Teeworlds up to and including 0.7.5 is vulnerable to Buffer Overflow. A map parser does not validate m_Channels value coming from a map file, leading to a buffer overflow. A malicious server may offer a specially crafted map that will overwrite client's stack causing denial of service or code execution.
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.