Facebook users are being targeted in a new man-in-the-browser (MitB) attack that attempts to steal money by duping the user into redeeming an e-cash voucher.

The malware is a new configuration of the Carberp Trojan, according to security firm Trusteer. Carberp replaces any Facebook page the user navigates to with a fake page notifying the victim that his or her Facebook account is “temporarily locked”.

The user is then asked to enter their name, email address, date of birth and password, followed by the number of a €20 Ukash voucher to “confirm verification” of their identity and unlock the account. Ukash is an e-money network that allows customers to anonymously exchange cash for vouchers that can be spent online.

The page claims the cash voucher will be “added to the user’s main Facebook account balance”. However, the credit is instead transferred to the Carberp bot master, who is able to use it as a cash equivalent.

”This clever man-in-the-browser (MitB) attack exploits the trust users have with the Facebook website and the anonymity of e-cash vouchers,” said Trusteer’s CTO Amit Klein.

“Unlike attacks against online banking applications that require transferring money to another account which creates an auditable trail, this new Carberp attack allows fraudsters to use or sell the e-cash vouchers immediately anywhere they are accepted on the internet.”

Klein told Techworld that Trusteer does not have a concrete idea of how many users have been affected. “However, we've seen botnets ranging in size from thousands to millions, so it's safe to say it's somewhere in between,” he said.

With the growing adoption of e-cash on the internet, Trusteer expects to see an increasing number of these types of attack, and social networks like Facebook provide cybercriminals with a large pool of victims that can be fairly easily tricked into divulging confidential account information.

Like card-not-present fraud, where cybercriminals use stolen debit and credit card information to make illegal online purchases, e-cash fraud is a low risk form of crime, explains Trusteer. With e-cash, however, it is the account holder who assumes liability for fraudulent transactions, rather than the financial institution.

Klein recommends using browser-based security tools that can secure communication between the computer and target website to block MitB attack methods like HTML injection, and prevent keylogging from grabbing data.

Carberp was first discovered in October 2010 by several security companies and noted for its ability to steal a range of data, as well as disguise itself as legitimate Windows files and remove antivirus software. It has been billed as a rival to Zeus, another well-known piece of malware.

Context Information Security warned yesterday that financial malware of this kind is becoming increasingly difficult to detect and eliminate, as it uses multiple layers of obfuscation and encryption to remain hidden and prevent analysis.