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[This post was co-written by J. Alex Halderman and Ed Felten.]

Over the weekend a Finnish researcher named Muzzy noticed a potential vulnerability in the web-based uninstaller that Sony offers to users who want to remove the First4Internet XCP copy protection software. We took a detailed look at the software and discovered that it is indeed possible for an attacker to exploit this weakness. For affected users, this represents a far greater security risk than even the original Sony rootkit.

The consequences of the flaw are severe. It allows any web page you visit to download, install, and run any code it likes on your computer. Any web page can seize control of your computer; then it can do anything it likes. That’s about as serious as a security flaw can get.

The root of the problem is a serious design flaw in Sony’s web-based uninstaller. When you first fill out Sony’s form to request a copy of the uninstaller, the request form downloads and installs a program – an ActiveX control created by the DRM vendor, First4Internet – called CodeSupport. CodeSupport remains on your system after you leave Sony’s site, and it is marked as safe for scripting, so any web page can ask CodeSupport to do things. One thing CodeSupport can be told to do is download and install code from an Internet site. Unfortunately, CodeSupport doesn’t verify that the downloaded code actually came from Sony or First4Internet. This means any web page can make CodeSupport download and install code from any URL without asking the user’s permission.

A malicious web site author can write an evil program, package up that program appropriately, put the packaged code at some URL, and then write a web page that causes CodeSupport to download and run code from that URL. If you visit that web page with Internet Explorer, and you have previously requested Sony’s uninstaller, then the evil program will be downloaded, installed, and run on your computer, immediately and automatically. Your goose will be cooked.

We have constructed a demonstration code package and web page that exploits this design flaw to install unwanted files on a target computer. The exploit does not actually harm the computer, but it demonstrates that hostile code can be run on a target computer, and that the hostile code can perform operations that should be forbidden. At present we are not releasing the demonstration exploit to the public.

CodeSupport was also installed as part of the original web-based updater that Sony released to remove First4Internet’s rootkit. Sony has since replaced the web-based version of the updater with a downloadable EXE or ZIP file; these are safe to use as far as we know. If you didn’t use the original web-based updater, and you haven’t requested the full uninstaller from Sony, then you are safe from this particular vulnerability, as far as we know.

How can you protect yourself against this vulnerability? First, for now don’t accept the installation of any software delivered over the net from First4Internet. (Eventually First4Internet may deliver a fix over the net. That may be worth installing.) That will keep CodeSupport off you machine, if it’s not already there.

To see whether CodeSupport is on your computer, try our CodeSupport detector page.

If you’re vulnerable, you can protect yourself by deleting the CodeSupport component from your machine. From the Start menu, choose Run. In the box that pops up, type (on a single line)

cmd /k del “%windir%downloaded program filescodesupport.*”

This is not an ideal solution – depending on your security settings, it may not prevent the software from installing again – but it’s better than nothing. We’ll have to wait for First4Internet to develop a complete patch.

UPDATE: USA Today reports that Sony will recall the affected CDs. Discs in the supply chain will not be sold, and customers who have already bought discs will be able to exchange them. Sony will announce details of the recall plan later in the week. We hope the plan will include distribution of cleanup tools to customers who still have potentially dangerous XCP software on their machines.