Skype lets hackers track your BitTorrent downloads

Scientists have devised a stealthy and low-cost way to track the internet protocol addresses of tens of thousands of Skype users, and link the information to their online activities just as the sharing of specific files over BitTorrent.

The method, which is laid out in a recently published academic paper, works even when Skype users have configured their accounts to accept calls only from people in their contact lists. It as well works against Skype users who aren't currently logged in, as long as they've used the VoIP (Voice over Internet Protocol) program in the past three days. The system is able to link an individual Skype user to specific BitTorrent activity, even when they share the IP address with others over a local area network that uses NAT, or network address translation.

"We have shown that it is possible for an attacker, with modest resources, to determine the current IP address of identified and targeted Skype user[s]," the 14-page paper stated. "In the case of Skype, even if the targeted user is behind a NAT, the attacker can determine the user's public IP address. Such an attack could be used for many malicious purposes, including observing a person's mobility or linking the identity of a person to his internet usage."

The IP addresses of individuals

After learning the IP addresses of individuals, the scientists tapped BitTorrent sites to track the specific downloads of addresses in their database. Even when one of the IP addresses was shared among many users on a single network, the method was able to single link a in a class by itself Skype user to a specific download by, among other things, collecting identifiers known as infohashes from BitTorrent networks.

The scientists said Google Talk, MSN Live and other real-time communication applications may as well be susceptible to the technique, nevertheless they singled Skype out for containing what they called "a major privacy vulnerability."

In a statement, Adrian Asher, chief information security officer in Microsoft's Skype division, said: "We value the privacy of our users and are committed to making our products as secure as possible. Such as with typical internet communications software, Skype users who are connected may be able to determine each other's IP address. Through innovation and development, we will continue to make advances in this area and improvements to our software."

"One solution that would go a long way is to design the VoIP system so that the callee's IP address is not revealed until the user accepts the call," it stated. "With this property, Alice would not be able to inconspicuously call Bob. In addition, if Alice is a stranger, and Bob configures his client to not accept calls from strangers, at that time this design would prevent any stranger from tracking him, conspicuously or if not."