Chip and Skim: Cloning EMV Cards with the Pre-play Attack

Abstract

Abstract-EMV, also known as "Chip and PIN", is the leading system for card payments worldwide. It is used throughout Europe and much of Asia, and is starting to be introduced in North America too. Payment cards contain a chip so they can execute an authentication protocol. This protocol requires point-of-sale (POS) terminals or ATMs to generate a nonce, called the unpredictable number, for each transaction to ensure it is fresh. We have discovered two serious problems: a widespread implementation flaw and a deeper, more difficult to fix flaw with the EMV protocol itself. The first flaw is that some EMV implementers have merely used counters, timestamps or home-grown algorithms to supply this nonce. This exposes them to a "pre-play" attack which is indistinguishable from card cloning from the standpoint of the logs available to the card-issuing bank, and can be carried out even if it is impossible to clone a card physically. Card cloning is the very type of fraud that EMV was supposed to prevent. We describe how we detected the vulnerability, a survey methodology we developed to chart the scope of the weakness, evidence from ATM and terminal experiments in the field, and our implementation of proof-of-concept attacks. We found flaws in widely-used ATMs from the largest manufacturers. We can now explain at least some of the increasing number of frauds in which victims are refused refunds by banks which claim that EMV cards cannot be cloned and that a customer involved in a dispute must therefore be mistaken or complicit. The second problem was exposed by the above work. Independent of the random number quality, there is a protocol failure: the actual random number generated by the terminal can simply be replaced by one the attacker used earlier when capturing an authentication code from the card. This variant of the pre-play attack may be carried out by malware in an ATM or POS terminal, or by a man-in-the-middle between the terminal and the acquirer. We explore the design and implementation mistakes that enabled these flaws to evade detection until now: shortcomings of the EMV specification, of the EMV kernel certification process, of implementation testing, formal analysis, and monitoring customer complaints. Finally we discuss countermeasures. More than a year after our initial responsible disclosure of these flaws to the banks, action has only been taken to mitigate the first of them, while we have seen a likely case of the second in the wild, and the spread of ATM and POS malware is making it ever more of a threat.

Relay Cost Bounding for Contactless EMV Payments

“…So, for PaySafe we must stop a single run of the protocol from accepting multiple GPO messages with different nonces. Clearly, our protocol only works if the nonces are actually unpredictable, unfortunately some EMV terminals have been shown to use predictable numbers [5].…”

Harvesting High Value Foreign Currency Transactions from EMV Contactless Credit Cards Without the PIN

“…The very recent "Chip and Skim" attack [1] is similar to our attack in that it could be operated on a large scale and it extracts money from the victim's account. It would be interesting to explore the possibility of using our mobile phone contactless-transactioncollecting app as the "skimming" platform for the Chip and Skim attack.…”

Attack surfaces for mobile devices

“…Hence, there has been a growing revelation and understanding of the vulnerabilities. EMV can be cracked [21], as can GSM [22]. Applications using these protocols should consider cellular and NFC attack surfaces in addition to the conventional higher-level, TCP/IP stack protocols.…”