Next.js Missing Origin Validation in WebSockets Vulnerability - CVE-2026-27977

Next.js is a React framework for building full-stack web applications. Starting in version 16.0.1 and prior to version 16.1.7 in next dev cross-site protection for internal websocket endpoints could treat Origin: null as a bypass case even if allowedDevOrigins is configured allowing privacy-sensitive/opaque contexts (for example sandboxed documents) to connect unexpectedly. 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 still allow connections from any origin. The issue is fixed in version 16.1.7 by validating Origin: null through the same cross-site origin-allowance checks used for other origins. If upgrading is not immediately possible do not expose next dev to untrusted networks and/or block websocket upgrades to /_next/webpack-hmr when Origin is null at the proxy.