A flaw was found in Next.js, a framework for building web applications. A remote attacker can exploit this vulnerability by manipulating Host-associated headers when a Server Action forwards or redirects a request. This can lead to Server-Side Request Forgery (SSRF), where the server is tricked into making requests to an attacker-controlled host. Such an attack could potentially expose internal system information or bypass authorization controls.

Next.js is a React framework for building full-stack web applications. In versions 14.1.1 through 15.5.20 and 16.0.0 through 16.2.10, when a Server Action forwards or redirects a request, an attacker can cause the server to send that outbound request to a malicious host (Server-Side Request Forgery). This requires the attacker's request to control Host-associated headers. In some configurations, it's also possible to obtain internal values that weaken middleware/proxy authorization. Applications that use Server Actions are affected when the incoming host header is not fixed to a trusted value. This typically occurs on custom servers, or on deployments not behind a proxy that pins the host. Managed hosting pins the host upstream and is not affected; next start and standalone output do the same from version 14.2 onward. This issue has been fixed in versions 15.5.21 and 16.2.11.

Next.js: Server-Side Request Forgery in Server Actions on custom servers