The two-part Microsoft component -- an add-on dubbed ".Net Framework Assistant" and a plug-in named "Windows Presentation Foundation" -- have been blocked by Mozilla as a precautionary measure, said Mike Shaver, the company's head of engineering.
"Because of the difficulties some users have had entirely removing the add-on, and because of the severity of the risk it represents if not disabled, we contacted Microsoft today to indicate that we were looking to disable the extension and plug-in for all users via our blocklisting mechanism," Shaver said in an announcement posted Friday night to the company's security blog.
Mozilla maintains an add-on/plug-in blocking list that automatically bars risky software from being used by Firefox. The open-source company first used the blocker in 2007. Mozilla has used the tool only nine times, including Friday's blocking of the Microsoft add-on and plug-in. In May 2008, for example, Mozilla added a Vietnamese language pack for Firefox to the blocking list when the pack was found to contain a worm.
According to Shaver, Microsoft gave Mozilla the go-ahead to block the .Net Framework Assistant and the Windows Presentation Foundation.
Last week, Microsoft's security team acknowledged that its software -- which had been silently installed in Firefox as far back as February 2009 -- contained a critical vulnerability that could be used by hackers to hijack Windows PCs. The same vulnerability also affected all versions of Internet Explorer (IE), including the newest version, IE8.
Microsoft maintained that users who applied the patches it issued last week as part of a record-setting security update would protect Firefox users from attack. However, the MS09-054 bulletin, which provided details on the vulnerability, said nothing about Firefox. Later last Tuesday, Microsoft expanded on MS09-054 in a blog post by security engineers , and confirmed that Firefox was affected because of the add-on and plug-in.