New configurations of the Shylock financial malware inject attacker-controlled phone numbers into the contact pages of online banking websites, according to security researchers from antivirus vendor Symantec.
By doing this, the attackers attempt to trick victims into calling them instead of the bank if they become suspicious during an online banking session, Symantec researcher Alan Neville said in a blog post on Tuesday.
Security researchers have advised users for years to call their banks in order to verify the authenticity of any unusual error messages or requests they encounter while performing online banking operations. This type of attack could defeat that basic defense.
"While the exact motive of the attackers is not clear, we speculate that it is either an attempt to extract sensitive login credentials from victims during a telephone conversation or an attempt to block victims from notifying their bank of a problem with their account, giving the attackers more time to perform activities," Neville said.
The new Shylock variant targets banks from the U.K., Neville said. "The numbers being used by the attacker are easy to create online and are disposable."
The Shylock malware, named after a character from Shakespeare's The Merchant of Venice, was first discovered in September 2011 and its main purpose is to steal online banking credentials and other financial information.
Like most financial Trojan programs, Shylock is capable of injecting rogue content into websites accessed from infected computers. The injected content is customized for every targeted website and is pulled from a configuration file.
Shylock attackers are known for being creative with their scams. Back in February, researchers from security vendor Trusteer reported that a Shylock variant was modifying online banking websites to inform users that their computers couldn't be identified and they needed to speak with a bank representative in order to verify their account information.
The malware then injected a Web chat window into the browsing session, connecting targeted users with the attackers instead of their bank's employees.