Subscriptions for WooCommerce <= 2.0.0 - Authenticated (Contributor+) Privilege Escalation via '_wps_plan_user_role' Membership Plan Meta

The Subscriptions for WooCommerce plugin for WordPress is vulnerable to Privilege Escalation in versions up to, and including, 2.0.0. This is due to the `save_meta_boxes()` function persisting the `_wps_plan_user_role` membership plan meta from `$_POST` without an allowlist that excludes privileged roles — the only validations applied, `sanitize_key()` and `wp_roles()->is_role()`, both accept `'administrator'` as a valid value, and the UI's `disabled` attribute on the role dropdown is a client-side-only control trivially bypassed via DevTools or a direct POST request; additionally, because the `wps_membership_plan` custom post type is registered with `capability_type => 'post'`, any user who can edit posts satisfies the `current_user_can('edit_post', $post_id)` guard in `save_meta_boxes()`. This makes it possible for authenticated attackers, with Contributor-level access and above, to escalate their privileges to Administrator by storing `'administrator'` as the role granted on membership acquisition, which the Pro companion plugin then applies via `add_role()` during membership lifecycle events. Successful exploitation requires the Subscriptions for WooCommerce Pro companion plugin to be active, as it is the component that reads the stored `_wps_plan_user_role` meta via `get_post_meta()` and calls `add_role()` to apply the role during membership lifecycle events.