Researchers determined that the victims of a backdoor developed by the advanced persistent threat (APT) group Turla are more numerous than originally expected.

The threat group recently employed the backdoor to access the foreign offices of two European countries and a major defense contractor, according to Slovakian IT security company ESET. Those victims received less publicity than Germany’s Federal Foreign Office, which the group breached after compromising the network of the country’s Federal College of Public Administration.

The most recent versions of Turla’s invention went after targets’ inboxes by subverting Microsoft Office’s Messaging Application Programming Interface (MAPI). They were fully controllable by email and didn’t rely on a conventional command-and-control (C&C) server. Instead, the backdoors used specially crafted PDF files in email attachments to fulfill a series of commands such as data exfiltration. The most recent variant from April 2018 was also capable of executing PowerShell commands by leveraging Empire PSInject.

Turla’s Threat Innovation Continues

In 2017, ESET observed Turla leveraging another backdoor called Gazer to target embassies and government organizations around the world. A year later, researchers found evidence that the threat group was bundling the backdoors with a legitimate Adobe Flash Player installer and using URLs and IP addresses that appeared identical to Adobe’s actual infrastructure.

Given ESET’s most recent findings, Turla is showing no signs of slowing down its efforts to spy on promising targets and secretly infect networks with malware for as long as possible.

How to Block an Email-Borne Backdoor

To defend against this and other backdoor threats, security teams should monitor for the indicators of compromise (IoCs) listed in the IBM X-Force Exchange threat advisory. Security experts also recommend following the National Institute of Standards and Technology’s (NIST) cybersecurity framework and conducting security awareness training to educate employees about email-based threats.

Sources: ESET, ESET(1), ESET(2)