This was an interesting Skype presentation given a few days ago at BlackHat Europe: http://www.blackhat.com/presentations/bh-europe-06/bh-eu-06-biondi/bh-eu-06-biondi-up.pdf It details the workings of Skype through heavy reverse engineering, and also walks through exploiting an actual Skype vulnerability toward the end. Enjoy! Cheers, Gary