How; creating a theme they can carry out a "Pass-the-Hash" attack.
The ability to install themes from other sources allows them to create malicious themes that, when opened, redirect users to a page asking them to enter their credentials.
Windows allows users to change themes from Settings - Personalization by right-clicking on the currently active theme. Then select "Save theme for sharing". This will create a ".deskthemepack" file that you can share via email or as you wish. The new file can be installed.
Intruders can also create a ".theme" file where the default wallpaper setting leads to a malicious site that requires authentication. There unsuspecting users give their credentials, which are sent with NTLM encryption to the site. The passwords are then read using a special decryption software.
The researcher mentioned a way of protection for these attacks. Extensions such as ".theme", ".themepack" and ".desktopthemepackfile" should be disabled.
Bayne says the findings were reported to the Microsoft Security Response Center (MSRC). However, the error was not corrected because it was "design-specific". It is not known if the company intends to fix the error after this revelation or if it changes the file structure for the topics to prevent redirects to sites that require authentication.
Considering that most users are logged into their accounts with a Microsoft account in Windows 10, stealing credentials puts a lot of user data at risk - such as email, OneDrive or even Azure data.