Authentication Bypass via Information Disclosure

Introduction

In this lab, the objective was to gain access to an administrative interface protected by authentication controls. Through systematic enumeration and analysis of application behavior, I identified a trust issue involving client-controlled headers combined with information disclosure through an unexpected HTTP method.

This write-up explains the methodology, findings, exploitation process, and root cause behind the vulnerability.

Initial Enumeration

The first step involved enumerating application endpoints and observing how the server responded to different requests.

During testing, I discovered the following endpoint:

GET /admin

The server returned:

HTTP/1.1 401 Unauthorized

A 401 Unauthorized response indicated that the endpoint existed but required additional authorization rather than being inaccessible or nonexistent.

HTTP Method Testing

To understand whether the endpoint handled different HTTP methods differently, I tested multiple request methods against /admin.

Most methods returned unsuccessful responses; however, one method behaved unexpectedly:

TRACE /admin

The server responded with:

HTTP/1.1 200 OK

The TRACE method is generally intended for diagnostic purposes and reflects the request back to the client. While reviewing the response, I noticed an interesting header:

X-Custom-IP-Authorization: [your-public-ip-address]

This suggested that the application might rely on a custom header for authorization decisions.

Identifying the Trust Boundary Issue

At this stage, I hypothesized that the application trusted the value inside X-Custom-IP-Authorization.

If the application expected local traffic for administrative access, replacing the value with localhost could potentially bypass restrictions.

I modified the header value to:

X-Custom-IP-Authorization: 127.0.0.1

Since this header needed to be included consistently across requests, I configured automatic header injection using Burp Suite.

Burp Suite Match & Replace Configuration

Configuration used:

Type: Request Header

X-Custom-IP-Authorization: 127.0.0.1

This ensured the header was automatically appended to all outgoing requests.

Exploitation Process

After enabling automatic header injection, I authenticated normally through the application and revisited the target endpoint.

Request:

TRACE /admin
X-Custom-IP-Authorization: 127.0.0.1

Following this, I refreshed the authenticated account page using:

GET /my-account?id=username

After refreshing, the administrative functionality became available and the lab objective was completed successfully.

Root Cause Analysis

The vulnerability stemmed from two security weaknesses:

1. Information Disclosure Through TRACE

The TRACE method exposed internal authorization-related behavior by reflecting information that should not have been externally useful.

2. Trusting Client-Controlled Headers

The application trusted a user-supplied header for authorization decisions.

Because clients can modify request headers freely, relying on them for security-sensitive access controls creates opportunities for privilege escalation and authentication bypass.

Impact

This vulnerability allowed unauthorized users to gain access to restricted administrative functionality by manipulating request headers.

Potential impacts include:

• Administrative privilege escalation
• Unauthorized access to sensitive functionality
• Security boundary bypass
• Increased attack surface through exposed internal mechanisms

Small inconsistencies in application behavior frequently lead to larger security findings.