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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 such 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 also works against Skype users who aren't currently logged in, as long as they've used the VoIP 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.

The scientists found that it was relatively easy to find the ID of most Skype users when their email address and birth name are known to the attacker. Additional information, such as the target's city of residence, sex, or age, brought greater accuracy to the task.

They then called the target's Skype account using a customized system that sent specially crafted packets. By examining the headers of the data that was returned, they had no trouble determining the person's IP address. Because the scientists prevented a TCP, or transmission control protocol, connection from being fully established during the probing, targets had no idea their Skype accounts were being tracked. The scientists devised the system so that it could track 10,000 people for about $500 per week.

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 unique 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 also be susceptible to the technique, but 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. Just as with typical internet communications software, Skype users who are connected may be able to determine each other's IP address. Through research and development, we will continue to make advances in this area and improvements to our software.”

The research paper, which is titled I Know Where You are and What You are Sharing, made several recommendations for improving Skype's ability to conceal the identity of its users.

“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. Moreover, if Alice is a stranger (that is, not on Bob's contact list), and Bob configures his client to not accept calls from strangers, then this design would prevent any stranger from tracking him, conspicuously or otherwise.”

A PDF of the paper is here. ®

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