The Serious engine, as used in (1) Alpha Black Zero Intrepid Protocol 1.04 and earlier, (2) Nitro family, and (3) Serious Sam Second Encounter 1.07 allows remote attackers to cause a denial of service (server crash) via a large number of UDP join requests that exceeds the maximum player limit, as originally reported for Alpha Black Zero.
https://exchange.xforce.ibmcloud.com/vulnerabilities/17545
http://securitytracker.com/id?1011454
http://secunia.com/advisories/12687