Crack the Gate 1 — picoCTF Write-up | Authentication Bypass via Hidden HTTP Header

Introduction

While practicing web security challenges on picoCTF / CyLab Academy, I came across an interesting challenge called Crack the Gate 1.

At first, it looked like a normal login challenge. The email address was already provided, but the correct password was unknown.

Instead of trying to guess or brute-force the password, I decided to explore the application and understand how the authentication process worked.

This challenge turned out to be a great example of how a small developer mistake can lead to an authentication bypass.

Challenge: Crack the Gate 1 Category: Web Exploitation Platform: picoCTF / CyLab Academy

Exploring the Login Page

After launching the challenge, I was presented with a simple login page.

The email field was already populated with:

ctf-player@picoctf.org

The only missing information was the correct password.

Rather than trying random passwords, I decided to inspect the application for anything interesting.

Inspecting the Source Code

One of the first things I like to check in a web challenge is the page source.

Developers sometimes accidentally leave useful information in HTML comments, JavaScript files, API endpoints, or debugging code.

When I opened the source code, I found this suspicious comment:

ABGR: Wnpx - grzcbenel olcnff: hfr urnqre "K-Qri-Npprff: lrf"

Just below it was another comment:

Remove before pushing to production!

That immediately caught my attention.

The text didn't look completely random, so I started looking at how it might have been encoded.

Identifying ROT13

The hidden message was encoded using ROT13.

ROT13 is a simple substitution cipher where each letter is replaced by the letter 13 positions ahead in the alphabet.

For example:

A → N B → O C → P

ROT13 is useful for hiding text from casual viewing, but it does not provide any real security.

Now the challenge became much more interesting.

The developer had apparently created a temporary authentication bypass using a custom HTTP header:

X-Dev-Access: yes

Understanding the Vulnerability

Normally, a login system should verify the supplied email and password before granting access.

The expected process is:

Email + Password → Server Validation → Authentication → Access

However, this application contained another authentication path.

The server appeared to trust the following HTTP header:

X-Dev-Access: yes

This is insecure because HTTP headers are controlled by the client.

A user can intercept their own request and modify or add headers before sending it to the server.

Simply keeping the name of a special header secret does not make it secure.

Testing with Burp Suite

To test the discovered behavior, I used Burp Suite.

I submitted the login request using the provided email address and a random password.

For example:

Email: ctf-player@picoctf.org

Password: 123

I then intercepted the request and added the custom header discovered in the source code:

X-Dev-Access: yes

After adding the header, I sent the modified request to the server.

Authentication Bypass Successful

The server returned a successful response even though I had not provided the correct password.

The response contained:

"success": true

It also returned information about the authenticated user and the challenge flag.

And with that, the challenge was solved! 🚩

What Was the Vulnerability?

The main vulnerability was an Authentication Bypass caused by an insecure developer backdoor.

The complete flow was:

Login Page → Inspect Source Code → Find Hidden Comment → Decode ROT13 → Discover Developer Header → Modify HTTP Request → Authentication Bypass → Flag

The application trusted a client-controlled HTTP header instead of relying only on proper authentication.

Why Is This Dangerous?

Imagine a developer creates a temporary authentication bypass while testing an application.

For example:

X-Dev-Access: yes

The developer intends to remove it before deployment but accidentally leaves it enabled in production.

If an attacker discovers this functionality, they may be able to bypass the normal login process without knowing a valid password.

In this challenge, the problem was even easier to discover because information about the bypass was left inside the application's source code.

This is why security through obscurity should never replace proper security controls.

How to Prevent This

Temporary authentication bypasses and debugging functionality should always be removed before deploying an application to production.

Authentication and authorization decisions should be performed securely on the server side.

Developers should also avoid exposing sensitive internal information through HTML comments or client-side JavaScript.

Development and production environments should be properly separated, and production code should be reviewed for debugging features before deployment.

Most importantly, custom HTTP headers controlled by the client should never be trusted as proof of authentication.

What I Learned

This challenge helped me understand several important web security concepts:

• HTML comments can reveal sensitive developer information.
• ROT13 is obfuscation, not encryption.
• HTTP headers can be modified by the client.
• Burp Suite can be used to inspect and modify HTTP requests.
• Authentication should never depend on hidden client-controlled values.
• Debugging and development backdoors can become serious security vulnerabilities.

It also reminded me that web exploitation doesn't always require complicated payloads.

Sometimes, simply understanding the application and carefully inspecting its source code is enough to discover a serious vulnerability.

Conclusion

Crack the Gate 1 was a simple but informative web exploitation challenge.

What initially looked like a password problem turned out to be an authentication logic vulnerability.

By inspecting the source code, finding the hidden developer comment, decoding the ROT13 message, and modifying the HTTP request using Burp Suite, I was able to bypass the authentication mechanism and retrieve the flag.

The main takeaway from this challenge is:

Never rely on hidden client-side information or secret HTTP headers as an authentication mechanism.

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