Security researcher Jimmy Bayne with a Publication on Twitter announced a gap in their theme settings Windows 10 which can allow malicious users to steal your credentials.
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 archive “.deskthemepack” that you can share via email or however you like. 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 reports that these findings were communicated to Microsoft Security Response Center (MSRC). Ωστόσο, το σφάλμα δεν διορθώθηκε επειδή ήταν “feature by design". It is not known if the company plans to fix the bug after this disclosure or if it changes the file structure for the themes 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.
