Vulnerability Details 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.
Exploit prediction scoring system (EPSS) score
EPSS Score 0.0
EPSS Ranking 4.3%
CVSS Severity
CVSS v3 Score 5.4
References
Products affected by CVE-2026-27977
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cpe:2.3:a:vercel:next.js:16.0.1
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cpe:2.3:a:vercel:next.js:16.0.10
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cpe:2.3:a:vercel:next.js:16.0.2
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cpe:2.3:a:vercel:next.js:16.0.3
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cpe:2.3:a:vercel:next.js:16.0.4
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cpe:2.3:a:vercel:next.js:16.0.5
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cpe:2.3:a:vercel:next.js:16.0.6
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cpe:2.3:a:vercel:next.js:16.0.7
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cpe:2.3:a:vercel:next.js:16.0.8
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cpe:2.3:a:vercel:next.js:16.0.9
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cpe:2.3:a:vercel:next.js:16.1.0
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cpe:2.3:a:vercel:next.js:16.1.1
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cpe:2.3:a:vercel:next.js:16.1.2
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cpe:2.3:a:vercel:next.js:16.1.3
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cpe:2.3:a:vercel:next.js:16.1.4
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cpe:2.3:a:vercel:next.js:16.1.5
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cpe:2.3:a:vercel:next.js:16.1.6