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June's "Patch Tuesday" (June 12) is here, but it is likely many Windows 10 users have not yet applied these updates. If you have not, just be sure not to leave your laptop lying around! The patches in this cycle fix a code execution vulnerability using the default settings for Windows 10 and the "Cortana" voice assistant. We'll detail how this vulnerability can be used to execute code from the locked screen of a fully patched Windows 10 machine (RS3 at the time of our original submission, and confirmed on RS4 prior to this patch cycle). The vulnerability was submitted to Microsoft as part of the McAfee Labs Advanced Threat Research team's responsible disclosure policy, on April 23. Attribution for this vulnerability submission goes to Cedric Cochin, Cyber Security Architect and Senior Principle Engineer.

In this post, we will address three vectors of research that have been combined by Microsoft and together represent CVE-2018-8140. The first of these is an information leak, but we'll culminate with a demo showing full code execution to log in to a locked Windows device!

Using "Hey Cortana!" to Retrieve Confidential Information
Personal digital assistants such as Siri, Alexa, Google Assistant, and Cortana have become commodities in many technologically inclined houses. From telling jokes, to helping with the grocery list, to turning on the kitchen lights, these robotic voices are beginning to feel oddly more and more personal as they expand their roles in our daily lives. However, we should consider the increased risk of built-in digital personal assistants when looking at new attack vectors for laptops, tablets, and smartphones. Our research on Microsoft's Cortana voice assistant began after reading about the "BadUSB" attacks demonstrated by industry researchers. We decided to take this a step further and ended up finding and reporting to Microsoft several issues related to Cortana.

If you have spoken with Cortana, you may have noticed that "she" is very helpful for a number of simple tasks: providing definitions, or looking up corporations, movies, artists, or athletes. She can even do math! In Windows 10, on the most recent build at the time of submission, we observed that the default settings enable "Hey Cortana" from the lock screen, allowing anyone to interact with the voice-based assistant. This led to some interesting behavior and ultimately vulnerabilities allowing arbitrary code execution.

We begin this analysis with a quick look into Windows indexing. If you have ever opened the advanced view of the Windows Indexing control panel, and navigated to the File Types tab, you will see a long list of file extensions. For each of them you will find details about the associated filter used by the indexing process. Essentially you have the "file properties filter" and several other filters that could all be summarized as "file properties and file content filter."

This means the index process will crack open the files and index their content, including some strings present in these documents. Let's keep that in mind for later as we continue.

Using this knowledge, we wanted to try to access the same menu that you would see when using a Cortana search on an unlocked device.

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