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.