fastify-csrf is an open-source plugin helps developers protect their Fastify server against CSRF attacks. Versions of fastify-csrf prior to 3.1.0 have a "double submit" mechanism using cookies with an application deployed across multiple subdomains, e.g. "heroku"-style platform as a service. Version 3.1.0 of the fastify-csrf fixes it. the vulnerability. The user of the module would need to supply a `userInfo` when generating the CSRF token to fully implement the protection on their end. This is needed only for applications hosted on different subdomains.

This affects the package fastify-csrf before 3.0.0. 1. The generated cookie used insecure defaults, and did not have the httpOnly flag on: cookieOpts: { path: '/', sameSite: true } 2. The CSRF token was available in the GET query parameter