CVE-2025-29927 Nginx + Next.js Middleware Bypass
Introduction
This security research demonstrates a critical vulnerability in middleware handling within web applications. The PoC illustrates how a specially crafted HTTP request can bypass redirection logic to access restricted content, such as administrative dashboards.
Table of Contents
Prerequisites
To follow this PoC, you'll need:
A target server running a vulnerable configuration
HTTP client tools like
curl, Burp Suite, or custom scriptsBasic knowledge of HTTP protocols and headers
Understanding of middleware architectures
Vulnerability Overview
CVE ID: CVE-2025-29927
CVSS Score: 8.2 (High)
Vulnerability Type: Authentication Bypass / Access Control
Affected Components: Nginx + Next.js middleware configurations
This vulnerability exploits a middleware misconfiguration or logic flaw where adding a custom header (X-Middleware-Subrequest) alters the server's behavior, bypassing redirection mechanisms to access restricted content. This relates to how middleware processes subrequests or validates headers in the authentication flow.
Proof of Concept Steps
The PoC demonstrates two scenarios: an unsuccessful request followed by a successful bypass using a crafted header.
Step 1: Initial Request (Unsuccessful)
This step demonstrates the default server behavior when accessing the /dashboard endpoint without additional headers.
HTTP Request
Server Response
Analysis
Request: Standard GET request to
/dashboardwith typical browser headersResponse:
307 Temporary Redirectindicates protective mechanism preventing direct accessServer: Nginx/1.14.1 handling the redirection logic
Behavior: Normal authentication flow redirecting to login page
Step 2: Modified Request (Successful Bypass)
This step introduces the custom header that successfully bypasses the redirection mechanism.
HTTP Request
Server Response
Critical Analysis
Key Header:
X-Middleware-Subrequest: middleware:middleware:middleware:middleware:middlewareSuccess Response:
200 OKwith full dashboard content (30,174 bytes)Framework:
X-Powered-By: Next.jsreveals the application stackExploit Mechanism: Custom header tricks middleware into treating request as legitimate subrequest
Security Impact: Complete bypass of authentication/authorization controls
How to Replicate
Follow these steps to replicate the PoC using curl:
Step 1: Send Initial Request
Expected: 307 Temporary Redirect response
Step 2: Send Bypass Request
Expected: 200 OK with dashboard content
Security Impact
Authentication Bypass: Complete circumvention of login mechanisms
Unauthorized Access: Direct access to protected administrative interfaces
Data Exposure: Potential access to sensitive user data and system information
Privilege Escalation: Access to administrative functions without proper authorization
Compliance Violations: May violate data protection regulations and security standards
Mitigation Recommendations
Implement proper header validation in middleware configurations
Review and harden Next.js middleware authentication logic
Add input sanitization for custom HTTP headers
Implement defense-in-depth authentication mechanisms
Regular security audits of middleware configurations
Update to latest versions of Nginx and Next.js frameworks
References
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