How to Fix CVE-2021-42321: Security vulnerability in Microsoft Exchange Server

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-2021-42321?

Microsoft Exchange Server Remote Code Execution Vulnerability CVE-2021-42321 is a security vulnerability in Microsoft Exchange Server. 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 "microsoft"
Get-WmiObject Win32_Product | Where-Object { $_.Name -like "*Microsoft Exchange Server*" } | Select-Object Name, Version

Compare what you see against the Affected row above (Microsoft 15.01.0 to <15.01.2308.020; Microsoft 15.02.0 to <15.02.0792.019; Microsoft 15.0.0 to <15.01.2375.017; Microsoft 15.02.0 to <15.02.0986.014). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2021-42321

The primary fix is to upgrade Microsoft Exchange Server to the patched build. Use the commands for your platform below; the patched versions listed in the vendor advisory are: Microsoft Exchange Server 2016 Cumulative Update 21 15.01.2308.020; Microsoft Exchange Server 2019 Cumulative Update 10 15.02.0792.019; Microsoft Exchange Server 2016 Cumulative Update 22 15.01.2375.017; Microsoft Exchange Server 2019 Cumulative Update 11 15.02.0986.014.

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 Microsoft Exchange Server 2016 Cumulative Update 21 to find it).

Complete PowerShell remediation script (Windows)

# Fix script for CVE-2021-42321 affecting Microsoft Exchange Server 2016 Cumulative Update 21
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2021-42321-fix-$(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 Microsoft Exchange Server 2016 Cumulative Update 21"
    $pkg = winget list --id "Microsoft_Exchange_Serve" 2>$null
    Write-Host $pkg

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

    Write-Host "[3/4] Applying upgrade to Microsoft Exchange Server 2016 Cumulative Update 21 15.01.2308.020; Microsoft..."
    winget upgrade --id "Microsoft_Exchange_Serve" --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 "Microsoft_Exchange_Serve"
    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-2021-42321 affecting Microsoft Exchange Server 2016 Cumulative Update 21
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2021-42321-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 microsoft 2>/dev/null | grep -i version || echo "microsoft not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q microsoft || echo "microsoft not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2021-42321-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/microsoft /etc/microsoft.d /etc/microsoft.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: Microsoft Exchange Server 2016 Cumulative Update 21 15.01.2308.020; Microsoft...)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y microsoft
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y microsoft
elif command -v yum >/dev/null; then
    yum update -y microsoft
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 microsoft 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q microsoft
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 Exchange Server" `
    -Direction Inbound -Action Block -RemoteAddress Any `
    -Protocol TCP -LocalPort 443
New-NetFirewallRule -DisplayName "Allow Mgmt Microsoft Exchange Server" `
    -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 microsoft
sudo systemctl disable microsoft

How to verify the fix worked

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

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

Expected: the reported version is at or above Microsoft Exchange Server 2016 Cumulative Update 21 15.01.2308.020; Microsoft Exchange Server 2019 Cumulative Update 10 15.02.0792.019; Microsoft Exchange Server 2016 Cumulative Update 22 15.01.2375.017; Microsoft Exchange Server 2019 Cumulative Update 11 15.02.0986.014. 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-2021-36955: Windows Common Log File System Driver Elevation of Privilege — Windows Common Log File System Driver Elevation of Privilege
• How to Fix CVE-2021-1675: Windows Print Spooler Remote Code Execution in Windows 10 Version 1809 , Windows Print Spooler Remote Code Execution in Windows 10 Version 1809
• How to Fix CVE-2021-27065: Microsoft Exchange Server Remote Code Execution , Microsoft Exchange Server Remote Code Execution
• How to Fix CVE-2021-36942: Windows LSA Spoofing in Windows Server 2019 , Windows LSA Spoofing in Windows Server 2019
• How to Fix CVE-2021-42278: Elevation of Privilege in Microsoft Active Directory , Elevation of Privilege in Microsoft Active Directory

Is CVE-2021-42321 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?

8.8 (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-2021-42321
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-42321
• CISA KEV catalog entry: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• http://packetstormsecurity.com/files/166153/Microsoft-Exchange-Server-Remote-Code-Execution.html
• http://packetstormsecurity.com/files/168131/Microsoft-Exchange-Server-ChainedSerializationBinder-Remote-Code-Execution.html
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-42321

*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.*