The two-part Microsoft component -- the '.NET Framework Assistant' add-in and a plug-in for Microsoft's Windows Presentation Foundation rich media framework -- 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.
Mozilla, however, clearly felt that that was not enough, and took the unusual step of blocking the Microsoft add-on and plug-in. Multiple Computerworld staffers have confirmed that Firefox is now blocking the Microsoft software. "These add-ons have a high risk of causing stability or security problems and have been blocked, but a restart is required to disable them completely," the Firefox warning message reads.
The history of the .Net Framework Assistant and Windows Presentation Foundation software is tangled and contentious. Firefox users complained last February, and then again in May, when they found out that Microsoft had pushed the components to their browser as part of the .NET Framework 3.5 Service Pack 1 (SP1) update, which was delivered via Windows Update.
Users were furious that the software was installed without their approval. To add salt to the wound, the components were impossible to uninstall without editing the Windows registry, a chore most users avoid because any misstep could cripple the PC. Later, Microsoft issued a follow-on update that made it possible to uninstall or disable the components without a registry edit.
Mozilla has been aggressively pursuing risky add-ons and plug-ins of late. Last month, it warned Firefox users running outdated versions of Adobe's Flash Player to upgrade, then last week added a more thorough plug-in checking service to its arsenal.
The next edition of the browser, Firefox 3.6, will warn users when they visit a Web site that relies on one or more outdated plug-ins. A beta of Firefox 3.6 is set to launch Wednesday.