Vulnerability in Vercel Next.js

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.

Published · last modified .

CVSS v3 metric

CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.002 (10.2th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 10th percentile — 10.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-27977: held from 0.002 to 0.002.

EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · change: +0.000

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-27977?
CVE-2026-27977 is a medium-severity vulnerability in Vercel Next.js, classified under CWE-1385. CVSS score: 5.4/10. Published 2026-03-18.
How severe is CVE-2026-27977?
Medium severity. CVSS v3 base score is 5.4 out of 10.