Easy Digital Downloads <= 3.6.7 - Cross-Site Request Forgery to Payment Account Hijacking via 'square_tokens' Parameter

The Easy Digital Downloads plugin for WordPress is vulnerable to Cross-Site Request Forgery in all versions up to, and including, 3.6.7. This is due to missing nonce verification in the `handle_oauth_redirect()` function, which is registered on the `admin_init` hook and processes Square OAuth tokens from a user-supplied GET parameter without any CSRF token validation. This makes it possible for unauthenticated attackers to overwrite the store's Square payment gateway credentials by tricking a logged-in administrator into clicking a crafted link, potentially resulting in payment account hijacking.