinfo

A vulnerability marked as problematic has been reported in HAProxy up to 3.3.14/3.4.4/3.5-dev5. The affected element is an unknown function of the component HTTP/3 Multiplexer. The manipulation leads to request smuggling. This vulnerability is documented as CVE-2026-90678. The attack can be initiated remotely. There is not any exploit available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in HAProxy up to 3.3.14/3.4.4/3.5-dev5. This issue affects an unknown code of the component HTTP/3 Multiplexer. The manipulation with an unknown input leads to a request smuggling vulnerability. Using CWE to declare the problem leads to CWE-444. The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination. Impacted is integrity. The summary by CVE is:

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.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-90678 since 09/13/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. The technical details are unknown and an exploit is not publicly available.

Upgrading to version 3.5-dev6 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-76943). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Type

• Firewall Software

Name

• HAProxy

Version

• 3.3.0
• 3.3.1
• 3.3.2
• 3.3.3
• 3.3.4
• 3.3.5
• 3.3.6
• 3.3.7
• 3.3.8
• 3.3.9
• 3.3.10
• 3.3.11
• 3.3.12
• 3.3.13
• 3.3.14
• 3.4.0
• 3.4.1
• 3.4.2
• 3.4.3
• 3.4.4
• 3.5-dev5

License

• open-source

Website

• Product: https://github.com/haproxy/haproxy/

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