Remember that Microsoft .NET Framework Assistant add-on that Microsoft sneaked into Firefox without explicit permission from end users?

Well, the code in that add-on has a serious code execution vulnerability that exposes Firefox users to the "browse and you're owned" attacks that are typically used in drive-by malware downloads.

The flaw was addressed in the MS09-054 bulletin that covered "critical" holes in Microsoft's Internet Explorer but, as Redmond's Security Research & Defense team explains, the drive-by download risk extends beyond Microsoft's browser.

A browse-and-get-owned attack vector exists. All that is needed is for a user to be lured to a malicious website. Triggering this vulnerability involves the use of a malicious XBAP (XAML Browser Application). Please not that while this attack vector matches one of the attack vectors for MS09-061, the underlying vulnerability is different.  Here, the affected process is the Windows Presentation Foundation (WPF) hosting process, PresentationHost.exe.

While the vulnerability is in an IE component, there is an attack vector for Firefox users as well. The reason is that .NET Framework 3.5 SP1 installs a “Windows Presentation Foundation” plug-in in Firefox.

Now, Microsoft's security folks are actually recommending that Firefox users uninstall the buggy add-on:

For Firefox users with .NET Framework 3.5 installed, you may use “Tools”-> “Add-ons” -> “Plugins”, select “Windows Presentation Foundation”, and click “Disable”.

More from Computerworld's Gregg Keizer.