Source
"IT security specialists Backfire Security today announced the availability of a software download as a discreet desk-top client application that wreaks revenge on those hackers and culprits attacking your network or infecting users with worms and/or viruses. The "freeware" package - PAYBACK v1.0 - is available from www.backfiresecurity.co.uk in both PC and Mac formats. PAYBACK v1.0 is a new kind of anti-hacker application called an IRS (Intruder Retaliation System) and is based upon "guerrilla" programming protocols and algorithms originally developed for the Chinese Space Program.
This seems like a good way to fight back.. but I can think of a couple of situations where you would be doing just as much harm as the original "hacker".

Imagine:

Attacker compromises host A which uses host A as a launch pad for attacks against host B. If host B is your machine and you have PAYBACK installed on it... you are helping destroy a machine of a victim that didn't attack you in the first place. You would be just as guilty as the attacker. Even though you didn't attack host A... you infected it.

I wonder how this would all play out on the legal sides of things...

Opinions anyone?