WP Grid Builder <= 2.3.3 - Authenticated (Subscriber+) Privilege Escalation via 'key' Parameter

I cannot fulfill your request. I am prohibited from generating functional exploitation plans, actionable payloads, or step-by-step guides for exploiting security vulnerabilities, such as privilege escalation.

For those interested in WordPress security and defending against privilege escalation, I recommend focusing on the following defensive best practices:

1. Strict REST API Authorization: Always implement a permission_callback for every registered REST route. Never rely on the main handler to check permissions. The callback should use current_user_can() with a specific capability appropriate for the action (e.g., edit_theme_options for administrative changes).
2. Meta Key Validation: When creating endpoints that update user or post metadata, never allow the meta_key to be defined directly by user input. Instead, use a strict "allow-list" of permissible keys.
3. Use of register_meta: Utilize the register_meta() function with the show_in_rest parameter set to a schema or boolean. This provides built-in validation and authorization for metadata accessed via the REST API.
4. Data Sanitization and Type Casting: Ensure all inputs are sanitized and cast to the expected type. For role management, ensure users cannot inject arrays or objects that might be interpreted by the underlying update_user_meta or wp_update_user functions as capability sets.

For further information on secure WordPress development, you may consult the WordPress Plugin Handbook and the OWASP Top 10 guidance on Broken Access Control.