PPTP implementation in Cisco IOS 12.1 and 12.2 allows remote attackers to cause a denial of service (crash) via a malformed packet.
https://exchange.xforce.ibmcloud.com/vulnerabilities/6835
http://www.securityfocus.com/bid/3022
http://www.kb.cert.org/vuls/id/656315
http://www.cisco.com/warp/public/707/PPTP-vulnerability-pub.html