In Teeworlds before 0.6.5, connection packets could be forged. There was no challenge-response involved in the connection build up. A remote attacker could send connection packets from a spoofed IP address and occupy all server slots, or even use them for a reflection attack using map download packets.
https://www.debian.org/security/2018/dsa-4329
https://teeworlds.com/?page=news&id=12544
https://github.com/teeworlds/teeworlds/issues/1536
https://bugs.debian.org/911487
http://lists.opensuse.org/opensuse-security-announce/2019-08/msg00077.html
http://lists.opensuse.org/opensuse-security-announce/2019-07/msg00046.html