ctrlpacket.c in PoPToP PPTP server before 1.1.4-b3 allows remote attackers to cause a denial of service via a length field of 0 or 1, which causes a negative value to be fed into a read operation, leading to a buffer overflow.
http://www.securityfocus.com/archive/1/319428
http://www.securityfocus.com/archive/1/317995
http://www.novell.com/linux/security/advisories/2003_029.html
http://www.kb.cert.org/vuls/id/673993
http://www.debian.org/security/2003/dsa-295
http://sourceforge.net/project/shownotes.php?release_id=138437