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Firefox blocks insecure .Net add-on--awkwardly

A security vulnerability with a Microsoft add-on for Firefox prompts Mozilla to block it--then scramble to let people with patched browsers override the block.

Mozilla on Friday disabled a Microsoft plug-in for Firefox called the .Net Framework Assistant because of a security problem--then scrambled to give people with patched systems an override option.

Mike Shaver, Mozilla's vice president of engineering, announced the first step late Friday night on his blog. "It's recently surfaced that it has a serious security vulnerability, and Microsoft is recommending that all users disable the add-on," Shaver said. "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. Microsoft agreed with the plan, and we put the blocklist entry live immediately."

This warning sign greeted Firefox users after Mozilla blocked use of a Microsoft add-on.
This warning sign greeted Firefox users after Mozilla blocked use of a Microsoft add-on. Screenshot by Stephen Shankland/CNET

The .Net Framework Assistant add-on lets Firefox use Microsoft's ClickOnce technology for installing applications that run on its .Net programming foundation. The add-on already was something of a thorn in the sides of some Firefox users: it was automatically installed via Windows Update with the .Net Framework 3.5 Service Pack 1 without telling the user the add-on was being installed or giving an option. More hackles were raised because it wasn't compatible with Firefox 3.5, Shaver said, and because removing it initially required people to edit their Windows Registry--a technically onerous task for most people.

Firefox checks a Mozilla server periodically for a list of add-ons to avoid. Although Mozilla's blocking move was intended to protect users, it caused other problems. Shaver indicated that Firefox's changed behavior irked some system administrators.

That led Justin Angel, a former Silverlight program manager at Microsoft, to tweet, "When business users can't use their core business functionality--they uninstall stuff."

One issue was that Mozilla's add-on blocking technology couldn't tell if people had patched their software and so weren't vulnerable anymore. "We can't distinguish patched from unpatched, so we're blocking it while we sort that out," Shaver twittered. Over the weekend, Mozilla worked to remedy the situation.

"Pushing a change to our blocklist software that will let Firefox 3.5 users override the blocking of .NET FA/WPF plugin if they're patched," Shaver tweeted Sunday. But a few hours later, he added, "We're still working on the blocklist tweaks to help enterprises override the blocking of the WPF plugin, stay tuned!"

Update 6:47 p.m. PDT: Crisis partially averted, apparently. At about 6:10 p.m., Shaver tweeted, "MSFT confirmed that the .NET Framework Assistant is not exploitable, so we've removed it from the blocklist; one down!"

Update 8:34 p.m. PDT: There's still another blocked Microsoft add-on that's vulnerable, one that concerns the Windows Presentation Foundation (WPF), which also is installed with the .Net service pack. Shaver said it was more serious.

"We're hard at work on improving the experience for (especially enterprise) users who wish to override the blocking of the WPF plugin before we remove it from the blocklist," Shaver said in a Sunday night blog post that announced the other plug-in had been removed from the Firefox blocked add-on list.