A flaw was found in Next.js. Unauthenticated users can exploit this vulnerability by accessing publicly served client artifacts, which may disclose Server Action IDs. This bypasses authentication on pages where these endpoints are used, leading to information disclosure. While primarily an enumeration primitive, this disclosure can increase overall risk when combined with other vulnerabilities.

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, Next.js applications using App Router, Server Actions (use server) or use cache endpoints can be disclosed bypassing any authentication on the pages where these endpoints are usually used. Server Action IDs can be disclosed to unauthenticated users via publicly served client artifacts (for example, static chunks containing action references). Affected users are applications using App Router and Server Actions. By itself, this disclosure is typically a recon/enumeration primitive; however, it can increase risk when combined with other weaknesses. This issue has been fixed in versions 15.5.21 and 16.2.11.

Next.js: Unauthenticated disclosure of internal Server Function endpoints