exim is one of the most used MTAs on mail-enabled servers on the internet and default on Debian/Ubuntu(?).

Original author(s) Philip Hazel (started 1995)
Developer(s) The Exim Maintainers

Thus you can expect hackers, crackers and NSA to target it.

Exim < 4.86.2 – Privilege Escalation

„This vulnerability could be exploited by attackers who have local access to the
system to escalate their privileges to root which would allow them to fully
compromise the system.“

„To exploit this setting and gain the effective root privilege of the SUID binary,
attackers can inject PERL5OPT perl environment variable, which does not get
cleaned by affected versions of Exim.“

https://www.exploit-db.com/exploits/39549/

what version of mail server you are running you can test with telnet connecting to the SMTP port of your server:

telnet domain.com 25
# or
telnet domain.com 587

some examples:

Connection closed by foreign host.
user@debian9:~$ telnet mx.freenet.de 587
Trying 195.4.92.211...
Connected to mx.freenet.de.
Escape character is '^]'.
220 mx0.freenet.de ESMTP Exim 4.85 Thu, 06 Jul 2017 11:27:46 +0200
^C
quit
Connection closed by foreign host.
user@debian9:~$ telnet mail.gmx.net 587
Trying 212.227.17.190...
Connected to mail.gmx.net.
Escape character is '^]'.
220 gmx.com (mrgmx002) Nemesis ESMTP Service ready
quit
221 gmx.com Service closing transmission channel
Connection closed by foreign host.
user@debian9:~$ telnet mail.your-server.de 587; # hetzner
Trying 78.46.5.205...
Connected to mail.your-server.de.
Escape character is '^]'.
220 sslproxy02.your-server.de ESMTP Exim 4.84_2 Thu, 06 Jul 2017 11:29:00 +0200
quit
221 sslproxy02.your-server.de closing connection
Connection closed by foreign host.

PS: All Cisco devices have NSA-backdoors factory build-in… have PHUN! 😀

Videos: Mail Server Conference Berlin 2009

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