Researchers in Berlin have demonstrated the ability to block calls and text messages intended for nearby users of the same GSM cellular network by making only a few changes to a standard phone.
By modifying a phone’s embedded software, the researchers can dupe the network out of delivering incoming calls or SMS messages to the intended recipients. One phone could theoretically block service to all subscribers served by base stations within a location area, says Technical University of Berlin's Jean-Pierre Seifert.
The researchers modified the firmware on the baseband processor, which controls how a phone communicates with a network’s transmission towers. Ordinarily, a cellular tower pages nearby devices to locate the one that should receive a call or SMS sent over the network, and the intended recipient's phone replies with a confirmation before the communication proceeds. However, the modified firmware responds to the cellular tower's paging before a victim’s phone can, preventing the intended recipient from receiving the communication.
Just 11 modified phones could shut down service of Germany's third-largest cellular network operator in a location area.
"All those phones are listening to all the paging requests in that area, and they are answering 'it's me,' and nobody in that cell will get an SMS or a phone call," Seifert says.
From Technology Review
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