Next.js is a React framework that can provide building blocks to create web applications. A Server-Side Request Forgery (SSRF) vulnerability was identified in Next.js Server Actions. If the Host header is modified, and the below conditions are also met, an attacker may be able to make requests that appear to be originating from the Next.js application server itself. The required conditions are 1) Next.js is running in a self-hosted manner; 2) the Next.js application makes use of Server Actions; and 3) the Server Action performs a redirect to a relative path which starts with a /. This vulnerability was fixed in Next.js 14.1.1.
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.