WooCommerce - PDF Vouchers <= 4.9.3 - Authentication Bypass to Voucher Vendor

The WooCommerce - PDF Vouchers plugin for WordPress is vulnerable to authentication bypass in versions up to, and including, 4.9.3. This is due to insufficient verification on the user being supplied during a QR code login through the plugin. This makes it possible for unauthenticated attackers to log in as any existing Voucher Vendor user on the site, if they have access to the user id.