A flaw was found in HAProxy before 2.0.6. In legacy mode, messages featuring a transfer-encoding header missing the "chunked" value were not being correctly rejected. The impact was limited but if combined with the "http-reuse always" setting, it could be used to help construct an HTTP request smuggling attack against a vulnerable component employing a lenient parser that would ignore the content-length header as soon as it saw a transfer-encoding one (even if not entirely valid according to the specification).

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-18277
• https://git.haproxy.org/?p=haproxy-2.0.git;a=commit;h=196a7df44d8129d1adc795da020b722614d6a581
• https://nathandavison.com/blog/haproxy-http-request-smuggling
• https://usn.ubuntu.com/4174-1/
• https://www.mail-archive.com/[email protected]/msg34926.html
• http:https://lists.opensuse.org/opensuse-security-announce/2019-12/msg00016.html
• http:https://lists.opensuse.org/opensuse-security-announce/2019-12/msg00019.html
• https://lists.debian.org/debian-lts-announce/2022/05/msg00045.html