This chapter tackles assessment of VPN services found running on network boundaries. Increasingly, VPN services provide access for both branch offices and home users, using IPsec, Microsoft PPTP, and SSL. These VPN service endpoints are under threat from information leak, buffer overflow, DoS, and offline password-grinding attacks, which are detailed in the following sections.
VPN technologies and their underlying protocols fill entire books already. One book I used to research IPsec key exchange and authentication protocols is IPSec: Securing VPNs by Carlton R. Davis (McGraw-Hill). If you require detailed low-level information about IPsec and its various modes and protocols, you should read a book dedicated to the subject. Here I tackle the key IPsec protocols and mechanisms at a high level and discuss known remotely exploitable weaknesses and attacks.
Standard Internet Protocol (IP) packets are inherently insecure. IPsec was designed to provide security options and enhancements to IP, and to negate the following security weaknesses:
IP spoofing and packet-source forgery issues
Modification of data within IP packets
Most IPsec implementations use the Internet Key Exchange (IKE) service to provide authentication and key exchange when establishing and maintaining an IPsec connection. Some older IPsec implementations use manual keying, but this is now considered obsolete. After authenticating and negotiating ...