A new vector for pass-the-hash attacks has been discovered targeting Windows 10 personalisation themes.

A security researcher has published details of a potential issue with the design of Windows 10 themes that can be exploited to harvest Windows and Microsoft Account login credentials.

A Windows 10 theme is a collection of customisation settings for Windows colours, desktop pictures and sound effects. These settings can be shared in a ‘.deskthemepack’ file via email or downloaded from a website.  The theme settings file is little more than a text file and is easily customised as it follow the familiar ‘.ini’ file format used by Windows.

By setting the destination of a required resource (such as the desktop wallpaper image) to be a remote location controlled by the attacker such as:

http://www.badactor/images/danger.jpg

Windows will automatically attempt to authenticate an SMB connection and provide the Windows user and hashed password to that endpoint.   The attacker can then harvest the hash to attempt to crack it or attempt a man in the middle attack by forwarding the hash to authenticate against another service in the target network.

If the logged in Windows user account is a Microsoft 365 account rather than a local machine account, the behaviour is the same – providing a means to harvest potentially more valuable Microsoft 365 account details for later cracking or abuse.

According to the researcher, Microsoft stated that the software is behaving as designed and has no plans to make a change to thwart this attack vector.

The problem is not really with the implementation of Desktop Themes in Windows, but rather the way the SMB service tries to be helpful by automatically attempting to connect to remote devices.  Given that SMB links are usually used for connections within the same Active Directory domain, Security Managers could block internet bound traffic at the perimeter firewall on port 445 which is used for SMB connections to reduce the threat of SMB abuse for credential harvesting.