Millions of Internet-connected machines running the open source Exim mail server may be vulnerable to a newly disclosed vulnerability that, in some cases, allows unauthenticated attackers to execute commands with all-powerful root privileges.

The flaw, which dates back to version 4.87 released in April 2016, is trivially exploitable by local users with a low-privileged account on a vulnerable system running with default settings. All that’s required is for the person to send an email to “${run{…}}@localhost,” where “localhost” is an existing local domain on a vulnerable Exim installation. With that, attackers can execute commands of their choice that run with root privileges.

The command execution flaw is also exploitable remotely, albeit with some restrictions. The most likely scenario for remote exploits is when default settings have been made such as: