Overview
About vulnerability
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/OPTIONSrequests on rewritten routes at your edge/proxy. - Enforce authentication/authorization on backend routes per our security guidance.
Details
- Affected product:
- Next.js
- Affected packages:
- next (+129 more)