Author: Syed Jan Muhammad Zaidi
Title: Independent Security Researcher | Full-Time Penetration Tester
Twitter: @th3_d4rkninj4
LinkedIn: syed-jan-muhammad-zaidi

👋 Introduction

Hey hackers and infosec enthusiasts,

In this write-up, I’ll walk you through a simple yet effective trick I used to bypass a 403 Forbidden response on a restricted subdomain during a private bug bounty engagement.

Understanding HTTP response behavior is crucial in reconnaissance. A seemingly “blocked” endpoint might still hold gold—if you know how to dig deeper.

Let’s break it down.

The HTTP 403 status code means that the server understands the request, but it refuses to authorize it. In most cases, this is used to restrict access to confidential or internal resources that aren’t meant for public access.

403s are often returned due to:

• Missing authentication
• IP address or location-based restrictions
• Server-level deny rules
• Referrer-based blocking
• Or simply misconfigured access policies

But sometimes, they’re just misleading clues waiting to be bypassed.

🕵️‍♂️ Reconnaissance Phase

While participating in a private invite-only bug bounty program, I started the recon phase with my standard routine. For the sake of privacy, let’s refer to the target as redacted.com.

I used Subfinder to enumerate all subdomains and then piped the output into httpx to check for live hosts and their status codes:

subfinder -d redacted.com -silent | httpx -sc
 

As expected, I discovered a long list of subdomains. But one subdomain stood out:

subdomain.redacted.com → 403 Forbidden

Immediately, it caught my attention. A restricted response like 403 usually means “there’s something behind this wall worth exploring.”

🧪 Tried and Failed — Open Source Bypass Tools

My next step was to attempt a direct 403 bypass using well-known open-source tools:

• bypass-403
• bypxx

However, none of these tools worked for this case. They returned the same 403 response regardless of header injection or HTTP method tampering.
 

💡 The CNAME Trick — An Idea That Paid Off

That's when I had a thought:

“What if I can access this subdomain via its underlying CNAME record?”

Some infrastructure setups rely on CNAMEs that resolve to services hosted on other domains or CDNs. Sometimes, these alternate paths aren’t locked down properly.

So I used the dig command to check the DNS records for the subdomain:

dig subdomain.redacted.com
 

Boom 💥 — the subdomain pointed to a CNAME entry, which revealed a different domain.

I took that CNAME and accessed the application directly through the resolved domain, bypassing the original 403 restriction entirely.

And guess what?

The resource loaded successfully!

I was able to access content that was previously restricted on subdomain.redacted.com — just by changing the entry point.

📝 Responsible Disclosure & Recognition

I immediately documented the steps and submitted a detailed report to the program’s triage team.

✅ The issue was acknowledged and resolved within a week.
✨ Additionally, the bypass technique was highlighted and appreciated by Intigriti, one of Europe’s leading bug bounty platforms.

🧠 Key Takeaways

• 403 Forbidden ≠ Dead End. It’s often a starting point for deeper exploration.
• Subdomain enumeration combined with DNS analysis (e.g., CNAME lookups) can help uncover misconfigured access policies.
• Don't rely solely on automated tools—critical thinking and creativity often lead to the breakthrough.
• Always validate and report responsibly.

📬 Let’s Connect

Got similar stories or tricks you’ve used? Let’s share knowledge and grow together in the hacker community.

🔗 Twitter: @th3_d4rkninj4
🔗 LinkedIn: Syed Jan Muhammad Zaidi

Stay curious, stay ethical, and hack smart! 🛡️