Users of Google smartphone wallets were being warned on Friday that there is a way to crack pass codes intended to thwart thieves from going on illicit shopping sprees.

Zvelo Labs researcher Joshua Rubin was featured in a video at the company's website demonstrating software that quickly figures out a Google Wallet personal identification number (PIN), provided the crook has the smartphone.

Rubin said that Google has been alerted to the vulnerability and is moving swiftly to fix it. He has not made his wallet "Cracker" application public.

"Google Wallet allows only five invalid PIN entry attempts before locking the user out," Rubin said in a blog post.

"With this attack, the PIN can be revealed without even a single invalid attempt," he continued. "This completely negates all of the security of this mobile phone payment system."

Google declined an AFP request for comment.

"Once attackers get your PIN, they have full access to any credit card information stored in the app and they can use your phone to make purchases," McAfee security firm researcher Jimmy Shah said in a blog post.

"As a user of Google Wallet, the main security you see is the PIN," McAfee added.

"What makes Wallet easy for you to use now makes it easy for attackers to use; they can now spend your money and credit just as if your phone were an ATM card."

Rubin dismissed the threat of hackers picking Google Wallets remotely, explaining that physical access is needed to get priority access to controls in a process called "rooting."

Security specialists advise Google Wallet users not to "root" smartphones, and to enable security features such as full-disk encryption and screen locks.

Google Wallet is available only on Nexus S and Galaxy Nexus smartphones. Google said it planned to expand the feature to more Android phones.

Google Wallet uses a near field communication (NFC) chip embedded in a phone to allow a user to "tap-and-pay" for purchases at a checkout register equipped with the PayPass system from CitiMasterCard.

Customers can also use a Google Prepaid card to pay for purchases, topping up the Google card with any payment card, and take advantage of Google Offers, the Mountain View, California-based company's online discount coupon program.

In addition to allowing for mobile payments, Google Wallet allows consumers to pay using gift cards and to redeem promotions such as discounts or coupons.