Software Engineering Institute

Easy VPN Server

Cisco IOS Easy VPN Server allows an IOS device to function as a VPN concentrator, providing authentication and encrypted access to network resources. Easy VPN Server was introduced in IOS 12.2(8)T.

IPSec

IPSec is a set of standards developed by the IETF that provides data confidentiality, integrity, and authentication at the IP layer. IPSec is used by applications such as Virtual Private Networks (VPNs).

Internet Key Exchange (IKE)

IKE (RFC2409) is a protocol that negotiates and provides authenticated keying material for security associations (SAs) in a protected manner. IKE is accomplished by using a combination of ISAKMP (RFC2408) and other protocols. ISAKMP provides a framework for internet key management. The IKE negotiation process consists of two phases. Phase 1 establishes an ISAKMP SA. Phase 2 is used to create SAs for other security protocols.

XAUTH

Extended Authentication (XAUTH) is an extension to IKE. It is defined in the expired document draft-ietf-ipsec-isakmp-xauth-06.txt. XAUTH allows IKE to use existing unidirectional authentication mechanisms after the Phase 1 SA has been established. XAUTH has been rejected by the IETF due to security flaws.

The Problem

When certain packets are sent to a Cisco Easy VPN server on port 500/udp, XAUTH authentication may be completed. This may allow access to network resources. Because XAUTH authentication takes place after the IKE Phase 1 negotiation, an attacker must know the shared group key to successfully perform this attack.

A remote attacker may be able to gain unintended access to the private network on the affected device.

Apply a patch or upgrade

Please refer to the "Software Versions and Fixes" section of the Cisco Security Advisory for more information on upgrading.

Do not use XAUTH

XAUTH contains several security flaws and has been rejected by the IETF.

Use strong group passwords

Use strong group passwords as described in the "Workarounds" section of the Cisco Security Advisory. This may help prevent a brute-force attack against the key.