7. Swamp your email

Posted on 12 May 2008 at 15:06

Can spammers use your email server, through the miracle of open relaying, to deliver junk mail or host malware, leaving you to take the flak? If you have security software installed the answer should be "no" because, as Jaime Lyndon A Janeza points out, this will also "act as email proxies and hook into the mailing process to ensure your system isn't being used to relay spam".

Nevertheless, he says it's worth testing this theory with the open relay test from SpamHelp (www.spamhelp.org/shopenrelay). Meanwhile, John Safa points us towards a simple relay test at MXToolbox (www.mxtoolbox.com/diagnostic.aspx), which will connect to a mail server via SMTP, perform an open relay test and verify that the server has a reverse DNS record. It also tests response times for the mail server and, if you know your domain but not the mail server address, it will even do an MX lookup.

If you prefer not to involve third-party testing facilities, ethical hacker Tony Fogerty suggests connecting to mail server SMTP service on TCP port 25 using a Telnet or Netcat client and sending email to your own account with the following code, where "IP" is the IP address of the server:

telnet IP 25

helo

mail from

rcpt to

enter message here, followed by a full stop on a new line

.

Also, check whether your email server divulges your internal users by supporting the EXPN and VRFY commands (for example, VRFY administrator -> admin@example.org):

' telnet 25

help - does it show EXPN / VRFY to be supported?

EXPN administrator

VRFY admin'

Finally, check if it's possible to send a spoofed email through the email server. For example, SMTP server for domain "example.org" receives an external email from john.smith@example.org. Does your email server drop this email? It should do.

8. Probe for viruses

Hack it yourself

Author: Davey Winder