Vulnerability in Haproxy
CVE-2026-33555
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 4.0 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N.
EPSS exploit prediction
EPSS: 0.006 (45.3th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 45th percentile — 45.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.006 · change: +0.003
Affected products
- Haproxy — versions 2.6
Weakness classification (CWE)
Public proof-of-concept exploits
References
- www.haproxy.org (Product)
- www.haproxy.com/documentation/haproxy-aloha/changelog/ (Release Notes)
- github.com/haproxy/haproxy/commit/05a295441c621089ffa4318daf0dbca2dd756a84 (Patch)
- www.mail-archive.com/haproxy@formilux.org/msg46752.html (Release Notes)
- r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html (Exploit, Third Party Advisory)
Frequently asked questions
- What is CVE-2026-33555?
- CVE-2026-33555 is a medium-severity vulnerability in Haproxy, classified under Improper Handling of Length Parameter Inconsistency. CVSS score: 4.0/10. Published 2026-04-13.
- How severe is CVE-2026-33555?
- Medium severity. CVSS v3 base score is 4.0 out of 10.
- Is CVE-2026-33555 known to be exploited?
- 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.