source: http://www.securityfocus.com/bid/5768/info

Mac OS X is the BSD-based operating system distributed and maintained by Apple.

It has been discovered that some types of links, when clicked on, may result in the execution of arbitrary commands. Due to the improper handling of some links, a user clicking on a link containing special characters and embedded commands could cause the execution of the commands in the link to be carried out in a terminal.app window. These commands would be executed in the security context of the user.

telnet://|ls -la