Last year while monitoring our SMTP stats, I noticed an unusual amount of input traffic to our Webserver. Curiosity got the best of me, so I started looking at the Web logs only to find thousands of attacks targeted at a Perl script known as FormMail.pl. A short time later, I started getting thousands of bounced email and complaints from hundreds of people blaming me for acting as a Spam relay. I questioned how this could be, as my Web server wasn't even listening on the SMTP port… or so I thought.
The FormMail.pl Hack
A Spam hack many administrators are still not aware of, takes advantage of a well-known perl script called FormMail.pl, used for processing Web-based forms. Originally written by Matt Wright, the popular version 1.6 allows an attacker to send a malicious URL to the Web Server thus accessing the mail binary and turning your otherwise harmless machine into a spam Mail relay. Millions of Web servers around the world still use version 1.6, despite the release of newer more secure replacements.
Until recently, FormMail.pl was considered reasonably safe due to the implementation of "Referrers" that [only] allowed access based on a specific ruleset of allowable domains and IP's.
# Config code
# @referers allows forms to be located only on servers
# which are defined in this field. This security fix
# from the last version which allowed anyone on any
# server to use your FormMail script on their web site.
A single machine could handle several thousand referrers at a time including Virtual Hosts. The script however, wasn't entirely perfect; not that it didn't work, but it was very easy to hack.
Detecting the Hack
The worst part of this exploit is the administrator usually won't suspect anything's wrong until he starts receiving mail informing him about his so-called Open Relay - that is, if he bothers to read it.
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