This article says that the ever-popular Skype has a security risk. Essentially, Dmitry Goroshevsky, founder-CEO of Popular Telephony, which has a server-less peer-to-peer VoIP software called Peerio, stated that Skype is a big security risk for corporate networks.
I'll have to investigate this some more, but for now, check out the article.
Om Malik on Broadband » Skype, a security risk for corporate networks?