Vulnerability · Published Apr 13, 2026 · Assigned by mitre
CVE-2026-33555
Medium 4.0
In plain English
A medium-severity security flaw in HAProxy from HAProxy. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 24% of all known vulnerabilities). Ready-made attack tooling exists: public proof-of-concept code. Fixed versions are available - upgrade the affected package.
Official description
An issue was discovered in HAProxy before 3.3.6. The HTTP/3 parser does not check that the received body length matches a previously announced content-length when the stream is closed via a frame with an empty payload. This can cause desynchronization issues with the backend server and could be used for request smuggling. The earliest affected version is 2.6.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | haproxy | 2.6.12-1+deb12u4 | DEBIAN-CVE-2026-33555 |
| Debian:13 | haproxy | 3.0.11-1+deb13u3 | DEBIAN-CVE-2026-33555 |
| Debian:14 | haproxy | 3.2.15-1 | DEBIAN-CVE-2026-33555 |
| Ubuntu:24.04:LTS | haproxy | 2.8.16-0ubuntu0.24.04.2 | UBUNTU-CVE-2026-33555 |
| Ubuntu:25.10 | haproxy | 3.0.12-0ubuntu0.25.10.4 | UBUNTU-CVE-2026-33555 |
| Ubuntu:26.04:LTS | haproxy | 3.2.9-1ubuntu2.1 | UBUNTU-CVE-2026-33555 |
Affected products (as reported)
Weakness: CWE-130 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N
References
- https://www.haproxy.org
- https://www.haproxy.com/documentation/haproxy-aloha/changelog/
- https://github.com/haproxy/haproxy/commit/05a295441c621089ffa4318daf0dbca2dd756a84
- https://www.mail-archive.com/haproxy@formilux.org/msg46752.html
- https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html
- https://lists.debian.org/debian-lts-announce/2026/10/msg00036.html