Cellular network hijacking for fun and profit
The Reg cut-out-and-keep guide
Instant island of connectivity
This is what Ousama Abushagur and his team did in Misurata, though their kit was scaled to handle a lot more calls than the backpack version shown above, the principle remains the same. The brought-in kit was plugged into an Ericsson BSC that wasn't being used, providing an instant island of connectivity.
Next step up is the MSC, which will control a number of BSCs. MSCs are better secured, just hitting the reset button isn't going to work this time. Our engineers reckoned that older kit had exploitable vulnerabilities but that access to the passwords would be hugely beneficial.
From the MSC, the down-chain BSCs and BTSs can be remotely managed, so if the information is available then it's a much better point of attack. From a single MSC one can provide (or deny!) connectivity to large swaths of a country from a single location, assuming the remote locations still have power.
The next step is to get the users connected, ideally without having to issue replacement SIMs to all of them.
Have a squiz at the local subscriber list...
In setting up Free Libyana, Abushagur managed to grab the identities of local subscribers from a captured VLR (Visitor Location Registry). That enabled customers to be added to the new HLR (necessary for them to make and receive calls).
Without the customer data, your best bet is to change the ID of your new network, making every handset think it is suddenly roaming internationally. Roamed-to networks aren't expected to have the customer's details, but they are expected to have access to the results of cryptographic ciphers, which can only take place on the customer's original network.
That's because the cryptography in GSM is based around a shared secret which is embedded in the SIM and stored in the operator's Authentication Server (AuS). Authentication Servers are generally secured, so even if you grabbed one (unlikely in this instance as it will be located at the operator's hub), you would still need the passwords that will be known to only a few employees.
What's worse is that the same key is used to generate a session key to encrypt the communications, so without access to the AuS you are forced to run the network without any encryption at all.
What you end up with is a network similar to those run in countries not trusted (by the USA) to have proper encryption, to whom software using strong cryptography can't be exported. That gets people connected, but it's problematic from a business point of view as SIM-cloning becomes possible and clandestine listening becomes easy.
But once that's all working, then you can start the slow process of issuing replacement SIMs, and rebuilding the rest of the country, which hopefully will be a good deal easier now that everyone can talk to each other. ®