A vulnerability in the Layer 2 Tunneling Protocol (L2TP) parsing function of Cisco IOS (12.0 through 12.4 and 15.0 through 15.6) and Cisco IOS XE (3.1 through 3.18) could allow an unauthenticated, remote attacker to cause an affected device to reload. The vulnerability is due to insufficient validation of L2TP packets. An attacker could exploit this vulnerability by sending a crafted L2TP packet to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a denial of service (DoS) condition. This vulnerability affects Cisco devices that are running a vulnerable release of Cisco IOS or Cisco IOS XE Software if the L2TP feature is enabled for the device and the device is configured as an L2TP Version 2 (L2TPv2) or L2TP Version 3 (L2TPv3) endpoint. By default, the L2TP feature is not enabled. Cisco Bug IDs: CSCuy82078.