Grav Form Plugin before 9.1.13 Open Redirect via form.value() Twig

The Grav form plugin (getgrav/grav-plugin-form) before 9.1.13 contains an open redirect vulnerability. Since v9.1.11, the redirect process action evaluates user-supplied form data inside Twig expressions, and Grav::redirect() accepts external URLs without origin validation. When a form blueprint defines a redirect target such as redirect: "{{ form.value('next') }}" using an attacker-controllable field, an unauthenticated form submitter can supply a value like https://evil.com to cause a 302 redirect to an arbitrary external site, enabling phishing.