The American Civil Liberties Union has revealed that the FBI no longer feels the need for judicial or operator oversight when deploying base station-faking technology to detect mobile phones.
This capability is nothing new, but previously it was assumed that a warrant was needed before the FBI started tracking phones and that the network operator would need to be informed. These are optimistic assumptions in this age of fear embodied in the USA's Patriot* Act, as shown in the documents obtained by the ACLU. Instead the FBI just gets a pen-trap order, granted under the Foreign Intelligence Surveillance Act, and Bob's your uncle.
The technology, known as Triggerfish, pretends to be a base station so that mobile handsets happily connect to it and identify themselves. A Triggerfish deployment can wait for a specific handset to pass by, or just grab nearby identities for later analysis.
When the GSM security model was created there was no attempt to confirm the identity of the base station - it never occurred to the designers that fake base stations would exist so all the authentication is one-way. Worse still, while the IMSI (unique to the subscriber) is not supposed to be sent over the network, the standard allows a base station to claim to have "lost" the corresponding TMSI (temporary equivalent) and thus ask the handset to resend the IMSI in the clear.
Setting up fake base stations is pretty rare, but clearly the US authorities aren't alone in doing it, which is why the 3G GSM standard requires base stations to authenticate themselves too - making Triggerfish once again inoperable without network operator collusion, at least where 3G is being used by default. ®
*Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism Act of 2001, since you ask.