"Knowledge of this is critical if you use Skype in any situations where your location needs to remain secure or simply if you are interested in personal privacy," said Nick Furneaux, managing director of computer forensic services provider CSITech, in a blog post.
Knowledge about the vulnerability first surfaced last week in a Pastebin post from Russian hackers. The instructions involve using a patched version of deobfuscated Skype 5.5, and then enabling debug logging by altering a few registry keys. Reviewing the log file will divulge active users, and entering one of those usernames into Skype's "add a contact" feature--but not sending a request to add them as a contact--would then let an attacker click on the name and see their IP address information. Running that information through the whois service, meanwhile, can detail the user's city, country, and service provider.
The Pastebin post also includes a 19-line Perl script that automates the process of searching in the debug log. "I've tested this and it does what it says on the tin," meaning the script works as advertised. "I was able to extract the external and internal IPs of a friend in the U.S. to within a few miles of his house, a buddy in Asia to within a few streets, and my own to just a few miles down the road," said Furneaux.
[ Can the Middle East eavesdrop on Skype? See Skype Protocol Cracked. ]
But the bigger concern is that being able to discern someone's internal and external IP address "provides the basis for a direct probe and then attack of any individual on Skype's global address book," he said.
A related website recently debuted, which automated the Skype username lookup process. But the site, Skype-IP-Finder, was offline Wednesday, apparently due to a service-provider takedown. "This domain and website have been suspended because of abuse or copyright reasons," read a notice posted on the site. Similarly, according to news reports, some Skype users who have tested out the bug--or undocumented feature--have seen their accounts terminated by Skype, which was bought last year by Microsoft.
Furneaux said that any peer-to-peer based service, such as Skype, might--by design--reveal the IP addresses of anyone that a user connects to, for example, while having a conversation or transferring files. "But at least you are in a conversation with a 'known' person," he said. By contrast, the attack technique "can be used by and against anyone with a Skype account, regardless of whether they are a buddy," he said. "I hope that Skype takes a serious look at this, simply proxying contact requests would likely solve it, which wouldn't be awfully hard for them."
"We are investigating reports of a new tool that captures a Skype user's last known IP address," said Adrian Asher, director of product security at Skype, in a statement. But he likewise warned that the service, by its nature, can reveal details about connected users. "This is an ongoing, industry-wide issue faced by all peer-to-peer software companies. We are committed to the safety and security of our customers and we are taking measures to help protect them."
This isn't the first privacy-related bug to surface on Skype. Last year, academic researchers released a paper, "I Know Where You Are And What You Are Sharing," that detailed techniques for probing Skype users' credentials without their knowledge, providing an attacker knew their target's birthdate and birth name, reported The Register.
"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] (if the user is currently active)," according to the paper. "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."
Using the gleaned information, researchers also were able to correlate BitTorrent downloading activity with Skype accounts, meaning they could positively identity people who were simultaneously using BitTorrent and Skype.
The researchers said the bug could be fixed by not disclosing any IP information until a Skype user accepted an incoming call.
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