Solving the CyLab Security Academy (formerly picoCTF) Crack the Gate 1 challenge taught me that a challenge name can be the biggest red herring of all. “Crack the Gate” sounds like a brute-force password challenge — and that’s exactly what I wasted time on before finding the real answer hidden in a HTML comment.
Article Overview
This is a writeup for Crack the Gate 1, a Web Exploitation challenge on CyLab Security Academy (formerly picoCTF), rated Easy. The challenge presents a login page with no obvious entry point. The solution involves finding a ROT13-encoded message in the HTML source that reveals a developer’s debug bypass — a single custom HTTP header that unlocks the application without any credentials.
In this article you’ll learn how to identify ROT13 encoding from character patterns, decode it in Python, and inject a custom HTTP header using Burp Suite to bypass a login form entirely.
Introduction
When a CTF challenge is called “Crack the Gate,” your brain jumps to one place: password cracking. Dictionary attacks, Burp Suite Intruder, maybe hydra. That’s where I went first, and it cost me a solid chunk of time before I realized the gate wasn’t locked with a password at all.
The actual solution was a forgotten developer shortcut — a temporary bypass left in the code, encoded in ROT13 inside an HTML comment. Barely obfuscated, completely overlooked unless you check the page source before reaching for your wordlist.
What makes this challenge worth writing about isn’t the ROT13 itself (that’s trivial once you spot it). It’s the lesson about where to look first. Checking the source code before any active attack is basic web recon — but “Crack the Gate” made me skip that step and go straight to hammering the login form.
Challenge Overview
The challenge gives you a URL. Launch the instance and you get a login page — username and password fields, a submit button, nothing else. No “forgot password” link, no registration form, no error messages that distinguish between wrong username and wrong password. Just a blank login screen.
My first read: this is a brute-force problem. The name “Crack the Gate” reinforced that. I started with the obvious credentials — admin/admin, admin/password, root/root — and got nothing. No useful error messages either. Just a generic “incorrect” response that gave me no information about whether the username even existed.
That’s when I should have stopped and checked the source. Instead, I started thinking about wordlists.
Step 1 — The Brute-Force Rabbit Hole
I opened Burp Suite and captured the login request. The POST body looked like username=admin&password=admin — straightforward enough for Intruder. I set up a cluster bomb attack targeting both fields, loaded a short wordlist of common credentials, and started the attack.
The responses all looked the same. Same status code, same response length. No differentiation between “wrong username” and “wrong password.” This is the sign that brute force won’t work: if you can’t tell whether you’ve got the username right, you’re searching a two-dimensional space with no feedback.
I also didn’t know the username. “admin”? “jack”? “user”? Without a leaked username, a real brute-force attack against a blind login form is essentially impractical in a CTF timeframe. I stopped the Intruder attack after a few minutes and stepped back.
Wrong approach, wasted maybe ten minutes. But that failure made me actually look at the application instead of throwing credentials at it.
Step 2 — Reading the Page Source
I right-clicked and hit “View Page Source.” It’s the most basic thing you can do in a web challenge, and I probably should have done it before opening Burp at all.
The HTML was minimal — just a login form. But buried near the top, there was an HTML comment that looked like this:
<!-- ABGR: Wnpx - grzcbenel olcnff: hfr urnqre "K-Qri-Npprff: lrf" -->My first reaction was that it looked like gibberish. But it wasn’t hex (no 0-9 in the right density), wasn’t Base64 (no padding, no uppercase/lowercase mix), and the words were too short and regular to be random noise. All letters, consistent word lengths, spaces in the right places.
That pattern points to a simple letter substitution cipher — either a Caesar cipher or ROT13 specifically. The tell: when all the characters are alphabetic (no digits, no special symbols beyond punctuation), and the ciphertext looks like it has a natural word structure, ROT13 is almost always the answer in CTF.
Step 3 — Decoding ROT13 in Python
ROT13 is a Caesar cipher with a shift of 13. Because the alphabet has 26 letters, ROT13 is its own inverse — applying it twice gives you back the original. That’s why Python’s codecs module has built-in support for it.
You can spot ROT13 quickly with a mental check: take any letter from the ciphertext and count forward 13 positions. “ABGR” → A+13=N, B+13=O, G+13=T, R+13=E → “NOTE”. Confirmed. I wrote a quick script to decode the whole comment:
import codecs
cipher = '<!-- ABGR: Wnpx - grzcbenel olcnff: hfr urnqre "K-Qri-Npprff: lrf" -->'
decoded = codecs.decode(cipher, 'rot_13')
print(decoded)Output:
<!-- NOTE: Jack - temporary bypass: use header "X-Dev-Access: yes" -->There it is. Someone named Jack left a development shortcut in the code — a custom HTTP header X-Dev-Access: yes that bypasses the login entirely. They encoded it in ROT13, which is barely more secure than plaintext. The message says “temporary bypass.” That means this was probably added during testing — “I’ll remove it before we go live” — and then it wasn’t removed.
Step 4 — Injecting the Header with Burp Suite
Back to Burp Suite, but this time for something useful. I captured the POST request to the login form, sent it to Repeater, and added the header manually:
POST /login HTTP/1.1
Host: [challenge-host]
Content-Type: application/x-www-form-urlencoded
X-Dev-Access: yes
username=anything&password=anythingThe credentials don’t matter at this point. The server checks for the header before it even looks at the username and password. I hit “Send” in Repeater and the response came back immediately — HTTP 200, with the full authenticated page and the flag visible in the response body. No redirect to a login failure page, no 403, just a clean successful response.
