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Over a three-year period of testing VPNs, NTA Monitor has discovered that 90% of remote access VPN systems have exploitable vulnerabilities. The tests were mainly carried out for large organisations, including financial institutions that had their own in-house security teams. The common belief is that VPN systems are invulnerable, when in fact they are frequently the weak link in an otherwise secure system.
Many remote access VPNs have vulnerabilities that allow valid usernames to be guessed through a dictionary attack, because they respond differently to valid and invalid usernames. One of the basic requirements of a username/password authentication scheme is that an incorrect login attempt should not leak information as to whether the username or password was incorrect, because the attacker can then deduce if the username is valid or not. However, many VPN implementations ignore this rule.
The fact that VPN usernames are often based on people's names or email addresses makes it relatively easy for an attacker to use a dictionary attack to recover a number of valid usernames in a short period of time.
During VPN security testing, NTA Monitor has found many usernames in this way. It is believed this VPN guessing issue is a new discovery and several vendors have been notified. However the vendors have not always implemented fixes after notification so many systems are still vulnerable.
Once a valid password is obtained using IKE Aggressive Mode it is possible to obtain a hash from the VPN server and use this to mount an offline attack to crack the associated passwords. As this attack is offline, it does not show on the VPN server log or cause account lockout. It is also extremely fast - typically several hundred thousand guesses per second:
VPNs carry sensitive information over an insecure network and remote access VPNs often allow full access to the internal network, while VPN traffic is usually invisible to IDS monitoring. With increasing security in other areas e.g. more organisations installing firewalls, moving Internet servers onto the DMZ and automatically patching servers, the VPN becomes a more tempting target.
The majority of VPN vendors still allow their implementations to leak information about valid usernames and do not lock out accounts after a number of failed attempts. This does not happen on operating system login and should not occur on VPN implementations.
NTA Monitor recommends that VPNs should be tested regularly to ensure they are secure. Tools such as NTA Monitor's updated ike-scan can help to test a VPN but it is quite complex and needs to be fully understood in order to be used effectively.
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