Microsoft Windows AppX Installer Spoofing and Remote Code Execution Vulnerability (CVE-2021-43890) Analysis & Mitigation Guide
Executive & SecOps Summary
SECOPS ASSESSMENTSystems running Microsoft Windows with App Installer (AppX/MSIX) and the ms-appinstaller URI scheme enabled
Successful exploitation allows threat actors to deliver malware families such as Emotet, Trickbot, and Bazaloader via phishing, leading to complete system compromise.
Apply the latest official Windows App Installer updates and disable the ms-appinstaller protocol handler via Group Policy.
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Update the Windows App Installer to the latest secure version using the Windows Package Manager (Winget). Run the following command in an elevated PowerShell session:
# Upgrade Windows App Installer via Winget
winget upgrade --id Microsoft.AppInstaller2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, disable the ms-appinstaller protocol handler via the registry to block the attack vector.
# Disable the ms-appinstaller protocol handler via Registry
$registryPath = 'HKLM:\SOFTWARE\Policies\Microsoft\Windows\AppInstaller'
if (-not (Test-Path $registryPath)) {
New-Item -Path $registryPath -Force | Out-Null
}
New-ItemProperty -Path $registryPath -Name 'EnableAppInstallerProtocol' -Value 0 -PropertyType DWord -Force3. Log Audit & IoC Verification Commands
Use the following commands to verify the installed version of App Installer and check if the mitigation policy is correctly applied.
# Check the installed App Installer package version
Get-AppxPackage -AllUsers -Name 'Microsoft.DesktopAppInstaller' | Select-Object Name, Version
# Verify if the protocol handler is disabled (Value should be 0)
Get-ItemProperty -Path 'HKLM:\SOFTWARE\Policies\Microsoft\Windows\AppInstaller' -Name 'EnableAppInstallerProtocol' -ErrorAction SilentlyContinueBlock Kernel Privilege Escalation & Ransomware Payloads Before You Can Reboot
Protect unpatched Windows, Linux, and macOS workloads against in-memory exploitation and lateral movement with autonomous EDR.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2021-43890) is a spoofing flaw in the Windows AppX/MSIX installer. The root cause lies in the handling of the ms-appinstaller:// URI scheme, which allows web browsers to directly invoke the local App Installer. An attacker can spoof the user interface of the installer to misrepresent the signature status or the source of the package, tricking users into believing they are installing a legitimate application.
2. Exploitation Mechanics & Attack Scenario
An attacker hosts a specially crafted malicious AppX/MSIX package on an external server and crafts a phishing campaign containing a link with the ms-appinstaller protocol. When a victim clicks the link, the local App Installer is triggered, displaying a spoofed installation prompt that mimics trusted software. Once the user clicks 'Install', the installer fetches and executes the malicious payload (such as Emotet or Trickbot) in the background.
3. Infrastructure & Business Impact
This vulnerability has been actively exploited in the wild by financially motivated threat actors (listed in CISA KEV). If an administrative user falls victim to this attack, the threat actor can gain elevated privileges, move laterally across the corporate network, deploy ransomware, and exfiltrate sensitive business data.
Official Vendor Advisories & References
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.
Frequently Asked SecOps Questions (CVE-2021-43890)
FAQQ1. How critical is CVE-2021-43890 and is it actively exploited in the wild?
CVE-2021-43890 is rated CVSS 7.1 (HIGH) with a 30-day EPSS exploitation probability of 11.3%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-43890?
Primary impacted vendors include microsoft across windows, app_installer. Scope summary: All Windows 10, 11, and Windows Server systems utilizing the Windows App Installer with the ms-appinstaller protocol enabled
Q3. What temporary mitigations or workarounds exist for CVE-2021-43890 before patching?
[Mitigation] Apply the latest official Windows App Installer updates and disable the ms-appinstaller protocol handler via Group Policy. Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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