Published Apr 13, 2026Sources checked Nov 14, 2026
5.8MEDIUMCVSS out of 10
What this means
Review the available evidence
CVE-2026-33555 and is rated Medium severity with a CVSS score of 5.8. It is not in the current CISA KEV record we collected. That does not prove exploitation has not occurred.
What to do next
Remediation and response
Microsoft Security Response Center guidance
Vendor revision: Apr 29, 2026
CBL-Mariner Releases
Vendor-listed releases
2.9.11-5
CISA KEVNot listedBased on the latest collected catalog
EPSS0.3%Estimated 30-day exploitation probability
Ransomware useNot markedCISA KEV ransomware field
Threat actors0Source-linked actor relationships
Overview
What is 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.