The Half-Life engine before July 7 2004 allows remote attackers to cause a denial of service (server or client crash) via an empty fragmented packet.

Half-Life Server 1.1.1.0 and earlier allows remote attackers to cause a denial of service (resource exhaustion) via multiple responses to the initial challenge with different cd_key values, which reaches the player limit and prevents other players from connecting until the original responses have timed out.

Format string vulnerability in Half Life dedicated server build 3104 and earlier allows remote attackers to execute arbitrary commands by injecting format strings into the changelevel command, via the system console or rcon.