source: http://www.securityfocus.com/bid/4516/info

xNewsletter is a script that allows web users to subscribe to a newsletter. It is written in PHP and will run on most Unix and Linux variants, as well as Microsoft Windows operating systems.

xNewsletter does not sanitize dangerous characters from form field input such as the e-mail address of the newsletter recipient. It has been demonstrated that this condition may be exploited to cause multiple instances of the same e-mail address to be written to the datafile. An attacker may effectively trick the script into mail bombing an arbitrary e-mail address.

It has also been demonstrated that the attacker may cause arbitrary data to be written to the datafile in such a way that it cannot be removed using the facilities provided by xNewsletter. The malformed data must be removed from the datafile manually.

These two consequences of insufficent validation of form input may be exploited in conjunction with each other. 

Subscribe with the following code to create an almost undeletable
entry:
<?php inlcude("text.txt");?>testmail@localhost.de

Subscribe with the following address to add an address more than one time:
(testmail@localhost.de)%testmail@localhost.de%testmail@localhost.de%testmail@localhost.de 