Skype has been forced to turn off a video-sharing feature in its software because it could be misused to launch a self-copying worm attack against Skype users, security researchers said Tuesday.
A bug in the software, which was first reported last Thursday by security researcher Aviv Raff, stems from the way Skype uses an Internet Explorer component to render HTML.
Skype's video-sharing feature allows users to share videos hosted on two sites -- Dailymotion.com and Metacafe.com -- while chatting with other Skype users.
Last week Raff showed how attackers could exploit the bug to run unauthorized software on a Skype user's PC. But on Tuesday, the security researcher said the flaw was more serious than he'd first thought. It can "be triggered by simply visiting a Web site, or clicking on a link from your instant messaging application," he wrote in a blog posting, "Which basically means that this vulnerability is now wormable."
Skype appeared to have pulled the video feature from its client software on Tuesday as a result of the bug. Users who attempted to click on the "videos" button within a chat window were greeted with a message that the feature was unavailable "because of some security concerns."
"Our brightest engineers are rattling their wrenches to make things all right and bring the beloved videos back. Soon," the message read. "Sorry about this."
Skpe representatives did not return calls seeking comment. Last week, Skype spokesman Villu Arak confirmed that there was a security problem for Skype 3.5 and 3.6 users who visited the Dailymotion.com Web site, but users were still able to share videos using Metacafe.com.
On Tuesday, however, Skype pulled the video feature altogether after being informed of the new problem, Raff said.
For Raff's attack to work, an attacker would have to post a maliciously encoded video file to either of the Metacafe or Dailymotion Web sites. Metacafe said Tuesday that it's "highly unlikely" that this kind of malicious video would make it through the site's content-filtering process.
In a statement, the company said it expects Metacafe videos to be available to Skype users as early as Wednesday morning.
Raff said that because the attack could lead to a widespread worm outbreak, it would be better for Skype to fix the underlying problem before bringing Metacafe back online.
Raff believes that Dailymotion was probably susceptible to this type of attack as well, although he was unable to confirm this after Skype cut off access to the Web site.
The problem lies in the fact that Skype uses a Windows Internet Explorer component with inappropriate security settings, researchers say. Instead of processing pages it renders with the more secure "Internet Zone" security setting, Skype uses IE's "Local Zone" security setting, usually reserved for more trustworthy content.
Until Skype engineers make some changes to their software, more of these problems will continue to pop up, Raff said.
Another security researcher who has been studying the flaw agreed.
"If they keep their Skype client running in the Local Zone of IE, we will see more of these," said Petko Petkov of GNU Citizen via instant message. "Before killing Metacafe, anyone who owns the server would have been able to own every Skype user on the planet."