July 24, 2013, 2:38 AM — A pair of severe security problems in millions of SIM cards should be easy for operators to fix, according to the German security researcher who found the issues.
Karsten Nohl of Security Research Labs in Berlin previewed research earlier this week that millions of SIM cards are likely still using an outdated, 1970s-era form of encryption to authenticate over-the-air (OTA) software updates.
Nohl found it was possible to trick some kinds of SIM cards into divulging an encrypted 56-bit DES (Data Encryption Standard) key, which can be decrypted using a regular computer. He discovered that by sending a bogus OTA update to a phone, some SIMs returned an error code containing the weak key.
A device could then be sent spyware which accesses critical phone data through the card's Java Virtual Machine, a software framework present on almost every SIM sold worldwide.
Nohl said in an interview Tuesday that 500 million phones, regardless of make, could be vulnerable, based on his sample of 1,000 SIM cards from a variety of operators, mostly in Europe.
But the weak encryption problem and mistake of returning an error code with a weak key can be fixed in the same way it can be exploited: through an OTA update.
SIM cards come in a wide variety of configurations. Operators will send manufacturers such as Gemalto specifications for SIM cards to be used on their network. Many SIM cards carry older configurations and technology, such as DES, that date back more than a decade, Nohl said.
For some vulnerable SIMs, it may be possible to switch off the DES encryption and turn on Triple DES, a more secure form of encryption that is now used, Nohl said.
Users won't even know their phones are updated, as operators frequently send out updates that are invisible to people using special SMS codes to change, for example, roaming settings, he said. An OTA update can also fix phones that return the revealing error message.
Operators can also make a key adjustment to their SMS centers, which process all SMS messages. Since the SMS codes carrying software updates are very specific, operators can adjust their firewalls to only allow those types of codes to be sent to their users if the codes originate from their servers, Nohl said.
Since so many operators are affected, Nohl said his lab contacted the GSM Association trade group with details of the research, which has issued advisories.
Although there was potential for arguments between operators and SIM card vendors over who was to blame, "everybody was extremely constructive in working to fix the problem, and there was no pointing fingers," Nohl said.