An attacker can exploit the 'populate_via_query_string()' function, which improperly processes the 'rwmb_frontend_field_object_id' GET parameter without authorization checks. This allows the attacker to overwrite the content of any post on the target WordPress site using 'wp_update_post()'. By injecting a malicious '[mb_user_profile_register]' shortcode into a post, the attacker leverages the mb-user-profile component's failure to validate the 'role' and 'auto_login' shortcode attributes. This process permits the registration or modification of user accounts, granting the attacker administrative access to the WordPress environment. This vulnerability affects Meta Box AIO versions up to 3.11.0, Meta Box Frontend Submission up to 4.5.6, and Meta Box User Profile up to 3.11.0.

Impact

Successful exploitation allows unauthenticated attackers to gain full administrative control over the affected WordPress installation. This can lead to unauthorized access to sensitive site data, modification of content, installation of malicious plugins or themes, and potential lateral movement into the hosting infrastructure.

Recommendation

Prioritize the immediate update of the Meta Box AIO plugin, Meta Box Frontend Submission, and Meta Box User Profile to the latest patched versions released by the vendor. Conduct a forensic audit of posts and pages for unexpected shortcode injections, specifically looking for the '[mb_user_profile_register]' shortcode in posts modified after the plugin update threshold.