In what might be conceived as one of Microsoft’s new year resolutions, it has disclosed that it's turned off the ms-appinstaller protocol handler by default. The change is designed to make installing apps easier, but it also makes installing malware easier.
Typically, an app needs to be on a device before it can be installed, which normally means that a user has to download it first. To save time and disk space, Microsoft introduced the ability to install applications directly from a web server, without downloading it first. It relies on links that use the ms-appinstaller URI (Uniform Resource Identifier) scheme, which are handled by App Installer rather than a web browser. When software is installed this way users don't see SmartScreen or browser warnings about downloaded executables.
Microsoft reports that it observed malicious activity where criminals tricked users into installing malware using ms-appinstaller links, allowing them to bypass mechanisms like SmartScreen that are designed to keep users safe.
Several cybercriminals were found selling a malware kit as a service that abuses the MSIX file format and ms-appinstaller protocol handler. They distribute signed malicious MSIX application packages using websites accessed through malicious advertisements for legitimate popular software.
The abuse was first noticed in November 2023. Several known groups were using it in different scenarios, all of which lead users to landing pages that mimicked legitimate software vendor sites, where the malware was available via ms-appinstaller links or malicious MSIX installers.
Cybercriminals used four different techniques to spread their malware:
The criminal groups identified as using these methods were all initial access brokers (IABs). IABs are individuals or organizations that specialise in providing ransomware gangs with access to company networks.
Malicious installers can be spotted by looking at the publisher information in the ms-appinstaller prompt.
Image courtesy of Microsoft
To manually disable ms-appinstaller on your network, set the Group Policy EnableMSAppInstallerProtocol
to disabled.
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