CVE-2026-90678
Received Received - Intake

HTTP/3 Request Smuggling in HAProxy

Publication date: 2026-09-13

Last updated on: 2026-09-13

Assigner: MITRE

Description
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.

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Meta Information

Published
2026-09-13
Last Modified
2026-09-13
Generated
2026-09-13
AI Q&A
2026-09-13
EPSS Evaluated
N/A
NVD
CVE-2026-90678
EUVD
EUVD-2026-76943

Affected Vendors & Products

Showing 2 associated CPEs
Vendor Product Version / Range
haproxy haproxy From 3.3.0 (inc) to 3.4.4 (inc)
haproxy haproxy From 3.5-dev1 (inc) to 3.5-dev5 (inc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-130 The product parses a formatted message or structure, but it does not handle or incorrectly handles a length field that is inconsistent with the actual length of the associated data.

Attack-Flow Graph

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Executive Summary

CVE-2026-90678 is a vulnerability in HAProxy versions 3.3.0 through 3.4.4 and 3.5-dev1 through 3.5-dev5. It involves HTTP/3 (H3) frame handling where an attacker can exploit truncated frames to perform HTTP request smuggling. The issue occurs when an HTTP/3 request lacks a Content-Length header and declares more payload than it delivers. This causes HAProxy to announce a larger chunk size than the actual data, leading to desynchronized backend connections and potential request smuggling.

Detection Guidance

Detecting this vulnerability requires checking if your HAProxy instance is running with HTTP/3 (QUIC) support and if it is configured with a QUIC bind listener. Inspect HAProxy logs for connection errors or desynchronized states. Use packet capture tools like tcpdump or Wireshark to analyze HTTP/3 traffic for truncated DATA frames or FIN signals without matching payload.

Impact Analysis

This vulnerability allows remote unauthenticated attackers to bypass security controls, such as path-based access restrictions, by smuggling malicious requests. It can cause legitimate users' requests, including sensitive data like Authorization headers, to be lost or misrouted. The impact includes unauthorized access, data leakage, or service disruption, depending on the backend configuration.

Compliance Impact

This vulnerability could lead to violations of GDPR or HIPAA by enabling unauthorized access to sensitive data or loss of audit trails. GDPR requires protecting personal data, while HIPAA mandates secure handling of health information. Exploitation may result in data breaches, non-compliance penalties, or legal consequences due to compromised data integrity and confidentiality.

Mitigation Strategies

Immediately upgrade HAProxy to a patched version that includes the fix for CVE-2026-90678. If upgrading is not possible, disable HTTP/3 (QUIC) support in HAProxy by removing QUIC bind listeners and ensure no HTTP/1.1 backend connections use chunked transfer coding. Monitor logs for signs of exploitation attempts.

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