The PPP implementation in Cisco IOS 12.2 and 15.0 through 15.2, when Point-to-Point Termination and Aggregation (PTA) and L2TP are used, allows remote attackers to cause a denial of service (device crash) via crafted network traffic, aka Bug ID CSCtf71673.
http://www.securitytracker.com/id?1027005
http://www.cisco.com/en/US/docs/ios/15_1/release/notes/151-2TCAVS.html