Microsoft says that it has disabled the MSIX ms-appinstaller protocol and is working on a fix after a security bug was discovered that allows an attacker to spoof App Installer.
According to a Microsoft Tech Community blog, an attacker could spoof App Installer to install a package that a user did not intend to install.
The bug, tracked as CVE-2021-43890, is being actively exploited by attackers using specially crafted packages that include the malware family known as Emotet/Trickbot/Bazaloader, Microsoft explains on its security updates page.
Attackers would have to convince users to open the attachment when used in a phishing attack. Users with fewer privileges would be less impacted than administrators.
Microsoft patched the vulnerability in the December Patch Tuesday release, but has now disabled the protocol altogether. That means App Installer will not be able to install an app directly from a web server, and users will need to first download the app to their device and install the package with App Installer.
The company notes that this could increase the download size for some packages.
For now, Microsoft is recommending that organizations using the ms-appinstaller protocol on its website to update the link to the application by removing ‘ms-appinstaller:?source=’ so that the MSIX package or App Installer file will be downloaded to user’s machine.
According to Microsoft, the MSIX app package format essentially makes it easy for users to keep their applications up to date and ensure a smooth installation experience. The ms-appinstaller protocol handler is designed to enable users to easily install an app just by clicking a link on a website, providing a way for users to install an app without needing to download the entire MSIX package.
Microsoft says it is doing its due diligence to make sure that re-enabling the protocol can be done in a secure manner, which may include a Group Policy that would allow IT administrators to re-enable the protocol and control use of it within their organizations.
BleepingComputer reported that Emotet started spreading and infecting Windows 10 and Windows 11 systems in early December using malicious Windows AppX Installer packages that were camouflaged as Adobe PDF software.
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