Overlay Technique from Brazilian Banking Trojans Making Resurgence

Could we be sitting on the precipice of a shift in banking fraud? Some researchers say yes, if attackers follow the playbook of a little known attack technique that's plagued victims in Brazil under the radar for a couple of years now.

According to new analysis of the Boleto Trojan by researchers from Check Point Research this week, this new variant takes the very rare but effective tack of using malicious overlays triggered after a victim's legitimate bank sign-in in order to manually take over their account. This unorthodox mechanism has been around for a couple years now, but attackers are also layering in some more advanced features used by traditional banking Trojans.

Whereas the typical banking malware such as Trickbot or numerous flavors of Zeus usually rely upon tried-and-true phishing techniques that send victims to a fake login screen to mine credentials, this one goes a totally different route.

Once the malware makes it onto a victim's machine it sits and waits until the user logs into a legitimate online banking account. At that point it sends an SMS to the attacker and pops up a very legitimate looking overlay screen on top of the browser. The Trojan is programmed to match the browser type and the bank information so that it looks completely like the bank's branding and the message of the overlay is meant to distract the user--with a warning or long message that they'll need to read. Under that overlay, the account information is open and running, and the attacker actually uses that window of distraction to manually go into the user's account and run transactions from their account. All of this happens underneath the overlay.

"It's pretty smart and these guys probably will make a lot of money," says Balmas, who explains that with this method the attackers can even go after accounts that have used a multi-factor authentication token, getting around common account transfer limits of $1000 imposed on Brazilian accounts that don't use MFA.

This is essentially a new variant of the KL-Remote discovered by IBM Security Trusteer researchers way back in 2015.  

Balmas says that consumers and vendors should keep their eyes open, because even though this is still limited to South American victims, the situation could change if more bad guys believe this business model can work for them. While a lot of banking malfeasance online depends upon a high level of automation, attackers might be willing to put the work into this kind of manual attack if they feel they can carry out enough high-value fraudulent transfers. The fact that Brazilian criminals have been updating this technique with newer technical advancements might be a sign that security experts need to keep on the lookout for similar methods used in other regions, says Balmas, who believes that most of the world remains in the dark about Brazilian malware because its historically been considered technically pretty crude, and therefore not dangerous enough to warrant attention. 

"I think this rapid pace of new variants, ongoing maintenance, anti-evasion techniques, and overall technical advancements we are observing strengthens our point that it is now more possible than ever that these Brazilian malware may start looking to other victims at other parts of the world, and we would like to raise awareness for these new type of risks," he says. 

First discovered by researchers with Cisco Talos last month, Boleto initially drew a high level of scrutiny from the security community due to its sophistication in using a commercial packer, Themida, in order to obfuscate under-the-hood workings of the malware from threat analysis. Talos engineers gave their initial analysis, but stated that they were still working on unpacking the malware. 

"Using commercial packing platforms like Themida will continue to make analysis difficult for analysts and shows that some attackers are willing to obtain these types of commercial packers in an attempt to thwart analysis," they wrote.

Check Point's team managed to take the ball from there and presented additional details yesterday after finally getting the Trojan unpacked. As Balmas explains, commercial level packing is just the start. From top to bottom it is a sophisticated piece of malware. Other examples of modernization include process termination protection and removal of previously installed hooks. Additionally, the overlay graphics have seen a significant upgrade to improve authenticity.

"They put a lot of effort into doing great graphics, everything looks legit and everything feels right, only it's not," Balmas says.

Related content:

Join Dark Reading LIVE for two days of practical cyber defense discussions. Learn from the industry’s most knowledgeable IT security experts. Check out the INsecurity agenda here.