If you don’t want to use Burp Suite, you can do the same thing from the command line:
curl -s -X POST "http://[challenge-host]/login" \
-H "X-Dev-Access: yes" \
-d "username=anything&password=anything"The -H flag adds a custom header. The response will include the flag in the HTML.
Capture the Flag
Flag: picoCTF{brut4_f0rc4_83812a02}
Read that flag carefully: brut4_f0rc4. Brute force. The thing I wasted time on at the start. The challenge designers put the wrong approach directly in the flag. That’s either a lesson or just very on-the-nose CTF humor — probably both.
“Crack the Gate” led me to think about cracking credentials. The actual crack was reading a comment that took three seconds and a one-line Python script. Everything after that was mechanical.
Full Trial Process Table
| Step | Action | Command / Tool | Result | Why it failed / succeeded |
|---|---|---|---|---|
| 1 | Try common credentials | admin/admin, admin/password, root/root | Generic login failure | No correct credential; no username/password differentiation in error messages |
| 2 | Burp Suite Intruder — credential brute force | Cluster bomb on username + password fields | Uniform response lengths | No feedback signal; unknown username makes search space too large |
| 3 | View page source | Ctrl+U in browser | Found ROT13-encoded HTML comment | Developer left a debug bypass note in the source |
| 4 | Decode ROT13 | codecs.decode(cipher, 'rot_13') | Revealed X-Dev-Access: yes header instruction | ROT13 is symmetric and trivially reversible |
| 5 | Inject custom header via Burp Repeater | Add X-Dev-Access: yes to request headers | Server returned flag in response | Server-side bypass check matched the header value |
Short Explanations for Commands and Techniques Used
ROT13
ROT13 (“rotate by 13 places”) is a Caesar cipher with a fixed shift of 13. Because 26 ÷ 2 = 13, applying ROT13 twice returns the original text — it is self-inverse. Python’s codecs module supports it natively: codecs.decode(text, 'rot_13'). In bash: echo "ciphertext" | tr 'A-Za-z' 'N-ZA-Mn-za-m'.
ROT13 provides essentially no security — any text editor or command-line tool can reverse it instantly. It’s used to hide forum spoilers or make developer notes slightly less obvious at a glance. In CTF, any all-alphabetic ciphertext with natural word structure is almost certainly ROT13.
HTML Comments as Information Leakage
HTML comments (<!-- ... -->) are visible in the page source but don’t render in the browser. Developers use them for notes, debugging information, and TODO items. The browser hides them; attackers find them in seconds. Reviewing HTML comments is a standard step in web application security testing — comments have exposed API keys, internal URLs, credentials, and developer notes in real production applications.
A quick command to extract all HTML comments from a page automatically:
curl -s "http://[target-url]" | grep -o '<!--.*-->'In penetration testing, this is a standard first-pass recon move. For multi-line comments, grep -Pzo '(?s)<!--.*?-->' handles newlines. The OWASP Testing Guide (OTG-INFO-005) specifically lists HTML comment review as an information gathering technique under web security testing.
Custom HTTP Headers as Access Control Bypass
HTTP headers are key-value pairs sent with every web request. Applications can define and respond to custom headers. In development environments, header-based bypasses (X-Dev-Access, X-Internal-Request, X-Debug-Mode) are common shortcuts to skip authentication during testing. When these aren’t removed before production deployment, they become vulnerabilities. A web application firewall typically won’t block custom headers it doesn’t recognize, so this bypass often survives standard security controls.
For Burp Suite Repeater usage, see the official PortSwigger documentation.
Beginner Tips
- View page source before anything else. Ctrl+U (Firefox/Chrome) takes two seconds. Do this before opening Burp Suite. Developers leave things in comments constantly.
- All-letter ciphertext with word structure = try ROT13 first. No digits is the key tell. If the encoded text has spaces and consistent short-word patterns, ROT13 before any other Caesar variant.
- Unknown username + blind errors = brute force is the wrong tool. If you can’t distinguish “wrong username” from “wrong password,” you have no feedback signal. Stop and look for another entry point.
- Burp Suite Community Edition is free. The Repeater module for header injection and the Proxy for request capture are all you need for this category of web challenge.
- Challenge names are flavor text, not instructions. “Crack the Gate” implied credentials. The actual entry point was source inspection. Always do recon before committing to an approach.
What You Learn
The core skill this challenge builds is web recon discipline. Specifically: view page source as the first step, recognize ROT13 from all-alphabetic ciphertext with natural word spacing, inject custom HTTP headers via a proxy, and understand why “security by obscurity” consistently fails — encoding something in ROT13 doesn’t protect it from anyone who checks the source.
If I encountered this again, I’d go straight to View Source on any login page before thinking about credential attacks. The flag’s content — “brut4_f0rc4” — is a reminder built into the challenge that brute force was never the intended path. Source first, always.
In real-world penetration testing, finding debug headers or developer notes in HTML source is a Phase 1 recon finding. A simple curl -s <URL> | grep -i "<!--" can surface comments automatically. The technique scales from a CTF login page to a production API endpoint with the same command.
Further Reading
This challenge is part of the Web Exploitation series on CyLab Security Academy (formerly picoCTF). For another challenge in the same category that also rewards source code inspection over active attacks, see the Includes picoCTF Writeup — it hides the flag across CSS and JavaScript comments in a similar “look before you attack” pattern.
For a broader toolkit reference that complements web challenges, see CTF Forensics Tools: The Ultimate Guide for Beginners on alsavaudomila.com.
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