Skype for iOS Has Serious XSS Vulnerability

Posted by at 12:25 am on September 21, 2011

A security researcher going by “Phil P” and running the Superevr security blog has found a serious scripting vulnerability in the chat messaging feature of Skype versions 3.01 and earlier for the iPhone and iPod Touch that could execute malicious Javascript code without the user being fully aware, giving the attacker access to file contents of any file that the Skype app would have access to — such as a user’s address book.

The flaw comes as a result of the Skype program failing to properly encode the incoming users’ “Full Name” field, which means Javascript code can easily be placed in that field instead. The victim still receives a message from the attacking user, but with the name replaced by the first characters of the code string.

This flaw alone would not by itself cause an issue except that Skype also improperly allows the URI scheme used by the built-in WebKit browser in Skype to use the “file://” URL header, gaining access to the file system under Skype’s own permissions, which includes access to the iOS contact list. In the video below, Phil demonstrates the delivery of a Base-64 encoded JavaScript file (which is then decoded by the attacker’s server) and run to copy the device’s contact list back to the attacking server (in SQLite format).

Apple’s built-in iOS application sandboxing prevents the attack from going anywhere beyond those files that Skype has access to. Skype on iOS is set by default to only allow existing contacts to chat with a user, which would largely prevent the possibility of such an attack being seen “in the wild”; but if the “established contacts only” feature is turned off, it opens up the possibility of random chatters executing a malicious JavaScript on iOS devices. As the attack is executed over http, a firewall or blocking ports would not prevent the attack

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