Security Advisory 2718704: Update to Phased Mitigation Strategy

Hello,

At Microsoft, our commitment is to help ensure customer trust in their computing experience. That was the impetus for Trustworthy Computing, and central to that is the priority we place on taking the necessary actions to help protect our customers.

Yesterday, we issued Security Advisory 2718704 outlining the steps we took to help protect our customers from attacks using certain unauthorized digital certificates. This was the first of a series of actions in a phased mitigation strategy, where we invalidated the unauthorized certificates used in the targeted attack, thus addressing that issue and any potential copycat attacks. Because removing these certificates was a simple and straightforward action, we believe that this was the fastest way to protect the largest number of customers.

Since then, we’ve been working closely with customers to discuss the Advisory and the prioritizing of deployment. Some asked how soon they should deploy the update provided last night. Our firm guidance is that customers should apply the update as soon as possible for one simple reason: the fact that malware can be created by attackers and made to look like it is from Microsoft would result in the malware being installed. Removing these certificates is the best first step and the update released yesterday prevents these unauthorized certificates from being used to attack systems running Windows software.

The Flame malware used a cryptographic collision attack in combination with the terminal server licensing service certificates to sign code as if it came from Microsoft. However, code-signing without performing a collision is also possible. This is an avenue for compromise that may be used by additional attackers on customers not originally the focus of the Flame malware. In all cases, Windows Update can only be spoofed with an unauthorized certificate combined with a man-in-the-middle attack.

To increase protection for customers, the next action of our mitigation strategy is to further harden Windows Update as a defense-in-depth precaution. We will begin this update following broad adoption of Security Advisory 2718704 in order not to interfere with that update’s worldwide deployment. We will provide more information on the timing of the additional hardening to Windows Update in the near future.

Our investigation of this issue is ongoing and we will continue to provide further guidance as available, and take any appropriate actions to help protect our customers.

Thanks,
Mike Reavey
Senior Director, MSRC
Microsoft Trustworthy Computing