Microsoft announced on Friday that the ms-appinstaller protocol for MSIX has been disabled temporarily due to the fact that it has been abused by malware.
Microsoft in December informed users about CVE-2021-43890, which it described as a Windows AppX installer spoofing vulnerability. The tech giant warned at the time that cybercriminals had been exploiting the vulnerability using specially crafted packages to deliver Emotet, TrickBot and Bazaloader (BazarLoader) malware.
Microsoft announced patches and workarounds in December, but it’s still working to address the vulnerability and, in the meantime, it has decided to disable the protocol.
While Microsoft’s announcement suggests that the protocol has been disabled only now, developers complained about it being disabled in December, shortly after CVE-2021-43890 was disclosed.
MSIX is an app package format designed to make it easy for users to install apps and keep them updated. The ms-appinstaller protocol handler enables users to install an application simply by clicking a link on a website, without the need to download the full MSIX package.
Cybercriminals have been abusing this method to deliver their malware to users by tricking them into installing apparently-legitimate applications.
With the protocol handler disabled, the App Installer component in Windows will no longer be able to install an application directly from a web server. Users will have to download the app they want and then install the package.
“We recognize that this feature is critical for many enterprise organizations,” said Microsoft’s Dian Hartono. “We are taking the time to conduct thorough testing to ensure that re-enabling the protocol can be done in a secure manner. We are looking into introducing a Group Policy that would allow IT administrators to re-enable the protocol and control usage of it within their organizations.”
Those who have used the protocol will need to update the application link on their website.
Related: Threat Actors Abuse MSBuild for Cobalt Strike Beacon Execution
Related: Threat Actor Abuses Microsoft’s WHCP to Sign Malicious Drivers
Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.
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