CVE-2025-29927 in Next.js
Summary
by MITRE • 03/21/2025
Next.js is a React framework for building full-stack web applications. Prior to 14.2.25 and 15.2.3, it is possible to bypass authorization checks within a Next.js application, if the authorization check occurs in middleware. If patching to a safe version is infeasible, it is recommend that you prevent external user requests which contain the x-middleware-subrequest header from reaching your Next.js application. This vulnerability is fixed in 14.2.25 and 15.2.3.
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Analysis
by VulDB Data Team • 01/04/2026
This vulnerability affects Next.js applications that implement authorization checks within middleware components, creating a potential bypass mechanism that could allow unauthorized access to protected resources. The flaw specifically manifests when middleware-based authorization logic is present in the application architecture, where the framework fails to properly validate request authenticity during the middleware execution phase. The vulnerability stems from how Next.js handles internal subrequest headers, particularly the x-middleware-subrequest header that is typically used for internal routing within the framework's middleware system. Attackers can exploit this weakness by crafting malicious requests that appear to originate from legitimate internal processes, thereby circumventing the authorization mechanisms that should prevent unauthorized access to sensitive application functionality.
The technical implementation of this vulnerability involves the manipulation of HTTP headers that Next.js uses internally to track middleware subrequests. When a Next.js application processes requests through middleware, it relies on specific header validation to distinguish between external user requests and internal framework operations. However, the vulnerability allows attackers to inject or modify the x-middleware-subrequest header in external requests, enabling them to bypass the authorization checks that are meant to protect sensitive routes and data. This flaw represents a critical security gap in the framework's request processing logic, particularly affecting applications that depend on middleware for access control and authentication validation.
The operational impact of this vulnerability extends beyond simple unauthorized access, potentially enabling attackers to escalate privileges and gain access to restricted application features, user data, or administrative functions. Organizations using vulnerable Next.js versions may experience data breaches, unauthorized system modifications, or complete compromise of protected application resources. The vulnerability affects both Next.js 14.x and 15.x major versions, indicating a fundamental flaw in the framework's middleware authorization handling that requires immediate attention from development teams and security administrators. The exploitation of this vulnerability aligns with attack patterns documented in the attack tree methodology where internal bypass techniques are used to circumvent external security controls.
Security mitigations for this vulnerability include immediate patching to Next.js versions 14.2.25 or 15.2.3, which contain the necessary fixes to properly validate middleware subrequest headers. Organizations unable to patch immediately should implement network-level restrictions to block external requests containing the x-middleware-subrequest header from reaching their Next.js applications. This approach aligns with the principle of least privilege and defense in depth strategies recommended in cybersecurity frameworks. The vulnerability classification aligns with CWE-285 (Improper Authorization) and may map to ATT&CK techniques related to privilege escalation and defense evasion. Organizations should also conduct comprehensive security audits of their middleware implementations to ensure no other authorization bypass opportunities exist within their Next.js applications. The fix addresses the root cause by implementing proper header validation mechanisms that distinguish between legitimate internal framework operations and potentially malicious external requests, thereby restoring the intended authorization controls within the application architecture.