HAProxy <3.3.6 HTTP/3 Parser Desynchronization & Request Smuggling
CVE-2026-33555 Published on April 13, 2026
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.
Weakness Type
What is a length manipulation Vulnerability?
The software parses a formatted message or structure, but it does not handle or incorrectly handles a length field that is inconsistent with the actual length of the associated data. If an attacker can manipulate the length parameter associated with an input such that it is inconsistent with the actual length of the input, this can be leveraged to cause the target application to behave in unexpected, and possibly, malicious ways. One of the possible motives for doing so is to pass in arbitrarily large input to the application. Another possible motivation is the modification of application state by including invalid data for subsequent properties of the application. Such weaknesses commonly lead to attacks such as buffer overflows and execution of arbitrary code.
CVE-2026-33555 has been classified to as a length manipulation vulnerability or weakness.
Products Associated with CVE-2026-33555
Want to know whenever a new CVE is published for HAProxy? stack.watch will email you.
Affected Versions
HAProxy:- Version 2.6 and below 3.3.6 is affected.