CVE-2026-27977Network attack vector

Dev WebSocket hijacking via null-Origin bypass

Published Mar 17, 2026 · Updated Mar 18, 2026

Missing WebSocket origin validation in Next.js 16.0.1 through 16.1.6 allows remote attackers to read and alter HMR traffic. The development router skips parsing and allowlist validation when the Origin header equals null, allowing opaque or sandboxed browser contexts to reach the HMR channel. Exploitation requires a developer to visit attacker-controlled content while an affected next dev server is reachable and allowedDevOrigins is configured; next start is unaffected.

CVSS severity2.3
Low
EPSS probability0.17%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Missing WebSocket origin validation in Next.js 16.0.1 through 16.1.6 allows remote attackers to read and alter HMR traffic. The development router skips parsing and allowlist validation when the Origin header equals null, allowing opaque or sandboxed browser contexts to reach the HMR channel. Exploitation requires a developer to visit attacker-controlled content while an affected next dev server is reachable and allowedDevOrigins is configured; next start is unaffected.

The record

CVE
CVE-2026-27977
Published
Mar 17, 2026
Updated
Mar 18, 2026
Vendor
Vercel
Product
Next.js
Classifications
CWE-1385, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 17, 2026CVE publishedPublication date reported by the CVE source.
  2. Mar 18, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=>= 16.0.1, < 16.1.7

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Vercel · Next.jsversion=>= 16.0.1, < 16.1.7

Published CVSS scores

5.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
2.3GitHub, Inc.CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-1385Missing Origin Validation in WebSockets
T1189Drive-by Compromise

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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