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.
“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
(if the user is currently active),” 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 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.
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