Hackers Target Recent Vulnerability in Exim Mail Server

Cybercriminals are already targeting a recently disclosed vulnerability in the open-source Exim mail server, Cybereason reports. 

Tracked as CVE-2019-10149, the vulnerability was disclosed early this month, but it has existed in Exim since version 4.87 of the mail server, which was introduced in April 6, 2016. All Exim versions through 4.91 are vulnerable.

Exploitable both locally and remotely, the security flaw allows for arbitrary commands with execv(), as root.

Qualys, which discovered the vulnerability, explained last week that, while a local attacker could exploit the issue instantly, a remote attacker would need to keep an active connection to the vulnerable system for several days for successful exploitation.

Now, Cybereason reveals that it has detected an active, ongoing campaign targeting the vulnerability, and that the purpose of the attacks is the installation of a coin miner. However, multiple actors are targeting the security bug. 

Exim powers around 57% of the Internet’s email servers and is used within over half a million organizations. At the moment, there are more than 3.5 million machines at risk worldwide, Cybereason says. 

As part of the analyzed campaign, the actor first achieves remote command execution on the vulnerable machine, then deploys a port scanner to find additional servers to infect. Any existing coin miners are removed from the target, along with defense mechanisms against miners, and cronjobs are used for persistence. 

Several payloads are used for the different stages of the attack, including a Python-based port scanner that can infect discovered vulnerable servers with the initial script. An RSA authentication key is added to the SSH server, which allows the attacker to connect as root and take full control of the machine. 

The final stage of the infection involves the download of a password protected zip archive with password “no-password,” which contains an UPX-packed ELF file that hides another ELF executable, none other than the coin miner.

Cybereason’s security researchers notes that their investigation into the attacks is still ongoing and that additional details on the breadth of the campaign, the payloads being used, and more will be revealed as they are discovered. 

The attackers appear to have invested a lot in hiding the intentions of their newly-created worm, using services on the TOR network to host payloads and also creating deceiving files in an attempt to throw off researchers and system administrators. 

The targeted vulnerability has been addressed with the release of Exim version 4.92. To stay protected, users should apply the update as soon as possible. 

“Attackers have started probing for and experimenting with attacks against Exim systems vulnerable to CVE-2019-10149. […] It is critically important for those running Exim to upgrade to version 4.92 or apply the backported fix to vulnerable versions in order to prevent these newly discovered attacks from succeeding,” Satnam Narang, senior research engineer, Tenable, pointed in an emailed comment to SecurityWeek.