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Next.js has a Middleware / Proxy bypass through dynamic route parameter injection

High severity GitHub Reviewed Published May 6, 2026 in vercel/next.js • Updated May 14, 2026

Package

npm next (npm)

Affected versions

>= 15.4.0, < 15.5.16
>= 16.0.0, < 16.2.5

Patched versions

15.5.16
16.2.5

Description

Impact

Applications that rely on middleware to protect dynamic routes can be vulnerable to authorization bypass. In affected deployments, specially crafted query parameters can alter the dynamic route value seen by the page while leaving the visible path unchanged, which can allow protected content to be rendered without passing the expected middleware check.

Fix

We now only honor internal route-parameter normalization in trusted routing flows and ignore externally supplied parameter encodings that should never have been accepted from ordinary requests.

Workarounds

If you cannot upgrade immediately, enforce authorization in route or page logic instead of relying solely on middleware path matching.

References

@timneutkens timneutkens published to vercel/next.js May 6, 2026
Published to the GitHub Advisory Database May 11, 2026
Reviewed May 11, 2026
Published by the National Vulnerability Database May 13, 2026
Last updated May 14, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(9th percentile)

Weaknesses

Authentication Bypass Using an Alternate Path or Channel

The product requires authentication, but the product has an alternate path or channel that does not require authentication. Learn more on MITRE.

CVE ID

CVE-2026-44574

GHSA ID

GHSA-492v-c6pp-mqqv

Source code

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