Russian-speaking threat group TA505 has begun targeting individuals working at financial institutions in the US, United Arab Emirates, and Singapore with new malware for downloading a dangerous remote access Trojan (RAT).
Researchers at Proofpoint say they have observed TA505 sending out tens of thousands of emails containing the downloader — dubbed "AndroMut" — to users in the three countries. The group is also targeting users in South Korea in a separate but similar campaign.
In both campaigns, TA505 is using AndroMut to download "FlawedAmmyy," a full-featured RAT that allows the attackers to gain administrative control of an infected device to monitor user activity, profile the system, and steal credentials and other data from it.
FlawedAmmyy is malware that first surfaced in 2016 and is based on the leaked source code of a legitimate remote admin tool called Ammyy. TA505 has used it in previous campaigns.
"We regularly see the group Cobalt Strike using the legitimate Cobalt Strike penetration testing software in their attacks," says Chris Dawson, threat intelligence lead at Proofpoint. Threat actors also frequently abuse other remote admin tools, such as Team Viewer and VNC, in attacks. It is less common for a legitimate tool to be converted into standalone malware like FlawedAmmyy, he says.
The AndroMut downloader itself is new and first surfaced last month. The malware is written in C++ and appears to bear some resemblance to another downloader called Andromeda. AndroMut uses encryption and obfuscated API calls to evade detection and includes several anti-analysis features, including checks for sandboxes and emulators, checks for mouse movement, and checks for debuggers, Proofpoint said.
"TA505 has evolved over the last few years from an extremely high-volume actor dealing in global ransomware and banking Trojan campaigns to a targeted actor focused on regional campaigns and malware," Dawson says.
Its current malware portfolio includes multiple downloaders and sophisticated RATs. "Infections with tools like these are quiet," he notes. "Individuals and organizations often don't know that they are infected until the actor decides to install additional malware, steal credentials and identity information, or launch further attacks inside an organization."
The evasion and anti-analysis capabilities built into modern malware tools like AndroMut highlight the need for multilayered protections. In addition to securing emails and endpoint devices, organizations need to monitor for malware communication with command-and-control systems, Dawson notes.
For enterprises, the threat posed by TA505 appears to be growing, according to Proofpoint. The group is behind some of the largest email campaigns ever, including one to distribute the Locky ransomware.
Through 2017 and the first half of 2018, TA505 launched such massive campaigns that they dramatically affected global malicious email volumes, Dawson says. "The group saturated organizations with Locky ransomware and the Dridex banking Trojan," he notes.
When TA505 shifted to smaller — though still relatively large — campaigns distributing RATs and other malware, it triggered a similar shift in this direction among other attackers that continues today, Dawson says.