How to Fix CVE-2021-40449: Use-after-free in Microsoft Windows

WARNING: This vulnerability is on the CISA Known Exploited Vulnerabilities catalog (added 2021-11-17). Federal civilian agencies must remediate by 2021-12-01. Treat it as active exploitation, not theoretical.

What is CVE-2021-40449?

Win32k Elevation of Privilege Vulnerability CVE-2021-40449 is a use-after-free in Microsoft Windows. An attacker who can reach the vulnerable code path can use it to gain code execution, escalate privilege, or otherwise impact the system depending on the vector documented in the vendor advisory. CISA has confirmed in-the-wild exploitation, so treat the patch as urgent rather than optional.

Am I affected?

Run the version check that matches your platform:

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

Compare what you see against the Affected row above (Microsoft Windows 10 Version 1809 10.0.0 to <10.0.17763.2237; Microsoft Windows Server 2019 10.0.0 to <10.0.17763.2237; Microsoft Windows Server 2019 (Server Core installation) 10.0.0 to <10.0.17763.2237; Microsoft Windows 10 Version 1909 10.0.0 to <10.0.18363.1854 (and 4 more editions, see vendor advisory)). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2021-40449

The primary fix is to upgrade Microsoft Windows to the patched build. Use the commands for your platform below; the patched versions listed in the vendor advisory are: Windows 10 Version 1809 10.0.17763.2237; Windows Server 2019 10.0.17763.2237; Windows Server 2019 (Server Core installation) 10.0.17763.2237; Windows 10 Version 1909 10.0.18363.1854 (and 23 more SKUs, see vendor advisory).

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 Windows 10 Version 1507 to find it).

Complete PowerShell remediation script (Windows)

# Fix script for CVE-2021-40449 affecting Windows 10 Version 1507
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2021-40449-fix-$(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 Windows 10 Version 1507"
    $pkg = winget list --id "Windows_10_Version_1507" 2>$null
    Write-Host $pkg

    Write-Host "[2/4] Backing up configuration"
    $backup = "C:\Backup\Windows_10_Version_1507-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\Windows_10_Version_1507" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Applying upgrade to Windows 10 Version 1809 10.0.17763.2237; Windows Server 2019 10.0.17763.2237;..."
    winget upgrade --id "Windows_10_Version_1507" --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 "Windows_10_Version_1507"
    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-2021-40449 affecting Windows 10 Version 1507
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2021-40449-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 windows 2>/dev/null | grep -i version || echo "windows not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q windows || echo "windows not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2021-40449-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/windows /etc/windows.d /etc/windows.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: Windows 10 Version 1809 10.0.17763.2237; Windows Server 2019 10.0.17763.2237;...)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y windows
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y windows
elif command -v yum >/dev/null; then
    yum update -y windows
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 windows 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q windows
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 Microsoft Windows" `
    -Direction Inbound -Action Block -RemoteAddress Any `
    -Protocol TCP -LocalPort 443
New-NetFirewallRule -DisplayName "Allow Mgmt Microsoft Windows" `
    -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 windows
sudo systemctl disable windows

How to verify the fix worked

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

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

Expected: the reported version is at or above Windows 10 Version 1809 10.0.17763.2237; Windows Server 2019 10.0.17763.2237; Windows Server 2019 (Server Core installation) 10.0.17763.2237; Windows 10 Version 1909 10.0.18363.1854 (and 23 more SKUs, see vendor advisory). 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

Related fixes

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

• How to Fix CVE-2021-31201: Microsoft Enhanced Cryptographic Provider Elevation of Privilege — Microsoft Enhanced Cryptographic Provider Elevation of Privilege
• How to Fix CVE-2021-43890: Security vulnerability in App Installer — Security vulnerability in App Installer
• How to Fix CVE-2021-43226: Privilege Escalation in Windows 10 Version 1809 , Privilege Escalation in Windows 10 Version 1809
• How to Fix CVE-2021-28310: Win32k Elevation of Privilege in Windows 10 Version 1803 , Win32k Elevation of Privilege in Windows 10 Version 1803
• How to Fix CVE-2021-1647: Microsoft Defender Remote Code Execution , Microsoft Defender Remote Code Execution

Is CVE-2021-40449 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?

7.8 (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

• Official vendor advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40449
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-40449
• CISA KEV catalog entry: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• http://packetstormsecurity.com/files/164926/Win32k-NtGdiResetDC-Use-After-Free-Local-Privilege-Escalation.html
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-40449

*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.*