Security researcher Karsten Nohl has shown that if you send some mobile phones an SMS that appears to originate with the phone company, the phone will SMS back an error message containing sensitive info about its SIM. With this info, you can send another SMS that terminally compromises the phone, giving the attacker the ability to listen in on calls, read texts, and impersonate the phone's owner. He disclosed the vulnerability to the GSM association early, and on August 1 he'll present his work at Black Hat in Las Vegas. At the root of the problem is a reliance on an older, compromised form of crypto, DES:
For each message, the network and the phone verify their identities by comparing digital signatures. The message sent by Mr. Nohl deliberately used a false signature for the network. In three-quarters of messages sent to mobile phones using D.E.S. encryption, the handset recognized the false signature and ended communication.
But in a quarter of cases, the phone broke off the communication and sent an error message back to Mr. Nohl that included its own encrypted digital signature. The communication provided Mr. Nohl with enough information to derive the SIM card's digital key.
Mr. Nohl said he had advised the GSM Association and chip makers to use better filtering technology to block the kind of messages he had sent. He also advised operators to phase out SIM cards using D.E.S. encryption in favor of newer standards. He added that consumers using SIM cards more than three years old should get new cards from their carriers.