CVE-2026-33555
HAProxy HTTP/3 Request Smuggling Vulnerability
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.
INFO
Published Date :
April 13, 2026, 5:16 p.m.
Last Modified :
June 29, 2026, 3:28 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | MEDIUM | [email protected] | ||||
| CVSS 3.1 | MEDIUM | [email protected] |
Solution
- Update HAProxy to version 3.3.6 or later.
- Apply relevant security patches to the backend server.
- Configure HTTP/3 parser rigorously.
Public PoC/Exploit Available at Github
CVE-2026-33555 has a 2 public
PoC/Exploit available at Github.
Go to the Public Exploits tab to see the list.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-33555.
| URL | Resource |
|---|---|
| https://github.com/haproxy/haproxy/commit/05a295441c621089ffa4318daf0dbca2dd756a84 | Patch |
| https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html | Exploit Third Party Advisory |
| https://www.haproxy.com/documentation/haproxy-aloha/changelog/ | Release Notes |
| https://www.haproxy.org | Product |
| https://www.mail-archive.com/[email protected]/msg46752.html | Release Notes |
| https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html | Exploit Third Party Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-33555 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-33555
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
One zero-byte QUIC packet is enough to desynchronize HAProxy's backend connection pool and smuggle HTTP requests across unrelated users — even users on a completely different frontend protocol.
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Cathedral-Grade Security for AI Agents. Attack vectors updated daily. Local-first, zero API cost. MIT licensed.
Python
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-33555 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-33555 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Jun. 29, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N Added CPE Configuration OR *cpe:2.3:a:haproxy:haproxy:*:*:*:*:*:*:*:* versions from (including) 2.6.0 up to (excluding) 3.3.6 Added Reference Type MITRE: https://github.com/haproxy/haproxy/commit/05a295441c621089ffa4318daf0dbca2dd756a84 Types: Patch Added Reference Type MITRE: https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html Types: Exploit, Third Party Advisory Added Reference Type MITRE: https://www.haproxy.com/documentation/haproxy-aloha/changelog/ Types: Release Notes Added Reference Type MITRE: https://www.haproxy.org Types: Product Added Reference Type MITRE: https://www.mail-archive.com/[email protected]/msg46752.html Types: Release Notes Added Reference Type CISA-ADP: https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html Types: Exploit, Third Party Advisory -
CVE Modified by [email protected]
Jun. 17, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'HAProxy', 'product': 'HAProxy', 'versions': [{'status': 'affected', 'version': '2.6', 'lessThan': '3.3.6', 'versionType': 'custom'}], 'defaultStatus': 'unaffected'}] -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jun. 17, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-33555', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'poc'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-04-22T18:42:59.239377Z'} -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Apr. 22, 2026
Action Type Old Value New Value Added Reference https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html -
CVE Modified by [email protected]
Apr. 22, 2026
Action Type Old Value New Value Added Reference https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html -
New CVE Received by [email protected]
Apr. 13, 2026
Action Type Old Value New Value Added 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. Added CVSS V3.1 AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N Added CWE CWE-130 Added Reference https://github.com/haproxy/haproxy/commit/05a295441c621089ffa4318daf0dbca2dd756a84 Added Reference https://www.haproxy.com/documentation/haproxy-aloha/changelog/ Added Reference https://www.haproxy.org Added Reference https://www.mail-archive.com/[email protected]/msg46752.html