Summary When Next.js rewrites proxy traffic to an external backend, a crafted "DELETE"/"OPTIONS" request using "Transfer-Encoding: chunked" could trigger request boundary disagreement between the proxy and backend. This could allow request smuggling through rewritten routes. Impact An attacker could smuggle a second request to unintended backend routes (for example, internal/admin endpoints), bypassing assumptions that only the configured rewrite destination/path is reachable. This does not impact applications hosted on providers that handle rewrites at the CDN level, such as Vercel. Patches The vulnerability originated in an upstream library vendored by Next.js. It is fixed by updating that dependency’s behavior so "content-length: 0" is added only when both "content-length" and "transfer-encoding" are absent, and "transfer-encoding" is no longer removed in that code path. Workarounds If upgrade is not immediately possible: - Block chunked "DELETE"/"OPTIONS" requests on rewritten routes at your edge/proxy. - Enforce authentication/authorization on backend routes per our "security guidance" (https://nextjs.org/docs/app/guides/data-security).