Ruby on Rails: The authenticity_token can be reversed and used to forge valid per_form_csrf_tokens for arbitrary routes

When performcsrftokens is set to true, each form should protected against CSRF with a unique token that is not predictable by an attacker. Theperformcsrftoken is generated using a HMAC SHA-256 using a key that is exposed in a reversed authenticitytoken. The authenticitytoken is a Base64 encoding ...