Pierluigi Paganini May 28, 2020

The U.S. NSA warns that Russia-linked APT group known as Sandworm Team have been exploiting a critical flaw in the Exim mail transfer agent (MTA).

The U.S. National Security Agency (NSA) is warning that Russia-linked APT group tracked Sandworm Team has been exploiting a critical vulnerability (CVE-2019-10149) in the Exim mail transfer agent (MTA) software since at least August 2019.

The CVE-2019-10149 flaw, aka “The Return of the WIZard,” affects versions 4.87 to 4.91 of the Exim mail transfer agent (MTA) software. The issue could be exploited by unauthenticated remote attackers to execute arbitrary commands on mail servers for some non-default server configurations.

The flaw resides in the deliver_message() function in /src/deliver.c and it is caused by the improper validation of recipient addresses. The issue could lead to remote code execution with root privileges on the mail server.

“Russian military cyber actors, publicly known as Sandworm Team, have been exploiting a vulnerability in Exim mail transfer agent (MTA) software since at least last August.” reads the advisory published by the NSA. “The Russian actors, part of the General Staff Main Intelligence Directorate’s (GRU) Main Center for Special Technologies (GTsST), have used this exploit to add privileged users, disable network security settings, execute additional scripts for further network exploitation; pretty much any attacker’s dream access – as long as that network is using an unpatched version of Exim MTA.”

“NSA adds its encouragement to immediately patch to mitigate against this still current threat.”

GRU Main Center for Special Technologies (GTsST) hackers of 

Hackers belonging to the Unit 74455, under the Russian GRU Main Center for Special Technologies (GTsST), are exploiting the Exim issue after an update was issued in June 2019.

“The actors exploited victims using Exim software on their public facing MTAs by sending a command in the “MAIL FROM” field of an SMTP (Simple Mail Transfer Protocol) message.” states the advisory.

Below a sample “MAIL FROM” exploitation command published by the NSA:

Russian state-sponsored hackers leverage the vulnerability to download a shell script from a domain under their control and use it to “add privileged users, disable network security settings, update SSH configurations to enable additional remote access, execute an additional script to enable follow-on exploitation.”

NSA recommends patching Exim servers immediately by installing version 4.93 or newer.

“Update Exim immediately by installing version 4.93 or newer to mitigate this and other vulnerabilities. Other vulnerabilities exist and are likely to be exploited, so the latest fully patched version should be used. Using a previous version of Exim leaves a system vulnerable to exploitation. System administrators should continually check software versions and update as new versions become available.” concludes NSA. “Administrators can update Exim Mail Transfer Agent software through their Linux distribution’s package manager or by downloading the latest version from https://exim.org/mirrors.html.”

NSA’s advisory also includes Indicators of Compromise and instructions on how to detect exploit attempts and unauthorized changes.

Unfortunately, the number of vulnerable Exim installs exposed online is still high, querying Shodan for installs exposed online we can more than 2,481,000 servers, with more than 2,400,000 servers running the patched Exim 4.93 release. 

Pierluigi Paganini

(SecurityAffairs – APT, hacking)

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