Cisco IOS 12.0 through 12.4 allows remote attackers to cause a denial of service via (1) a malformed MGCP packet, which causes a device hang, aka CSCsf08998; a malformed H.323 packet, which causes a device crash, as identified by (2) CSCsi60004 with Proxy Unregistration and (3) CSCsg70474; and a malformed Real-time Transport Protocol (RTP) packet, which causes a device crash, as identified by (4) CSCse68138, related to VOIP RTP Lib, and (5) CSCse05642, related to I/O memory corruption.