---
id: CVE-2026-33555
title: An issue was discovered in HAProxy before 3.3.6
summary: >-
  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 des…
severity: medium
cvss: 4
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N'
cwe:
  - CWE-130
vendor: haproxy
product: haproxy
affected:
  - 'haproxy >= 2.6.0, < 3.3.6'
patched:
  - haproxy 3.3.6
published: '2026-04-13'
updated: '2026-06-29'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-33555'
references:
  - url: >-
      https://github.com/haproxy/haproxy/commit/05a295441c621089ffa4318daf0dbca2dd756a84
    label: cve@mitre.org
  - url: >-
      https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html
    label: cve@mitre.org
  - url: 'https://www.haproxy.com/documentation/haproxy-aloha/changelog/'
    label: cve@mitre.org
  - url: 'https://www.haproxy.org'
    label: cve@mitre.org
  - url: 'https://www.mail-archive.com/haproxy@formilux.org/msg46752.html'
    label: cve@mitre.org
  - url: >-
      https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
  - exploit-available
epss: 0.00577
epssPercentile: 0.45123
ingestedAt: '2026-06-29T15:48:27.651Z'
exploits:
  github: 1
  githubRepos:
    - 'https://github.com/r3verii/CVE-2026-33555'
  checkedAt: '2026-09-25T08:20:55.064Z'
exploitAvailable: true
---

## Overview

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

- `haproxy >= 2.6.0, < 3.3.6`

## Remediation

Upgrade past the affected range:

- `haproxy 3.3.6`