CVE-2026-27977

Description

## Summary In `next dev`, cross-site protection for internal websocket endpoints could treat `Origin: null` as a bypass case even if [`allowedDevOrigins`](https://nextjs.org/docs/app/api-reference/config/next-config-js/allowedDevOrigins) is configured, allowing privacy-sensitive/opaque contexts (for example sandboxed documents) to connect unexpectedly. ## Impact If a dev server is reachable from attacker-controlled content, an attacker may be able to connect to the HMR websocket channel and interact with dev websocket traffic. This affects development mode only. Apps without a configured [`allowedDevOrigins`](https://nextjs.org/docs/app/api-reference/config/next-config-js/allowedDevOrigins) still allow connections from any origin. ## Patches Fixed by validating `Origin: null` through the same cross-site origin-allowance checks used for other origins. ## Workarounds If upgrade is not immediately possible: - Do not expose `next dev` to untrusted networks. - Block websocket upgrades to `/_next/webpack-hmr` when `Origin` is `null` at your proxy.

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