The flaw in Skype could allow a skilled hacker to find out the IP address from which a user has logged in to Skype, thereby determining the location of Skype users. The company is trying to downplay the flaw, claiming that the ability to derive IP addresses was common with all web based communication clients. The flaw can reportedly be exploited without the user’s knowledge, and can be executed on a massive scale. The reserch team demonstrated the flaw by scheduling hourly calls to tens of thousands of users.
Source: https://thehackernews.com/2011/10/latest-security-flaw-in-skype-enables.html