Researchers discovered a critical Remote Code Execution vulnerability in the OpenSMTPD Mail server that allows attackers to completely take over the Linux systems remotely.
OpenSMTPD (OpenBSD SMTP Server) is an open-source project that runs in several Unix based operating systems including OpenBSD, FreeBSD,  NetBSD, Linux, macOS.
Newly discovered this RCE Vulnerability in OpenSMTPD lets remote attackers run arbitrary shell commands as root and remotely exploit the operating system that running with the vulnerable OpenBSD SMTP server.
“The vulnerability existing the OpenSMTPD since December 2015, the out-of-bounds read introduced and the vulnerability can be exploited either as root, after May 2018 (commit a8e22235, “switch smtpd to new grammar”); or as any non-root user, before May 2018.” said Qualys research.
The Vulnerability can be Tracked as CVE-2020-8794, and the vulnerability resides in OpenSMTPD’s client-side code that responsible to delivers mail to remote SMTP servers.
Researchers successfully tested this vulnerability in a different platform such as OpenBSD 6.6 (the current release), OpenBSD 5.9 (the first vulnerable release), Debian 10 (stable), Debian 11 (testing), and Fedora 31.
2 Different Scenario’s to Exploit The OpenSMTPD Mail Server
Researchers state that the vulnerability can be exploited by both client-side and server-side using two different scenarios.
In the client-side attack, Attackers take advantage of the OpenSMTPD default configuration to exploit the vulnerability remotely.
Once the Attacker compromised the servers and taking control of it remotely, he could execute arbitrary shell commands on the vulnerable OpenSMTPD installation.
It can be achieved by the attackers using various types of attacks such as a man-in-the-middle, DNS, or BGP attack.
Researchers from Qualys explained the following way to attackers to perform Server-side exploitation:
First, the attacker must connect to the OpenSMTPD server (which accepts external mail) and send a mail that creates a bounce. Next, when OpenSMTPD connects back to their mail server to deliver this bounce, the attacker can exploit OpenSMTPD’s client-side vulnerability. Last, for their shell commands to be executed, the attacker must (to the best of our knowledge) crash OpenSMTPD and wait until it is restarted (either manually by an administrator, or automatically by a system update or reboot).
Exploit is available for this vulnerability and it will be released on February 26, 2020, we will update you once the exploit PoC Code released.
Users are advised to download OpenSMTPD 6.6.4p1 that released on February 24, 2020, and apply the patch as soon as possible.
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