Executive Summary

The vulnerability in the MC4WP: Mailchimp for WordPress plugin (up to version 4.11.1) is due to missing authorization checks on the `_mc4wp_action` POST parameter. This parameter is trusted without validation, allowing unauthenticated attackers to manipulate the form to perform unsubscribe actions instead of subscribe actions.

Because the form ID is publicly exposed in the HTML source, attackers can obtain it and use the vulnerability to arbitrarily unsubscribe any email address from the connected Mailchimp audience without authentication.

The vulnerability was fixed in version 4.12.0 by removing the ability to unsubscribe users directly through a form. Instead, unsubscription now requires using the Mailchimp hosted form, which includes email verification to improve security.

Useful 0 Not Useful 0

Impact Analysis

This vulnerability allows unauthenticated attackers to unsubscribe any email address from your Mailchimp audience lists without permission.

The impact includes unauthorized removal of subscribers, which can disrupt your email marketing campaigns, reduce your audience reach, and potentially damage your communication efforts.

Since the attacker does not need to be authenticated and only requires the publicly available form ID, the attack can be automated and widespread.

The vulnerability has a CVSS v3.1 base score of 6.5, indicating a medium severity with low attack complexity and no privileges required.

Useful 0 Not Useful 0

Compliance Impact

I don't know

Useful 0 Not Useful 0

Detection Guidance

The vulnerability involves the Mailchimp for WordPress plugin processing the `_mc4wp_action` POST parameter without authorization, allowing unauthenticated unsubscribe actions if the form ID is known.

To detect exploitation attempts on your system or network, you can monitor HTTP POST requests targeting the vulnerable WordPress site that include the `_mc4wp_action` parameter set to "unsubscribe" along with a valid `_mc4wp_form_id`.

Suggested commands to detect such activity include:

• Using web server logs (e.g., Apache or Nginx) to search for POST requests containing `_mc4wp_action=unsubscribe`:
• grep -i '_mc4wp_action=unsubscribe' /var/log/apache2/access.log
• grep -i '_mc4wp_action=unsubscribe' /var/log/nginx/access.log
• Using network traffic capture tools like tcpdump or Wireshark to filter HTTP POST requests containing the parameter:
• tcpdump -A -s 0 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)' | grep '_mc4wp_action=unsubscribe'
• Checking WordPress logs or plugin debug logs for unusual unsubscribe actions or errors related to form submissions.

Note that the form ID is publicly exposed in the HTML source, so scanning for POST requests with the unsubscribe action and known form IDs can help identify exploitation attempts.

Useful 0 Not Useful 0

Mitigation Strategies

The primary mitigation is to update the Mailchimp for WordPress plugin to version 4.12.0 or later, where the unsubscribe functionality via forms has been removed.

This update removes the ability to unsubscribe users directly through forms and requires migrating to Mailchimp hosted forms that include email verification, enhancing security.

Additional immediate steps include:

• Disable or remove any existing unsubscribe forms in your WordPress site that use the vulnerable plugin versions.
• Monitor and block suspicious POST requests containing the `_mc4wp_action=unsubscribe` parameter at the web application firewall or server level.
• Review and restrict access to form IDs if possible, although they are publicly exposed in HTML source.
• Implement or enforce additional verification steps for unsubscribe actions, such as email confirmation, by migrating to Mailchimp hosted forms.

These steps will help prevent unauthorized unsubscribe actions and protect your Mailchimp audience lists.

Useful 0 Not Useful 0