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.