A NULL Pointer Dereference of CWE-476 exists in quassel version 0.12.4 in the quasselcore void CoreAuthHandler::handle(const Login &msg) coreauthhandler.cpp line 235 that allows an attacker to cause a denial of service.
https://www.debian.org/security/2018/dsa-4189
https://usn.ubuntu.com/4594-1/
https://security.gentoo.org/glsa/201806-04
https://github.com/quassel/quassel/blob/master/src/core/coreauthhandler.cpp#L236