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Skype: XSS vuln fix is on the way

Backend backdoor to be firmly plugged

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Updated Skype has promised to fix a cross-site scripting flaw that exposes Windows users of VoIP technology to potential attack.

The flaw was discovered by independent security researcher Levent Kayan, who warned that a hacker might be able to enter a string of JavaScript code into the "mobile phone" field. This would enable a hacker – provided he or she could trick a victim into adding them as a contact – to compromise the user's Skype account (or "trivially hijack session IDs", as Kayan puts it). Kayan (who has published a video on the bug) adds that the vulnerability also creates a possible mechanism to attack "underlying software and operating systems", a contention strongly disputed by Skype.

Skype said that the bug isn't very serious because it only allows access to the Skype home area, an area of the VoIP client that displays web content. Restrictions on web clients means that the flaw can't be used to inject malware, Skype contends. "The person reporting the bug has only demoed a simple javascript prompt, which is a long leap to loading malware, and the web area is not connected to a Skype account," Skype said.

Despite saying the bug is no big deal Skype promised an update address the vulnerability by the end of the week.

The server-side bug created a possible mechanism for miscreants to redirect Skype users to potentially malicious websites, providing they successfully tricked users into adding them as a contact, as the VoIP outfit explains in an update to its official security blog.

Skype for Windows is not correctly validating some fields of your contacts' profiles. What this means is if one of your Skype contacts has put some specific strings into their profile, it could result in your Skype Home area being redirected to another web page or a message being displayed.

In order for someone to cause these messages to be popped up or to redirect you to a website, they would first have to be one of your accepted Skype contacts. However, this vulnerability should not be there and there is a fix, which we are finalising testing of, that is due to be pushed out early next week.

Cross Site Scripting (XSS) flaws, in general, can be used to present content or pop-ups from potentially hostile websites as if the content had originated from other domains. The class of vulnerability is sometimes used as an adjunct to more highly evolved and subtle phishing scams.

Skype said the necessary fix will be applied without troubling its users with software updates, indicating the bug can be resolved by an update to backend systems alone. ®

Updated to Add

Skype have supplied us with a statement regarding this issue in which the firm says:

The XSS issue relates to the Skype home area of the Skype client. This area provides information on your most frequent contacts and their mood messages. It is correct that the fact that your friends could perform an XSS attack could result in them being able to get a web session token or cookie. But this token is extremely limited in what it can do.

It is totally separate to a Skype logon session, i.e. the account itself. In no way does it allow for hijacking of accounts, resetting of passwords [etc].

The Skype home area is an area of the client that displays web content. As such it has all the protections of standard web clients which prohibit the accessing of local systems or executing arbitrary code.

Mobile application security vulnerability report

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