Actively exploited in the wild
Microsoft Windows AppX Installer Spoofing Vulnerability
Microsoft - Windows · Listed in the CISA KEV since 2021-12-15. This indicates confirmed attacks in production environments.
Required action: Apply updates per vendor instructions.
CVE-2021-43890
HighCVSS 7.1KEVExploitation Probability (EPSS)
Very high risk95th percentile - higher than 95% of all known CVEs
Summary
A spoofing vulnerability in AppX installer for Microsoft Windows. An attacker could craft a malicious attachment for phishing campaigns and convince the user to open it. Users with fewer user rights could be less impacted than those with administrative rights. As of December 27, 2023, Microsoft updated the installer to disable the ms-appinstaller protocol by default.
Risk Assessment
The risk includes the possibility of system infection with malware via phishing, which could lead to data theft or system takeover.
Recommendation
It is recommended to update the AppX installer to the latest version, disable the ms-appinstaller protocol if not needed, and educate users about phishing.
Original NVD description (English source)
We have investigated reports of a spoofing vulnerability in AppX installer that affects Microsoft Windows. Microsoft is aware of attacks that attempt to exploit this vulnerability by using specially crafted packages that include the malware family known as Emotet/Trickbot/Bazaloader. An attacker could craft a malicious attachment to be used in phishing campaigns. The attacker would then have to convince the user to open the specially crafted attachment. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights. Please see the Security Updates table for the link to the updated app. Alternatively you can download and install the Installer using the links provided in the FAQ section. Please see the Mitigations and Workaround sections for important information about steps you can take to protect your system from this vulnerability. December 27 2023 Update: In recent months, Microsoft Threat Intelligence has seen an increase in activity from threat actors leveraging social engineering and phishing techniques to target Windows OS users and utilizing the ms-appinstaller URI scheme. To address this increase in activity, we have updated the App Installer to disable the ms-appinstaller protocol by default and recommend other potential mitigations.