What’s Vulnerable › Vulnerabilities

CVE-2026-33555

• Severity: medium (CVSS 5.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-04-13 · updated 2026-10-18
• Weakness: CWE-130

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 products

• HAProxy: from 2.6.0 before 3.3.6 — fixed in 3.3.6

How to fix

Upgrade to a fixed version: HAProxy 3.3.6. Check the vendor advisory for mitigations.

References

• https://github.com/haproxy/haproxy/commit/05a295441c621089ffa4318daf0dbca2dd756a84
• https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html
• https://www.haproxy.com/documentation/haproxy-aloha/changelog/
• https://www.haproxy.org
• https://www.mail-archive.com/haproxy@formilux.org/msg46752.html
• https://lists.debian.org/debian-lts-announce/2026/10/msg00036.html
• https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html

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