Gnireenigne

Is the reverse engineering - informaally: gnireenigne - by itself legal or illegal. I know that stealing someones code or algorith and using it in a product of your own especially if this piece of code is copyrighted is illegal. But the question is if I just reversed engineered this code, may be to find some vulnerability in it or to test some freaks disassebler I've written or any other reason, will this be illegal too or not.