Vulnerability record · CVE-2021-43890 · published 15 December 2021
CVE-2021-43890: Microsoft AppX Installer spoofing enables malware delivery via crafted packages
Microsoft · App Installer
CVE-2021-43890 is a spoofing vulnerability in the Microsoft Windows AppX installer. Attackers craft malicious packages delivered through phishing attachments or the ms-appinstaller URI scheme, and Microsoft has confirmed real-world abuse tied to Emotet, Trickbot and Bazaloader. It matters because the flaw was exploited in the wild and later prompted Microsoft to disable the ms-appinstaller protocol by default.
Description
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.
CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
Automated analysis
high priorityIt is in CISA KEV with confirmed in-the-wild exploitation and ransomware association, though exploitation requires user interaction and the CVSS score is 7.1.
What it is
CVE-2021-43890 is a spoofing vulnerability in the Microsoft Windows AppX installer. Attackers craft malicious packages delivered through phishing attachments or the ms-appinstaller URI scheme, and Microsoft has confirmed real-world abuse tied to Emotet, Trickbot and Bazaloader. It matters because the flaw was exploited in the wild and later prompted Microsoft to disable the ms-appinstaller protocol by default.
Impact
An attacker who convinces a user to open a crafted package can execute malicious code in the user's context, with greater impact if that user holds administrative rights. Microsoft states users with fewer rights could be less impacted.
Attack surface
Reached over the network through phishing attachments or the ms-appinstaller URI scheme; the CVSS vector (AV:N/AC:H/PR:L/UI:R) indicates low privileges and required user interaction, so the victim must open the crafted content.
Exploitation
Listed in CISA KEV since 2021-12-15 with known ransomware campaign use, and EPSS 30-day probability is about 10.3 percent (95th percentile). Microsoft and press references document active exploitation and abuse of the ms-appinstaller protocol.
What to do
- Apply the Microsoft security update and updated App Installer package per the MSRC advisory.
- Ensure the ms-appinstaller protocol handler is disabled by default, as Microsoft changed in December 2023.
- Block or restrict execution of untrusted AppX/MSIX packages and attachments at email and web gateways.
- Enforce least privilege so users do not operate with administrative rights.
- Train users to treat unexpected package attachments and install prompts as suspicious.
Detection
- Monitor for ms-appinstaller URI scheme invocations and AppX/MSIX package installs from email, browser or temp directories.
- Alert on Office or browser child processes spawning App Installer or package installation activity.
- Hunt for Emotet, Trickbot and Bazaloader indicators and follow-on payload execution after package installs.
- Review proxy and email logs for delivery of package attachments or links that trigger the installer.
This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.
Exploitation in the wild
CISA added CVE-2021-43890 to the Known Exploited Vulnerabilities catalog on 15 December 2021 as "Microsoft Windows AppX Installer Spoofing Vulnerability". CISA reports known use in ransomware campaigns. Required action: Apply updates per vendor instructions. Federal deadline 29 December 2021.
Affected products
1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.
References
Track CVE-2021-43890 inside VULONE
Watch it alongside the ransomware crews, C2 infrastructure and forum chatter that reference it, query it through the API and pull it into your SIEM over TAXII.
Related vulnerabilities
Same products first, then exploited flaws of the same weakness class.
Source: NIST National Vulnerability Database (record CVE-2021-43890), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.
