⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● High · CVSS 7.1 ⚠ ACTIVELY EXPLOITED — CISA KEV

How to Fix CVE-2021-43890: Security vulnerability in App Installer

⚡ At a glance
Severity7.1 (High)
Actively exploited?Yes, listed in CISA KEV (added 2021-12-15)
AffectedMicrosoft App Installer 1.0.0.0 to <publication
Fixed inApp Installer publication
Type (CWE)Not verified — see official advisory
WARNING: This vulnerability is on the CISA Known Exploited Vulnerabilities catalog (added 2021-12-15). Federal civilian agencies must remediate by 2021-12-29. Treat it as active exploitation, not theoretical.

What is CVE-2021-43890?

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.

Am I affected?

Run the version check that matches your platform:


# Windows
winget list | findstr /I "app"
Get-WmiObject Win32_Product | Where-Object { $_.Name -like "*App Installer*" } | Select-Object Name, Version

Compare what you see against the Affected row above (Microsoft App Installer 1.0.0.0 to <publication). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2021-43890

The primary fix is to upgrade App Installer to the patched build. Use the commands for your platform below; the patched version listed in the vendor advisory is: App Installer publication.

Windows (PowerShell, run as administrator)


# Check installed version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

# Apply latest Microsoft security updates (Windows Update)
Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck
Import-Module PSWindowsUpdate
Get-WindowsUpdate -MicrosoftUpdate
Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -AutoReboot

# Or via winget for application-level patches
winget upgrade --all --accept-source-agreements --accept-package-agreements

If the affected component is a third-party application, identify the package and run:


winget upgrade --id <vendor.product>

Replace <vendor.product> with the actual winget identifier (run winget search App Installer to find it).

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2021-43890 affecting App Installer
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2021-43890-fix-$(Get-Date -Format yyyyMMdd-HHmmss).log"
New-Item -ItemType Directory -Force (Split-Path $LogPath) | Out-Null
Start-Transcript -Path $LogPath -Append

try {
    Write-Host "[1/4] Detecting installed version of App Installer"
    $pkg = winget list --id "App_Installer" 2>$null
    Write-Host $pkg

    Write-Host "[2/4] Backing up configuration"
    $backup = "C:\Backup\App_Installer-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\App_Installer" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Applying upgrade to App Installer publication"
    winget upgrade --id "App_Installer" --silent --accept-source-agreements --accept-package-agreements
    # Fallback: Windows Update for OS-level fixes
    if ($LASTEXITCODE -ne 0) {
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verifying patched build"
    winget list --id "App_Installer"
    Write-Host "Fix applied. Reboot if prompted."
    exit 0
} catch {
    Write-Error "Patch failed: $_"
    exit 1
} finally {
    Stop-Transcript
}

Complete Bash remediation script (Linux)


#!/usr/bin/env bash
# Fix script for CVE-2021-43890 affecting App Installer
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2021-43890-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detecting installed version"
if command -v dpkg >/dev/null; then
    dpkg -s app 2>/dev/null | grep -i version || echo "app not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q app || echo "app not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2021-43890-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/app /etc/app.d /etc/app.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: App Installer publication)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y app
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y app
elif command -v yum >/dev/null; then
    yum update -y app
elif command -v zypper >/dev/null; then
    zypper --non-interactive patch --category security
fi

echo "[4/4] Verifying patched build"
if command -v dpkg >/dev/null; then
    dpkg -s app 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q app
fi
echo "Done. Restart any running daemons that loaded the old library."

If you can't patch immediately

If you cannot apply the patched version today, restrict exposure with one of the following runnable controls. None replace the patch.

Windows firewall isolation


# Allow only management subnet to reach the vulnerable service
New-NetFirewallRule -DisplayName "Restrict App Installer" `
    -Direction Inbound -Action Block -RemoteAddress Any `
    -Protocol TCP -LocalPort 443
New-NetFirewallRule -DisplayName "Allow Mgmt App Installer" `
    -Direction Inbound -Action Allow -RemoteAddress 10.0.0.0/8 `
    -Protocol TCP -LocalPort 443

Service-level fallback


# If the affected feature is optional, stop the service until the patch is applied
sudo systemctl stop app
sudo systemctl disable app

How to verify the fix worked


# Linux
app --version 2>/dev/null || dpkg -s app | grep -i version
rpm -q app 2>/dev/null || true


# Windows
winget list | findstr /I "app"
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above App Installer publication. Restart any services that loaded the old library (systemctl restart <service> on Linux, restart the Windows service or reboot when prompted). For network appliances, run show version on the device and confirm the build matches the patched release.

Frequently asked questions

Other vulnerabilities in the same area that are worth patching alongside this one:

Is CVE-2021-43890 actually being exploited?

According to the data sources above, yes, CISA has it listed as actively exploited. Either way, the fix is the same: apply the vendor patch.

Do I need to reboot after patching?

For OS or kernel updates, yes. For most userland packages a systemctl restart <service> is enough. Any process that loaded the old shared library keeps using it until restarted, so when in doubt, reboot.

What is the CVSS score?

7.1 (high). Refer to the vendor advisory for the exact vector string.

Where is the official advisory?

See the References section at the bottom of this page; the vendor's URL is the authoritative source for affected builds and patched versions.

References

*Written by Sai Kiran Pandrala. Assembled from the official vendor advisory, NVD record, and CISA KEV listing on 2026-05-25. Always confirm against the vendor's advisory before applying changes in production.*