Buffer overflow in Half Life dedicated server before build 3104 allows remote attackers to execute arbitrary commands via a long rcon command.
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 cdkey values, which reaches the player limit and prevents other players from connecting until the original responses have timed out.
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.