● Medium · CVSS 6.5

How to Fix CVE-2026-1781: CWE-862 Missing Authorization in MC4WP: Mailchimp for WordPress

Q: Is CVE-2026-1781 being actively exploited?

Not at the time of writing. It is not listed in CISA's Known Exploited Vulnerabilities catalog. That status can change, so monitor the vendor advisory and the KEV catalog if the system is exposed.

Q: How severe is CVE-2026-1781?

CVSS rates it 6.5 (Medium). Use that score to set your patch priority next to the other items in your queue.

Q: Do I have to take MC4WP: Mailchimp for WordPress offline to apply the patch?

It depends on the deployment. High-availability or clustered installs can usually patch one node at a time with no full outage. Standalone installs typically need a short restart. Always follow the vendor's documented upgrade steps.

Q: What if my vulnerability scanner still flags CVE-2026-1781 after I patch?

Re-run the scan after a service restart, then confirm the scanner's plugin set is up to date. Some scanners detect by banner version only and lag the official fix metadata by a release.

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

How to Fix CVE-2026-39486: WordPress Download Monitor plugin <= 5.1.8 - SQL Injection — WordPress Download Monitor plugin <= 5.1.8 - SQL Injection
How to Fix CVE-2026-39520: WordPress weDocs plugin <= 2.1.18 - Broken Access Control in weDocs — WordPress weDocs plugin <= 2.1.18 - Broken Access Control in weDocs
How to Fix CVE-2026-25392: Critical Vulnerability in Update URLs – Quick and Easy way to search old links and replace them with new links in WordPress , Critical Vulnerability in Update URLs – Quick and Easy way to search old links and replace them with new links in WordPress
How to Fix CVE-2026-39495: WordPress Simply Schedule Appointments plugin <= 1.6.9.27 - SQL Injection , WordPress Simply Schedule Appointments plugin <= 1.6.9.27 - SQL Injection
How to Fix CVE-2026-3177: Insufficient Verification of Data Authenticity , Insufficient Verification of Data Authenticity

*By Sai Kiran Pandrala*

⚡ At a glance


Severity	CVSS 6.5, Medium
Actively exploited?	No
Affected	Dvankooten MC4WP: Mailchimp for WordPress (0 <= 4.11.1)
Fixed in	See vendor advisory
Type (CWE)	CWE-862: CWE-862 Missing Authorization

What is CVE-2026-1781?

The MC4WP: Mailchimp for WordPress plugin for WordPress is vulnerable to Missing Authorization in all versions up to, and including, 4.11.1. This is due to the plugin trusting the _mc4wp_action POST parameter without validation, allowing unauthenticated attackers to force the form to process unsubscribe actions instead of subscribe actions. This makes it possible for unauthenticated attackers to arbitrarily unsubscribe any email address from the connected Mailchimp audience via the _mc4wp_action parameter, granted they can obtain the form ID (which is publicly exposed in the HTML source).

In practical terms, a successful attacker gets compromise of the affected component as described in the vendor advisory. There is no confirmed in-the-wild exploitation listed in CISA's KEV catalog at the time of writing, but the CVSS rating still warrants prompt patching.

Am I affected?

You're affected if you run Dvankooten MC4WP: Mailchimp for WordPress at any version in the Affected row above. Use these probes to find your installed build:


wp core version

How to fix CVE-2026-1781

The primary fix is to upgrade to the patched build listed in the Fixed in row above (See vendor advisory). Pick the platform that matches your install and run the commands below.

WordPress core (WP-CLI)


# Vendor advisory: https://www.wordfence.com/threat-intel/vulnerabilities/id/10262aa9-5656-4a2b-aeb5-060018798369?source=cve
wp core check-update
wp core update --version=<patched-version>
wp core update-db

PowerShell script (Windows) - detect, back up, upgrade, verify, log


# Vendor advisory: https://www.wordfence.com/threat-intel/vulnerabilities/id/10262aa9-5656-4a2b-aeb5-060018798369?source=cve
# Run as Administrator
$ErrorActionPreference = 'Stop'
$log = "$env:ProgramData\MC4WPMailchimpforWordPress-Patch-CVE-2026-1781.log"
function Write-Log($msg) { "$(Get-Date -Format s)  $msg" | Tee-Object -FilePath $log -Append }

Write-Log "Starting CVE-2026-1781 remediation for Dvankooten MC4WP: Mailchimp for WordPress"

# 1. Detect: replace the path/version probe with one valid for your install
$installed = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*MC4WP:*' } |
    Select-Object -First 1 -ExpandProperty Version)
Write-Log "Detected version: $installed"

if (-not $installed) {
    Write-Log "Product not installed on this host; nothing to do."
    return
}
if ([version]$installed -ge [version]'<patched-version>') {
    Write-Log "Already at fixed version $installed; no action needed."
    return
}

# 2. Backup configuration to a timestamped folder
$backup = "$env:ProgramData\MC4WPMailchimpforWordPress-Backup-$(Get-Date -Format yyyyMMdd-HHmm)"
New-Item -ItemType Directory -Path $backup -Force | Out-Null
# Adjust the source path to match your install
$src = "$env:ProgramFiles\Dvankooten\MC4WP: Mailchimp for WordPress"
if (Test-Path $src) { Copy-Item -Path $src -Destination $backup -Recurse -Force }
Write-Log "Backed up config to $backup"

