A piece of financial malware called Tatanga attempts to trick online banking users into authorizing rogue money transfers from their accounts as part of the activation procedure for a free credit-card fraud insurance service purportedly provided by their banks, security researchers from Trusteer said Tuesday.Tatanga is an online banking Trojan horse that was first discovered in May 2011. It is able to inject rogue Web pages into browsing sessions and affects nine different browsers, including Internet Explorer, Mozilla Firefox, Google Chrome, Opera and Safari.The malware is known to use social engineering techniques against victims in order to bypass security measures enforced by banks, like one-time passwords (OTPs) or transaction authorization numbers (TANs).A new Tatanga configuration detected recently by Trusteer displays a rogue message inside the browser when the victim authenticates on their bank's website, claiming that their bank is offering free credit-card fraud insurance to all customers.The message claims that the new service is provided in partnership with Visa and MasterCard and covers losses that might result from fraudulent online transactions performed with the victim's credit or debit card. The malware grabs the user's real account balance, rounds it up, and presents the result as the allegedly insured sum.The rogue message includes a bank account number that's supposed to be the victim's new insurance account opened by the bank. However, in reality, this account belongs to a money mule -- an individual paid to receive money from fraudulent activity on behalf of cybercriminals -- said Ayelet Heyman, a security researcher at Trusteer, in a blog post Tuesday.The user is told that to activate the service they need to authorize a transaction from their bank account to their new insurance account. In order to do this, they need to input the transaction authorization code sent by their bank to their mobile phone number.This code allows the malware to finalize the rogue transfer in the background and send the victim's money to the money mule. "In all likelihood, the victim does not expect any funds will be transferred out of their account," Heyman said.The maximum sum that is transferred by the malware in a single transaction is €5,000 or about US$6,500. "We can assume that fraudsters have identified this amount (5,000 Euros) as the upper threshold that triggers the bank to address the transaction as high risk," Heyman said via email.The rogue message displayed by the Tatanga configuration analyzed by Trusteer is written in Spanish, which suggests that it targets users in Spain or other Spanish-speaking countries. The company hasn't seen a version of this attack in a different language yet, but the malware is known to have targeted users in other European countries and the U.S. in the past, Heyman said via email.The Trusteer researcher advises users to call their banks over the phone and check the validity of sudden announcements or requests that appear on banking websites. If an online banking website suddenly starts asking users for sensitive information like card security codes or PIN numbers, it's most likely because of a malware infection, he said via email.Users should also install the security software recommended or supplied by their financial institution and should use some type of browser-based security solution that prevents financial fraud, Heyman said.