A recent update fixes a 0day vulnerability in the Windows AppX installer (CVE-2021-43890), who used the Emotet.
An add-on to the patch Tuesday, December 2021 for the AppX installer used on Windows, Microsoft has closed the Windows AppX Installer forgery vulnerability.
The gang behind Emotet took advantage of this vulnerability to infect systems. The following is a brief overview.
The AppX-Installer used in Windows 10 and Windows 11 to install apps, contains a serious design flaw.
An infected payload from Emotet appears as a trusted application in the installer dialog. The Trusted App statement appears in users without any digital signature being evaluated. Only when the user clicks on the Trusted App link does he get the publisher authentication notification you see in the tweet below from the actual app publisher.
The Emotet team is taking advantage of this security loophole to distribute its dropper that will install ransomware as a trusted Windows application.
As early as December 14, 2021, security researcher Will Dormann tweeted that ms-appinstaller's URI had been blocked by Microsoft with an update.
Note that as of today updates (CVE-2021-43890), the ms-appinstaller: URI has been disabled. pic.twitter.com/tk9KpmHCy8
- Will Dormann (@wdormann) December 14, 2021
Dormann posted the issue in a series of tweets from December 1, 2021. If Windows users were logged in as administrators, it could be abused because ms-appinstaller: is a URL protocol through which install AppX from almost anywhere. Even opening a Microsoft Office document was enough.
Ο Dormann συνέστησε τότε την ενεργοποίηση της πολιτικής ομάδας "Αποτροπή από χρήστες που δεν είναι διαχειριστές από την εγκατάσταση συσκευασμένων εφαρμογών Windows" ή την απενεργοποίηση του ms-appinstaller μέσω του αρχείου reg για όλους τους χρήστες.
Do your users log in as administrators? No? Good!
Set the "Prevent non-admin users from installing packaged Windows apps" group policy to avoid foot-shooting.
They do log in as admins? Interesting choice...
Maybe unregister ms-appinstaller for EACH user.https://t.co/kttJZfRgEX pic.twitter.com/QiYg8XhlPM- Will Dormann (@wdormann) December 2, 2021
But as of December 14, 2021, Microsoft published the security page Windows security AppX Installer spoofing Vulnerability CVE-2021-43890, which confirms all of the above and discloses the CVE.
Microsoft is aware of attacks that attempt to exploit this vulnerability with specially designed packages containing the Emotet / Trickbot / Bazaloader malware.
In addition, Microsoft has updated the AppX Installer (Desktop Installer):
- Microsoft Desktop Installer 1.16 for Windows 10 version 1809 or later
- Microsoft Desktop Installer 1.11 for Windows 10 version 1709 or Windows 10 version 1803