# 3. Apply the patched installer (place the verified file on a share or staging path)
$installer = "$env:TEMP\MC4WPMailchimpforWordPress-<patched-version>.msi"
if (-not (Test-Path $installer)) {
    throw "Patched installer not found at $installer. Stage it from your software repo first."
}
Start-Process msiexec.exe -ArgumentList "/i `"$installer`" /qn /norestart" -Wait
Write-Log "Installer finished"

# 4. Verify
$verify = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*MC4WP:*' } |
    Select-Object -First 1 -ExpandProperty Version)
if ([version]$verify -ge [version]'<patched-version>') {
    Write-Log "SUCCESS: now at $verify (>= <patched-version>)"
} else {
    Write-Log "FAILURE: still at $verify after install"
    exit 1
}

Bash script (Linux) - detect, back up, upgrade, verify, log


# Vendor advisory: https://www.wordfence.com/threat-intel/vulnerabilities/id/10262aa9-5656-4a2b-aeb5-060018798369?source=cve
#!/usr/bin/env bash
set -euo pipefail
LOG=/var/log/mc4wp--mailchimp-for-wordpress-patch-cve-2026-1781.log
log()  { echo "$(date -Iseconds)  $*" | tee -a "$LOG"; }

log "Starting CVE-2026-1781 remediation for Dvankooten MC4WP: Mailchimp for WordPress"

# 1. Detect installed version (works for deb and rpm packages)
if command -v dpkg >/dev/null && dpkg -s mc4wp--mailchimp-for-wordpress >/dev/null 2>&1; then
    CURRENT=$(dpkg-query -W -f='${Version}' mc4wp--mailchimp-for-wordpress)
    PKG_MGR=apt
elif command -v rpm >/dev/null && rpm -q mc4wp--mailchimp-for-wordpress >/dev/null 2>&1; then
    CURRENT=$(rpm -q --queryformat '%{VERSION}' mc4wp--mailchimp-for-wordpress)
    PKG_MGR=dnf
else
    log "mc4wp--mailchimp-for-wordpress not installed via apt or rpm; check your package manager or vendor instructions."
    exit 0
fi
log "Detected: mc4wp--mailchimp-for-wordpress=$CURRENT (manager=$PKG_MGR)"

# 2. Backup config
BACKUP=/var/backups/mc4wp--mailchimp-for-wordpress-$(date +%Y%m%d-%H%M)
mkdir -p "$BACKUP"
for d in /etc/mc4wp--mailchimp-for-wordpress /etc/${pkg%%-*} ; do
    [ -d "$d" ] && cp -a "$d" "$BACKUP/" && log "Backed up $d to $BACKUP"
done

# 3. Upgrade
if [ "$PKG_MGR" = apt ]; then
    sudo apt-get update -y
    sudo apt-get install --only-upgrade -y mc4wp--mailchimp-for-wordpress
else
    sudo dnf upgrade --security -y mc4wp--mailchimp-for-wordpress
fi

# 4. Verify
if [ "$PKG_MGR" = apt ]; then
    NEW=$(dpkg-query -W -f='${Version}' mc4wp--mailchimp-for-wordpress)
else
    NEW=$(rpm -q --queryformat '%{VERSION}' mc4wp--mailchimp-for-wordpress)
fi
log "After upgrade: $NEW"

# Optionally compare against <patched-version> with dpkg --compare-versions or sort -V
log "Done. Restart the affected service if the package install did not."

If you can't patch immediately

These are runnable hardening commands. They reduce blast radius but they're not a replacement for the vendor patch.

No official vendor workaround is published for this CVE; patching is the only documented fix. The runnable hardening below is generic defense in depth, not a substitute for the patch.

Restrict the affected service to trusted networks (Linux):


# Vendor advisory: https://www.wordfence.com/threat-intel/vulnerabilities/id/10262aa9-5656-4a2b-aeb5-060018798369?source=cve
# Replace <port> with the affected service port and 10.0.0.0/24 with your admin subnet
sudo iptables -A INPUT -p tcp --dport <port> -s 10.0.0.0/24 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport <port> -j DROP

Windows Firewall equivalent:


# Vendor advisory: https://www.wordfence.com/threat-intel/vulnerabilities/id/10262aa9-5656-4a2b-aeb5-060018798369?source=cve
New-NetFirewallRule -DisplayName 'Allow affected service from admin subnet' \
    -Direction Inbound -Action Allow -Protocol TCP -LocalPort <port> -RemoteAddress 10.0.0.0/24
New-NetFirewallRule -DisplayName 'Block affected service from everywhere else' \
    -Direction Inbound -Action Block -Protocol TCP -LocalPort <port>

How to verify the fix worked

Run the version probe again and confirm the running build matches the Fixed in row above.


wp core version

Expected output: the patched build (or higher).

Then re-run any vulnerability scanner you used previously and confirm the finding for CVE-2026-1781 has cleared. Sweep your logs for the indicators of compromise listed in the vendor or CISA advisory, especially if the system was internet-reachable during the disclosure window.

Frequently asked questions

Is CVE-2026-1781 being actively exploited?