ip_nat_pptp in the PPTP NAT helper (netfilter/ip_nat_helper_pptp.c) in Linux kernel 2.6.14, and other versions, allows local users to cause a denial of service (memory corruption or crash) via a crafted outbound packet that causes an incorrect offset to be calculated from pointer arithmetic when non-linear SKBs (socket buffers) are used.
https://exchange.xforce.ibmcloud.com/vulnerabilities/24204
http://www.vupen.com/english/advisories/2006/0220
http://www.trustix.org/errata/2006/0004
http://www.securityfocus.com/bid/16414
http://securityreason.com/securityalert/388