Noptin WordPress plugin: unauthenticated file inclusion can run PHP code

The Noptin – Newsletter, New Post Notifications & Email Automation plugin for WordPress is vulnerable to Local File Inclusion in all versions up to, and including, 4.4.3 via the 'noptin_fields[first_name]' parameter parameter. This makes it possible for unauthenticated attackers to include and execute arbitrary .php files on the server, allowing the execution of any PHP code in those files. This can be used to bypass access controls, obtain sensitive data, or achieve code execution in cases where .php file types can be uploaded and included. The public Manage Preferences page issues a nonce to unauthenticated visitors, meaning there is no effective authorization barrier — an unauthenticated attacker can store the malicious payload as their first_name and have it executed the next time any newsletter is sent by the site operator. Successful exploitation additionally requires an action by an administrator — the attacker cannot trigger the vulnerability on their own.