Your computer is hacked. Can you legally counter-hack the hackers?
A vigorous debate on the answer (and deep disagreement) in a series of posts at The Volokh Conspiracy. Short answer: likely not under present (somewhat uncertain) United States law (including the Computer Fraud and Abuse Act).
1st post in the series (Stewart Baker) here.
2nd post (Stewart Baker).
3rd post (Orin Kerr).
4th post (Stewart Baker).
5th post (Orin Kerr).
6th post (Stewart Baker).
7th post (Orin Kerr).