CVE-2026-109515 in MailOptin Plugin
Summary
by MITRE • 10/16/2026
The MailOptin – Popup, Optin Forms & Email Newsletters for Mailchimp, HubSpot, AWeber Etc. plugin for WordPress is vulnerable to Privilege Escalation in all versions up to, and including, 1.2.78.6 This is due to the `subscribe_to_email_list` endpoint accepting an attacker-supplied `optin_data[form_custom_field_mappings]` parameter without any allow-list validation on destination keys, allowing the attacker-controlled mapping to override the administrator's saved mapping and pass arbitrary `wp_users` column names — including `ID` and `user_pass` — directly into `wp_insert_user()` via the `subscribe()` function. This makes it possible for unauthenticated attackers to force `wp_insert_user()` into UPDATE mode on any existing account by supplying a target user's numeric `ID`, overwrite that account's password with a pre-hashed value of their choosing, and thereby take over any WordPress user account — including administrators — leading to full site compromise. Exploitation requires that a WordPress User Registration opt-in form is published somewhere on the site, though the form's UUID is visible in page source and no authentication, nonce, or capability check is required to reach the vulnerable endpoint.
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Analysis
by VulDB Data Team • 10/16/2026
The MailOptin plugin for WordPress contains a critical privilege escalation vulnerability affecting all versions up to 1.2.78.6. This flaw stems from an insecure direct object reference combined with improper validation of user-supplied input within the subscribe_to_email_list endpoint. The core technical issue lies in how the plugin handles the optin_data[form_custom_field_mappings] parameter, which is intended to map form fields to specific WordPress user metadata or columns. Instead of enforcing a strict allow-list for valid destination keys, the application accepts arbitrary values provided by the attacker. This lack of validation allows an unauthenticated actor to manipulate internal database operations by injecting column names that are typically protected from direct modification through standard registration forms.
The operational impact of this vulnerability is severe, as it enables full account takeover and subsequent site compromise without any authentication credentials. By exploiting the flaw in the subscribe function, an attacker can force wp_insert_user() into update mode for existing users rather than creating new ones. This is achieved by supplying a target user's numeric ID within the malicious payload. Once this mapping is processed, the attacker gains the ability to overwrite critical account attributes, most notably the user_pass column which stores password hashes. Because the plugin does not verify capabilities or validate nonces before processing these mappings, an unauthenticated remote attacker can directly inject their own pre-hashed password value into any existing WordPress user account.
This vulnerability aligns with CWE-269 Improper Privilege Escalation and CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component. From the perspective of the MITRE ATT&CK framework, this exploitation technique corresponds to T1078 Valid Accounts, specifically allowing an attacker to maintain persistence or escalate privileges within the WordPress environment. The ease of exploitation is heightened because the required form UUID is visible in the page source code, and no additional security checks such as CSRF tokens are implemented for this specific endpoint. Consequently, any visitor with access to a site containing a MailOptin opt-in form can execute this attack remotely.
Mitigation strategies must prioritize immediate patching of the MailOptin plugin to version 1.2.78.7 or later where these validation checks have been corrected. In environments where updating is not immediately feasible, administrators should disable user registration via public forms if possible, or implement server-side web application firewall rules that block requests containing suspicious patterns in form_custom_field_mappings parameters. Additionally, reviewing and restricting the capabilities of lower-privileged users can help limit the blast radius if an account is compromised, although this does not address the root cause of the insecure deserialization-like behavior inherent in the mapping logic.