Russian Sandworm Exploiting Exim Mail Servers

It has been found by the NSA that the Russian Spy Group called BlackEnergy is actively exploiting Exim mail servers with Sandworm.

The Exim mail server flaw can be exploited using a email containing a modified “MAIL FROM” field in a Simple Mail Transfer Protocol (SMTP) message. The Russians have been exploiting unpatched Exim servers 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.

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

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Patrick Domingues

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