The MailOptin – Popup, Optin Forms & Email Newsletters for Mailchimp, HubSpot, AWeber Etc. plugin...

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.