Researcher finds major encryption flaw in older mobile SIM cards

DES encryption flaw

Hundreds of millions of mobile handsets using older SIM cards are vulnerable to a flaw that allows attackers to gain complete control over affected handsets, German security expert Karsten Nohl has reported.

What Nohl has spent two years researching could turn out to be the most significant flaw in SIMs in many years when he reveals its full details at a presentation scheduled for the Black Hat security conference at the end of this month.

Nohl's briefings to the media and short website description have so far been light on detail, but the teasers he has been willing to put out sound alarming enough.

According to the Security Research Labs chief scientist, a quarter of 1,000 SIMs tested by his team had proved vulnerable to a weakness in the older DES (Data Encryption Standard) encryption used to secure them.

This made it possible to take control of a handset using a binary SMS text message to upload malware. This piggybacked on the carrier over-the-air (OTA) process through which SIMs are transparently updated via the JavaCard programming interface.

Attackers could use the method to eavesdrop on calls, sent premium text messages, and even in some circumstances, conduct payment fraud.

Only a proportion of the world's handsets used DES in its vulnerable form but this still amounted to 750 million, overwhelmingly older handsets, he said. Not all SIMs using DES seemed to have the flaw, but enough batches to cause major concern.

The size of the issue could define its seriousness with carriers unsure about Nohl's estimates. Most SIMs from recent years would use a different standard, Triple DES (3DES), and be immune to the attack.

The research had been sent to the GSM Association so it could be passed on to carriers, Nohl told media outlets.

"Cards need to use state-of-art cryptography with sufficiently long keys, should not disclose signed plaintexts to attackers, and must implement secure Java virtual machines," he wrote in a blog. "While some cards already come close to this objective, the years needed to replace vulnerable legacy cards warrant supplementary defenses."

Nohl has form when it comes to sniffing out mobile handset issues. Between 2009 and 2011 he publicised a number of security flaws in the GSM standard, most famously in its A5/1 encryption cipher, which could allow call interception between the handset and the network base station.

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