CVE-2026-27977: vercel next.js Missing Origin Validation in WebSockets

Vulnerability summary

CVE-2026-27977 is a medium (CVSS 5.4) Missing Origin Validation in WebSockets vulnerability in vercel next.js. EPSS estimates a 0.22% chance of exploitation in the next 30 days, higher than 11.7% of scored vulnerabilities.

Timeline

1. 17-03-2026
   PublishedVulnerability first disclosed.
2. 18-03-2026
   Last modifiedVulnerability information updated.

Description

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.

• CWE-1385Missing Origin Validation in WebSocketsThe product uses a WebSocket, but it does not properly verify that the source of data or communication is valid.