When researchers late last year revealed that the Google Wallet app stored sensitive user data in plain text locally on the device, they also gave the app credit for its PIN protection. But now that last line of defense has been exposed by another researcher, who this week released a proof-of-concept (PoC) for cracking the Google Wallet PIN.

Joshua Rubin, senior engineer with Zvelo, posted his PoC that demonstrates how he cracked Google Wallet's four-digit PIN, used to authorize and process mobile-phone payments. The PIN is considered the extra layer of security that a plain, old credit card wouldn't have. But Rubin poked a big hole in that strategy: "With this attack, the PIN can be revealed without even a single invalid attempt. This completely negates all of the security of this mobile phone payment system," Rubin said in a blog post.

In December, researchers at viaForensics said they had found that the app locally stores some payment card data in plain text, such as the cardholder's name, transaction dates, email address, and account balance.

Google Wallet lets consumers transact credit-card charges, redeem gift cards, and use loyalty membership cards in stores from their phones. To run the app, the users must type in their four-digit PINs when they launch the app.

While Google Wallet hides the full credit-card account number, the last four digits reside in plain text in the app's local SQLite database, viaForensics found. viaForensics also found that the app repels man-in-the-middle attacks, and at the time gave the app credit for being protected by a PIN to conduct transactions with the cards.

Zvelo's Rubin, who posted details on the PoC as well as a video of it, was able to brute-force the PIN, which was a long-integer "salt" plus a SHA256 hash.

Wallet uses near field communication (NFC), based on RFID technology, and it communicates with a so-called secure element (SE) stored in a chip on the device. "It's like the chip and pin model in European credit cards," says Tyler Shields, a security researcher with Veracode.

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