Exim, one of the four MTAs commonly used on Unix servers, is developed by Phillip Hazel at the University of Cambridge. It is the default on some Linux distributions, like Debian.
As usual, there are various guesstimates about the number of servers running Exim, with figures ranging from 57% or about 600,000 to 5.4 million being bandied about.
The vulnerability allows for remote command execution where an attacker could execute arbitrary commands as root. Versions 4.87 to 4.91 were said to be vulnerable, with a fix having been Incorporated into 4.92.
Already working on the blogpost – look for this key in your SSH server configuration to see if you were targeted: pic.twitter.com/Q6zVPuaVtU— Amit Serper (@0xAmit) June 13, 2019
The script would then reconfigure the OpenSSH configuration file to allow root logins along with an RSA public/private key pair.
"That means that if your server was exploited, the attackers have root access to your server via private/public key authentication," Serper said.
The original post about the vulnerability was released by Qualys Research Labs on 5 June, which said it was trivially exploitable in local and non-default cases, but with the default configuration an attack would take a long time to succeed.
Qualys said: "This vulnerability is exploitable instantly by a local attacker (and by a remote attacker in certain non-default configurations). To remotely exploit this vulnerability in the default configuration, an attacker must keep a connection to the vulnerable server open for seven days (by transmitting one byte every few minutes).
"However, because of the extreme complexity of Exim's code, we cannot guarantee that this exploitation method is unique; faster methods may exist."
Satnam Narang, senior research engineer at security outfit Tenable, said: "Attackers have started probing for, and experimenting with attacks against Exim systems vulnerable to CVE-2019-10149.
"Security researchers have observed active exploitation in the wild, one of which includes an attack resulting in permanent root access to vulnerable systems via SSH.
"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."
Advisories and fixes have been listed in a post from the National Vulnerability Database.