The infamous Carbanak malware is now capable of using Google services for command and control (C&C) communication, Forcepoint security researchers warn.
The malware is used by the Carbanak group (also known as Anunak), which was first exposed in 2015 as a financially motivated actor targeting mainly financial institutions. When first uncovered, the group was said to have stolen upwards of $1 billion from more than 100 banks across 30 countries. Historically, the group has been using targeted malware in their attacks, and researchers recently associated it with an attack campaign that leveraged weaponized Office documents hosted on mirrored domains for malware distribution.
The recent attack analyzed by Forcepoint Security Labs follows a similar path, as it uses a RTF document to distribute the Carbanak malware. The document was packed with an encoded Visual Basic Script (VBScript) previously associated with the Carbanak malware before.
The document features an embedded OLE object that contains a VBScript file. When the user opens the document, they are presented with an image that was designed to hide the embedded OLE object and trick the victim into clicking on it. As soon as the user double-clicks the image, a file open dialog for “unprotected.vbe” is launched, and the user is asked to run it. This results in the malware being executed.
More and more cybercriminals have been observed abusing the object linking and embedding (OLE) functionality in Microsoft Office instead of malicious macros for malware distribution. Most recently, the method was observed in a keylogger distribution campaign detailed last week, after Microsoft warned of the practice in June 2016.
In their attack, the Carbanak group packed the RTF document with malicous code in the form of an encoded VBScript file. According to Forcepoint, while this was the typical malware associated with the group, a new “ggldr” script module was also observed. Base64 encoded inside the main VBScript file along with various other VBScript modules, the script was designed to use Google services as a C&C channel.
“The ‘ggldr’ script will send and receive commands to and from Google Apps Script, Google Sheets, and Google Forms services. For each infected user a unique Google Sheets spreadsheet is dynamically created in order to manage each victim. The use of a legitimate third party service like this one gives the attacker the ability to hide in plain sight. It is unlikely that these hosted Google services are blocked by default in an organization, so it is more likely that the attacker will establish a C&C channel successfully,” Forcepoint’s Nicholas Griffin reveals.
When the malware first attempts to contact the hard-coded Google Apps Script URL with the user’s unique infection ID, the C&C responds that no spreadsheet exists for the user. Next, the malware sends two requests to another hard-coded Google Forms URL to create a unique Google Sheets spreadsheet and Google Form IDs for the victim. The next time the Google Apps Script is requested, the C&C responds with these unique details.
“The Carbanak actors continue to look for stealth techniques to evade detection. Using Google as an independent C&C channel is likely to be more successful than using newly created domains or domains with no reputation,” Griffin concludes.