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Picking Pockets, Wirelessly

The world’s most popular wireless smart card can be copied.

The world’s most popular wireless smart card has had a rough couple of years. The Mifare Classic, which is used in public-transit systems all over the world and to control access to many offices and buildings, has been the subject of intense scrutiny from security researchers. Last February, researchers from the University of Virginia cracked the encryption used to protect data on the card. Then, in August, a team from MIT showed how to get free rides on the MBTA transit system by exploiting weaknesses in the card. However, in both cases, physical access to the targeted card was required.

Next week, at the IEEE Symposium on Security and Privacy, in Oakland, CA, researchers from Radboud University, in the Netherlands, will demonstrate a new, even easier way to steal data from the smart card. Their attack, which requires only a cheap, off-the-shelf card reader and an ordinary computer, can pull sensitive data out of a card in less than a second–even if the attacker has no physical access to the card.

The attack builds on previous research and takes advantage of newly discovered flaws in the card’s design, explains Peter van Rossum, an assistant professor of computer science at Radboud. Key to the exploit is the way that the smart card communicates with a wireless reader. The radio signal received by the card provides it with enough power to respond. But both the card and the reader have to first prove their identity by sending a secret key.

The researchers use an off-the-shelf reader to make a series of strategic requests of a card. As the card tries to determine whether it should trust the reader, it inadvertently reveals enough information for the attacker to guess the correct secret key. Because so much information about the Mifare Classic is already publicly available, van Rossum believes that an attacker could pull together the necessary knowledge and equipment within a matter of weeks.

Van Rossum says that an attacker would most probably perform the attack on a card that she already owns–for example, to increase the balance on her subway card. But he says that being able to perform the attack wirelessly raises the possibility that the attacker could copy someone else’s card to gain unauthorized access to a building, for example.

“Previously, the nail was in the coffin. Now, this puts the coffin in the ground and buries it,” says Tadayoshi Kohno, an assistant professor of computer science at the University of Washington, in Seattle. He says that previously, some have claimed that it would be difficult to pull off an attack against the card in practice. Kohno hopes that this new work will put remaining objections to rest by showing how easy and inexpensive an attack can be.

“Apparently, nothing short of real-world hacks will convince operators to upgrade to better technology,” says Karsten Nohl, a security researcher now based in Berlin who reverse-engineered the algorithm inside the Mifare Classic while at the University of Virginia. “Now that the paper detailing a very practical attack is released, it won’t take long until we see many Mifare-based security systems being exploited.”

Last year, NXP, the company that makes the Mifare Classic, sued Radboud University in an attempt to stop the researchers from publishing their discoveries about the smart card. That effort failed, and the company, which coincidentally has offices near the university, is now working with the researchers to improve the security of the Mifare Plus, a successor to the Mifare Classic. Fixing all of the Mifare Classic’s security holes would require replacing infrastructure, according to van Rossum, but improvements can be made to the design of the Mifare Plus so that it will work with existing infrastructure in a more secure way.

Van Rossum is most concerned about hackers gaining access to important buildings. He says that since there is no fix for existing Mifare Classic infrastructure, companies and organizations that use the cards should improve other security practices that supplement the cards.

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