In Twisted before 19.2.1, twisted.web did not validate or sanitize URIs or HTTP methods, allowing an attacker to inject invalid characters such as CRLF.
http://lists.opensuse.org/opensuse-security-announce/2019-07/msg00030.html
http://lists.opensuse.org/opensuse-security-announce/2019-07/msg00042.html
https://github.com/twisted/twisted/commit/6c61fc4503ae39ab8ecee52d10f10ee2c371d7e2
https://usn.ubuntu.com/4308-1/