Yesterday, Microsoft released Windows 10 Insider Preview build 18305 to the Fast ring. It contains a ton of new features, but at the same time, the firm also released cumulative updates for all supported versions of Windows 10. These cumulative updates included a security fix for Internet Explorer, and now, Microsoft is releasing build 18305.1003 to the Fast ring with that exact same fix.
The security bulletin lists it as a remote code execution vulnerability, and it's in the scripting engine that handles objects in memory. If a script is used that corrupts the memory in a certain way, someone could execute foreign code on that machine. It can be used to install malware and more.
Obviously, it's recommended that you install this update right away, even if it doesn't affect many people. After all, the only reason that Internet Explorer is still bundled with Windows 10 is for businesses that need the legacy support. Still, if you're using the latest Windows 10 Insider Preview and you use Internet Explorer, you're going to want to check Windows Update.