If you are running WPA Enterprise with PEAP, or EAP/TTLS its about time you take a serious look at your client configuration! This weekend at Shmoocon in Washington D.C, Josh Wright and I gave a presentation that demonstrated how a very common, but incorrect client supplicant configuration can lead to the compromise of certain wireless networks and in some cases, provide Windows domain access.

Our AP impersonation attack on PEAP and EAP/TTLS relies on the client failing to properly validate the authentication server’s (RADIUS) TLS certificate. By default, the Windows Zero Configuration (WZC) wireless supplicant performs this validation by putting the trust of the network in the client’s hands. WZC will prompt the client to either continue or cancel upon connecting to the wireless network (similar to the way your web browser prompts you when accessing certain websites over HTTPS). Furthermore, the client may be mislead by this message as it only contains the signing authorities’ name (i.e Verisign) rather then the actual certificate name.

The severity of this issue is further escalated when the client is configured not to validate the server certificate at all. Unfortunately, this is the most common configuration I’ve seen used within organizations. It should be noted that because this is a configuration related attack, WZC is not the only vulnerable client supplicant. OSX’s client, Juniper’s Odyssey Client, and virtually every other wireless supplicant is vulnerable as well.

In either of these scenarios, FreeRADIUS-WPE (our modified version of the open source RADIUS server) can be used to gain access to the inner authentication credentials passed in the TLS tunnel that is established between client and the authentication server. These weak inner authentication protocols (i.e. PAP, MSCHAPv1, MSCHAPv2, etc..) rely on the outer TLS tunnel for protection, so without this protection they are greatly exposed to attack. In some cases these protocols reveal the client’s username and password in clear text, while other cases require a brute force attack. Due to active directory integration, these credentials may also be those used for domain authentication.

Finally, because this is the result of a client related issue, clients may be vulnerable in areas such as coffee shops, airports and other locations outside of the vicinity of the corporate wireless network.

When using WZC and other supplicants, you’ll want to make sure that the client clearly validates the server certificate by only trusting certificates that match the signing authority, and hostname of the RADIUS server. An example of the WZC configuration is below. This is also covered in Microsoft knowledge base article KB941123. For additional information on protecting yourself from this and other attacks, please see my 802.11 attacks whitepaper on Foundstone.com!