The National Security Agency today published an advisory warning of Russian military attackers exploiting vulnerable email servers. Sandworm, an intelligence group also known as GRU Unit 74455, has been targeting a flaw in Exim mail transfer agent (MTA) since at least August 2019.
Exim is a broadly used MTA software for Unix-based systems and comes preinstalled on some Linux distributions. In early June of last year, a patch was issued to fix a critical remote code execution vulnerability (CVE-2019-10149) in Exim. Attackers could exploit this by sending a specially crafted email to a machine using an unpatched version of Exim. They could execute commands with root privileges and install programs, modify data, and create new accounts.
When Sandworm targeted the flaw, the device receiving a malicious email would download and execute a shell script from an attacker-controlled domain. This script attempted to take a few actions: add privileged users, disable network security settings, update SSH configurations to enable additional remote access, and execute additional script to enable follow-on exploitation. This exploit could be used to achieve "pretty much any attacker's dream access," according to NSA.
The advisory does not share details about the organizations Sandworm has targeted or the motivation behind these attacks.
"When the patch was released last year, Exim urged its users to update to the latest version," NSA officials said in a statement. "NSA adds its encouragement to immediately patch to mitigate against this still current threat." Users are advised to install version 4.93 or newer.
Read more details in the full NSA advisory.