CVE-2026-1781: Mailchimp for WordPress Auth Bypass Flaw

CVE-2026-1781 Overview

The MC4WP: Mailchimp for WordPress plugin is vulnerable to Missing Authorization (CWE-862) in all versions up to and including 4.11.1. This vulnerability allows unauthenticated attackers to manipulate the _mc4wp_action POST parameter without proper validation, enabling them to force subscription forms to process unsubscribe actions instead of the intended subscribe actions. Attackers can arbitrarily unsubscribe any email address from connected Mailchimp audiences by exploiting this flaw, provided they can obtain the form ID, which is publicly exposed in the HTML source code of WordPress pages.

Critical Impact

Unauthenticated attackers can arbitrarily unsubscribe email addresses from Mailchimp audiences, potentially disrupting email marketing campaigns and subscriber lists without any authentication requirements.

Affected Products

• MC4WP: Mailchimp for WordPress plugin versions up to and including 4.11.1
• WordPress installations using vulnerable MC4WP plugin versions
• Mailchimp audiences connected to vulnerable MC4WP plugin installations

Discovery Timeline

• 2026-03-11 - CVE-2026-1781 published to NVD
• 2026-03-11 - Last updated in NVD database

Technical Details for CVE-2026-1781

Vulnerability Analysis

This vulnerability stems from a Missing Authorization flaw classified under CWE-862. The MC4WP plugin fails to implement proper authorization checks when processing the _mc4wp_action POST parameter. The plugin blindly trusts user-supplied input without validating whether the request originates from an authorized source or whether the action being performed is legitimate for the requesting user.

The vulnerability is particularly impactful because form IDs are publicly exposed in the HTML source code of WordPress pages where the Mailchimp subscription forms are embedded. This allows attackers to easily identify valid form IDs and craft malicious requests targeting specific email addresses for unsubscription.

Root Cause

The root cause lies in the class-form-listener.php component, which processes form actions without implementing proper authorization controls. The plugin accepts the _mc4wp_action parameter directly from POST data and executes unsubscribe operations without verifying that the requester has the authority to perform such actions. The process_unsubscribe_form() method iterates through configured Mailchimp lists and unsubscribes the provided email address without any authentication or authorization validation.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Identifying a target WordPress site using the MC4WP plugin
2. Extracting the form ID from the publicly visible HTML source code
3. Crafting a malicious POST request with the _mc4wp_action parameter set to unsubscribe
4. Including the target email address in the request payload
5. Submitting the request to force an unsubscription action

The following code shows the security patch that removed the vulnerable unsubscribe functionality:

     acceptsMultipleValues: true
   }, true)
 
-  choices = {
-    subscribe: 'Subscribe',
-    unsubscribe: 'Unsubscribe'
-  }
-  register(i18n.formFields, {
-    name: '_mc4wp_action',
-    type: 'radio',
-    title: i18n.formAction,
-    choices,
-    value: 'subscribe',
-    help: i18n.formActionDescription
-  }, true)
-
   register(i18n.formFields, {
     name: 'AGREE_TO_TERMS',
     value: 1,

Source: GitHub Security Commit

The server-side unsubscribe handler was also removed:

      */
     public function process_unsubscribe_form(MC4WP_Form $form)
     {
-        $mailchimp = new MC4WP_MailChimp();
-        $log       = $this->get_log();
-        $result    = null;
-        $data      = $form->get_data();
-
-        // unsubscribe from each lists
-        foreach ($form->get_lists() as $list_id) {
-            $result = $mailchimp->list_unsubscribe($list_id, $data['EMAIL']);
-        }
-
-        if (! $result) {
-            $form->add_notice($form->messages['error'], 'error');
-            $log->error(sprintf('Form %d > Mailchimp API error: %s', $form->ID, $mailchimp->get_error_message()));
-
-            // bail
-            return;
-        }
-
-        // Success! Unsubscribed.
-        $form->last_event = 'unsubscribed';
-        $form->add_notice($form->messages['unsubscribed'], 'notice');
-        $log->info(sprintf('Form %d > Successfully unsubscribed %s', $form->ID, $data['EMAIL']));
-
-        /**
-         * Fires right after a form was used to unsubscribe.
-         *
-         * @since 3.0

Source: GitHub Security Commit

Detection Methods for CVE-2026-1781

Indicators of Compromise

• Unexpected POST requests to WordPress pages containing _mc4wp_action=unsubscribe parameter
• Unusual patterns of unsubscription activity in Mailchimp audience logs
• Web server logs showing POST requests targeting MC4WP form endpoints with suspicious _mc4wp_action values
• Mailchimp API logs indicating bulk unsubscription operations from WordPress-connected integrations

Detection Strategies

• Monitor web server access logs for POST requests containing the _mc4wp_action parameter with unsubscribe value from unexpected sources
• Implement rate limiting detection for repeated form submissions targeting the same or multiple email addresses
• Review Mailchimp audience activity reports for anomalous unsubscription patterns that deviate from normal user behavior
• Deploy web application firewall (WAF) rules to flag or block requests attempting to manipulate the _mc4wp_action parameter

Monitoring Recommendations

• Enable detailed logging on WordPress sites using the MC4WP plugin to capture all form submission activity
• Configure Mailchimp webhook notifications to alert on unsubscription events for real-time monitoring
• Implement network traffic analysis to detect patterns consistent with automated exploitation attempts
• Review plugin version information regularly to ensure vulnerable versions are identified and tracked

How to Mitigate CVE-2026-1781

Immediate Actions Required

• Update the MC4WP: Mailchimp for WordPress plugin to the latest patched version immediately
• Review Mailchimp audience lists for any unauthorized unsubscriptions that may have occurred
• Audit web server logs for evidence of exploitation attempts against the vulnerable endpoint
• Re-subscribe any legitimate users who may have been maliciously unsubscribed

Patch Information

A security patch is available via the official plugin changeset. The fix completely removes the ability to unsubscribe through MC4WP forms, eliminating the attack vector. The patch removes both the frontend unsubscribe option from the form editor and the backend process_unsubscribe_form() method. For detailed technical information, refer to the Wordfence Vulnerability Report.

Workarounds

• Temporarily disable MC4WP subscription forms until the plugin can be updated
• Implement WAF rules to block POST requests containing _mc4wp_action=unsubscribe
• Use server-side input validation to reject unexpected values in the _mc4wp_action parameter
• Consider using Mailchimp's native embedded forms as an alternative until the WordPress plugin is patched

# Example .htaccess rule to block exploitation attempts
<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteCond %{REQUEST_METHOD} POST
    RewriteCond %{QUERY_STRING} _mc4wp_action=unsubscribe [NC,OR]
    RewriteCond %{HTTP:Content-Type} application/x-www-form-urlencoded
    RewriteRule .* - [F,L]
</IfModule>