Is CVE-2026-33555 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-33555 being exploited in the wild.

How severe is CVE-2026-33555?

Medium severity, with a CVSS score of 4, published 13 April 2026.

Is there a fix for CVE-2026-33555?

No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

How is CVE-2026-33555 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-33555?

Check whether you run HAProxy at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-33555: HAProxy request smuggling via HTTP/3 standalone FIN desynchronization

CVE-2026-33555 · Severity: medium · CVSS 4 · Published 2026-04-13

HAProxy, a widely used load balancer and proxy server, contains a vulnerability in how it handles modern HTTP/3 web traffic. An attacker can send a specially crafted request that tricks the proxy into miscalculating the size of the data being sent. This can lead to 'request smuggling,' where an attacker can interfere with other users' web sessions, potentially bypassing security controls or accessing sensitive data on backend servers.

Technologies: HAProxy. Vendors: HAProxy.

Quick answers

Is CVE-2026-33555 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-33555 being exploited in the wild.
How severe is CVE-2026-33555?: Medium severity, with a CVSS score of 4, published 13 April 2026.
Which products and versions are affected?: HAProxy 2.6 through 3.3.5.
Is there a fix for CVE-2026-33555?: No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-33555 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-33555?: Check whether you run HAProxy at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A vulnerability in HAProxy's HTTP/3 (H3) implementation arises when a QUIC STREAM frame is received with no payload but with the FIN bit set. In this 'standalone FIN' code path, the parser (h3_rcv_buf) skips standard H3 validation and calls qcs_http_handle_standalone_fin() directly, bypassing the h3_check_body_size() check. If a request includes a Content-Length header but provides no body data before the FIN bit, HAProxy forwards the request to the backend while the backend continues to wait for the missing bytes. This allows an attacker to desynchronize the connection pool, causing the backend to interpret the beginning of a subsequent user's request as the missing body of the attacker's request. The issue is fixed in versions 3.3.6, 3.2.15, 3.0.19, 2.8.20, and 2.6.25.

Affected products

HAProxy HAProxy 2.6 through 3.3.5

Timeline

2026-04-13: disclosed
2026-04-13: advisory
2026-04-14: other: Detailed technical writeup published by researcher
2026-06-29: other: NIST analysis completed

References

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