Redux Framework <= 4.5.15 - Authenticated (Subscriber+) Stored Cross-Site Scripting via multi_media Field and Client-Supplied checkuser_id

The Redux Framework plugin for WordPress is vulnerable to Stored Cross-Site Scripting in all versions up to, and including, 4.5.15 via the multi_media field. This is due to the field's renderer printing the stored value into an href attribute, a hidden input value and an HTML element without escaping, while every neighbouring attribute in the same method is wrapped in esc_attr(). It is compounded by Redux_Extension_Users::user_meta_save() evaluating a field's 'permissions' requirement against a user id taken from the request body ('checkuser_id') rather than the current user, so a Subscriber can write a field declared for 'manage_options'. This makes it possible for authenticated attackers with Subscriber-level access and above to inject arbitrary web scripts into their own profile record that execute when an administrator views that user's profile in the dashboard. Requires a theme or plugin that registers a multi_media field through the Redux Users API.