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Skype blamed an "unintentional communication oversight" for not notifying users a month ago that it had patched the Windows version of its voice over IP (VoIP) client software against a critical bug.

Company spokesman Villu Arak apologised for the blunder. "We strive to inform the public of vulnerabilities and malware that may affect Skype software," said Arak on the company's security blog Monday. "While this particular vulnerability was fixed, there was an unintentional communication oversight and we failed to bring the case to the public's attention. All we can do now is to apologise."

Arak was reacting to an advisory published last Thursday by 3Com's TippingPoint division and its bounty for bugs Zero Day Initiative. The bug, said TippingPoint, was in a Skype uniform resource identifier (URI) protocol handler that could be exploited to hijack a Windows machine by duping its owner into steering to a malicious Web site.

TippingPoint reported the bug to Skype on 2 November Skype patched it on 15 November, less than two weeks later, Arak said Monday. "The issue was fixed in the public release of Skype 3.6 for Windows," he said. "All versions of Skype for Windows updated or installed as of 15 November include the patch."

In a separate alert, Symantec spelled out how users can tell if they're running a vulnerable version of Skype. "Customers can check ... by clicking 'Help' > 'About Skype.' If you are running a version earlier than 3.6.0.216, then you should upgrade," the advisory read.

The vulnerability wasn't the first URI protocol handler bug that Skype has had to patch, nor is it the only application that has faced the issue this year. In May 2006, Skype fixed a command line parsing flaw in the URI handler of the Windows client, for example.

And other developers, including Mozilla and Adobe Systems, have patched their software against protocol handler errors multiple times since July. Mozilla fixed a Firefox URI bug as recently as two weeks ago. After a contentious debate over responsibility, Microsoft also issued its own fix for Windows on Nov. 13.

The current, patched version of Skype for Windows can be downloaded from the company's site.