A flaw was found in Next.js, a React framework for building web applications. This vulnerability allows a remote attacker to perform Server-Side Request Forgery (SSRF) or Open Redirect attacks. When a `rewrites()` or `redirects()` rule constructs an external destination hostname using attacker-controlled input, the application can be coerced into proxying requests to arbitrary hosts, leading to SSRF. Additionally, this misconfiguration can result in Open Redirects, potentially exposing users to phishing.

Next.js is a React framework for building full-stack web applications. In versions 12.0.0 through 15.5.20 and 16.0.0 through 16.2.10, a rewrites() or redirects() rule that builds its external destination hostname from request-controlled input can be pointed at an arbitrary hostname, regardless of the rule's hostname suffix. For a rewrite, Next.js proxies the request to that arbitrary host and serves the response from the application's origin, leading to Server-Side Request forgery. A redirects() rule configured this way is vulnerable to an Open Redirect. This issue has been fixed in versions 15.5.21 and 16.2.11.

Next.js: Server-Side Request Forgery in rewrites via attacker-controlled destination hostname