CVE-2026-1781 in MC4WP: Mailchimp for WordPress Plugin
Summary
by MITRE • 03/11/2026
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).
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Analysis
by VulDB Data Team • 03/16/2026
The vulnerability identified as CVE-2026-1781 affects the MC4WP: Mailchimp for WordPress plugin, specifically targeting versions up to and including 4.11.1. This represents a critical authorization flaw that fundamentally undermines the security of user subscription management within WordPress environments. The issue stems from the plugin's failure to properly validate incoming POST parameters, creating a pathway for malicious actors to manipulate form processing behavior without proper authentication credentials. The vulnerability manifests through the `_mc4wp_action` parameter which the plugin blindly trusts during form processing operations, allowing attackers to redirect form functionality from legitimate subscription to unauthorized unsubscribe actions.
The technical implementation of this vulnerability resides in the plugin's insufficient input validation mechanisms and lack of proper access control checks. When the plugin processes form submissions, it directly interprets the `_mc4wp_action` parameter without verifying whether the request originates from an authenticated user or contains valid authorization context. This design flaw creates a direct attack vector where unauthenticated adversaries can manipulate the form's intended behavior by simply altering the action parameter from its expected subscription value to an unsubscribe directive. The vulnerability is particularly concerning because the form ID required to execute the attack is publicly exposed in the HTML source code, making it easily discoverable through routine web scraping or manual inspection of web pages. This public exposure of form identifiers eliminates the need for complex reconnaissance phases and allows attackers to immediately target vulnerable installations.
The operational impact of this vulnerability extends beyond simple subscription management disruption and creates significant risks for Mailchimp audience management and user privacy. Attackers can systematically unsubscribe any email address from connected Mailchimp audiences, potentially causing mass disruption to marketing campaigns and user communication channels. The vulnerability enables large-scale audience manipulation that could result in loss of subscribers, compromised marketing analytics, and potential spam complaints that may affect sender reputation. Furthermore, the attack can be executed repeatedly and at scale, making it particularly dangerous for high-traffic websites where the volume of potential targets increases exponentially. The impact is amplified because the vulnerability affects the core functionality of email marketing automation within WordPress, potentially disrupting business operations that rely on consistent subscriber lists and campaign delivery.
Security practitioners should implement immediate mitigations including upgrading to patched versions of the MC4WP plugin, which would typically address the authorization bypass through proper parameter validation and access control checks. The vulnerability aligns with CWE-863, which describes "Incorrect Authorization" in software systems where access control decisions are made incorrectly or without proper validation. Organizations should also consider implementing additional defensive measures such as input sanitization at the web application firewall level and monitoring for unusual patterns in form submission parameters. The ATT&CK framework categorizes this vulnerability under T1078, which covers Valid Accounts and T1566, which covers Phishing, as attackers can exploit the vulnerability to manipulate user subscriptions and potentially harvest email addresses for further malicious activities. Network segmentation and application-level monitoring can help detect anomalous form processing patterns that may indicate exploitation attempts, while regular security audits should verify that all WordPress plugins maintain proper authorization mechanisms to prevent similar vulnerabilities from emerging in the future.