The well known worm Sasser has been one of the viruses which has received more attention in the press in the latest months. It’s author, an 18 years old student from Germany, after causing lots of troubles to many home users and small enterprises faces up to several years of prison. Sasser is not a well programmed virus, it’s success is entirely due to the exploit it implements, which was announced by Microsoft in one of their security bulletins. In this paper, we will reverse Sasser.B - the second of its variants - showing how it works and also how to clean your computer after infection. Warning: disconnect your computer from the network while working with Sasser, you are left as the only responsible of your mistakes.