Vulnerability Instance

Next.js: null origin can bypass dev HMR websocket CSRF checks
## Summary
In `next dev`, cross-site protections for internal development endpoints could treat `Origin: null` as a bypass case even when [`allowedDevOrigins`](https://nextjs.org/docs/app/api-reference/config/next-config-js/allowedDevOrigins) is configured. This could allow privacy-sensitive or opaque browser contexts, such as sandboxed documents, to access privileged internal dev-server functionality unexpectedly.

## Impact
If a developer visits attacker-controlled content while running an affected `next dev` server with [`allowedDevOrigins`](https://nextjs.org/docs/app/api-reference/config/next-config-js/allowedDevOrigins) configured, attacker-controlled browser code may be able to connect to internal development endpoints and interact with sensitive dev-server functionality that should have remained blocked.

This issue affects development mode only. It does not affect `next start`, and it does not expose internal debugging functionality to the network by default.

## Patches
Fixed by validating `Origin: null` through the same cross-site origin-allowance checks used for other origins on internal development endpoints.

## Workarounds
If upgrade is not immediately possible:
- Do not expose `next dev` to untrusted networks.
- If you use [`allowedDevOrigins`](https://nextjs.org/docs/app/api-reference/config/next-config-js/allowedDevOrigins), reject requests and websocket upgrades with `Origin: null` for internal dev endpoints at your proxy.