NSA Warns of Sandworm Backdoor Attacks on Mail Servers

sandworm exim attacks

The Russian spy group, a.k.a. BlackEnergy, is actively compromising Exim mail servers via a critical security vulnerability.

The Russia-linked APT group Sandworm has been spotted exploiting a vulnerability in the internet’s top email server software, according to the National Security Agency (NSA).

The bug exists in the Exim Mail Transfer Agent (MTA) software, an open-source offering used on Linux and Unix-like systems. It essentially receives, routes and delivers email messages from local users and remote hosts. Exim is the default MTA included on some Linux distros like Debian and Red Hat, and Exim-based mail servers in general run almost 57 percent of the internet’s email servers, according to a survey last year.

The bug (CVE-2019-10149) would allow an unauthenticated remote attacker to execute commands with root privileges on an Exim mail server, allowing the attacker to install programs, modify data and create new accounts. It’s also wormable; a previous campaign spread cryptominers automatically from system to system using a port sniffer. The bug was patched last June.

The NSA this week released a cybersecurity advisory on new exploit activity from Unit 74455 of the GRU Main Center for Special Technologies (GTsST), a division of the Russian military intelligence service, a.k.a. Sandworm, a.k.a. BlackEnergy. The APT has been linked to the Industroyer attack on the Ukrainian power grid as well as the infamous NotPetya attacks. According to Kaspersky, the group is part of a nexus of related APTs that also includes a recently discovered group called Zebrocy.

The flaw can be exploited using a specially crafted email containing a modified “MAIL FROM” field in a Simple Mail Transfer Protocol (SMTP) message. The APT has been exploiting unpatched Exim servers in this way since at least August, according the NSA’s advisory.

Once Sandworm compromises a target Exim server, it subsequently downloads and executes a shell script from a Sandworm-controlled domain to establish a persistent backdoor that can be used for reconnaissance, spying on mail messages, lateral movement and additional malware implantation.

“This script would attempt to do the following on the victim machine: Add privileged users; disable network security settings; update SSH configurations to enable additional remote access; and execute an additional script to enable follow-on exploitation,” according to the NSA, which didn’t disclose any details as to the victimology of the latest offensives.

Exim admins should update their MTAs to version 4.93 or newer to mitigate the issue, the NSA noted.

“This emphasizes the need for a good vulnerability management plan,” Lamar Bailey, senior director of security research at Tripwire, said via email. “CVE-2019-10149 has been out almost a year now and has a CVSS score above 9, making it a critical vulnerability. High-scoring vulnerabilities on a production email server are high risk and there should be plans in place to remediate them ASAP.”

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