EAP-AKA - Methods - EAP-FAST


EAP-FAST (Flexible Authentication via Secure Tunneling) is a protocol proposal by Cisco Systems as a replacement for LEAP. The protocol was designed to address the weaknesses of LEAP while preserving the "lightweight" implementation. Use of server certificates is optional in EAP-FAST. EAP-FAST uses a Protected Access Credential (PAC) to establish a TLS tunnel in which client credentials are verified. EAP-FAST has three phases. Phase 0 is an optional phase in which the PAC can be provisioned manually or dynamically, but is outside the scope of EAP-FAST as defined in RFC4851. PAC provisioning is still officially Work-in-progress, even though there are many implementations. PAC provisioning typically only needs to be done once for a RADIUS server, client pair. In Phase 1, the client and the AAA server use the PAC to establish a TLS tunnel. In Phase 2, the client credentials are exchanged inside the encrypted tunnel.

When automatic PAC provisioning is enabled, EAP-FAST has a slight vulnerability that an attacker can intercept the PAC and subsequently use that to compromise user credentials. This vulnerability is mitigated by manual PAC provisioning or by using server certificates for the PAC provisioning phase.

There is also a vulnerability where an attacker's AP can use the same SSID, reject the users PAC and supply a new one. Most supplicants can be set to prompt the user this credentials using the inner method to the hacker, who will then get either a cleartext password (EAP-FAST w/ GTC) or a vulnerable to dictionary attack MSCHAPv2 hash.

It is worth noting that the PAC file is issued on a per-user basis. This is a requirement in RFC 4851 sec 7.4.4 so if a new user logs on the network from a device, he needs a new PAC file provisioned first. This is one reason why it is difficult not to run EAP-FAST in insecure anonymous provisioning mode. The alternative is to use device passwords instead, but then it is not the user that is validated on the network.

EAP-FAST can be used without PAC files, falling back to normal TLS.

EAP-FAST is natively supported in Apple OS X 10.4.8 and newer. Cisco supplies an EAP-FAST module for Windows Vista and later operating systems which have an extensible EAPHost architecture for new authentication methods and supplicants.

EAP-FAST is defined in RFC 4851.

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