Ep. 6 – 403 Bypass & Request Smuggling: Tiny Tricks, Total Takeover

A single uppercase letter unlocked an admin panel. One malformed request hijacked user sessions.
In this episode, we break down two real-world exploits—a 403 bypass and a request smuggling attack—that turned small oversights into full system compromise. Learn how they worked, why they were missed, and what should have been done differently.

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Chapters

• 0:00 INTRO
• 1:18 FINDING #1 – The 403 Bypass That Led to Full Admin Control
• 8:17 FINDING #2 – Smuggling Requests, Hijacking Responses
• 16:35 OUTRO

Transcript

INTRO

What if one tiny URL change, something like just flipping a single letter, could turn a restricted admin page into an open door? No zero-days or fancy exploits—just one small tweak, and suddenly, you’ve got full admin control.

And what if you didn’t need to bypass authentication at all? just a single carefully crafted request that tricks the backend into processing user data the wrong way. One smuggled request, and suddenly, you’re hijacking sessions and injecting scripts.

I’m Amin Malekpour, and you’re listening to Hacked & Secured: Pentest Exploits & Mitigations.

Today, we’re breaking down two real-world exploits—a 403 forbidden error bypass that led to full admin control and a request smuggling attack that let an attacker hijack user sessions and inject malicious responses.

FINDING #1 – The 403 Bypass That Led to Full Admin Control

Picture this. You try to visit an admin page—the kind that should be locked down, and as expected, you get hit with a 403 Forbidden error. No way in. Security is doing its job, right? Well, not quite. Because with one tiny tweak—just changing a single letter in the URL—you bypass it completely. No crazy tricks, no fancy code, just that one small change, and boom, you’re in. Full admin access.

That’s exactly what Viktor Mares documented in his write-up on Medium.com. A 403 error bypass that turned a locked admin page into an open door leading to a full website takeover.

And here’s the best part—this wasn’t some secret zero-day or advanced exploit. No deep hacking skills were required. Just persistence, creativity, and testing things most people assume won’t work.

Alright, let’s break it down.

Viktor was doing a black-box pentest—which means no insider knowledge, no logins or credentials. Just him, a domain, and some subdomains to play with. His first move was to hunt for those sneaky admin panels hiding around and he found one fast. When he tried to access it, a 403 Forbidden error hits him. A 403 error usually means:

• The page is restricted—maybe only accessible to logged-in users.
• There’s IP allowlisting—so only specific IPs can get in.
• Server-level access controls are blocking it.

Sound locked down, right? But if you’ve done pentesting for a while, you know better—403 error doesn’t always mean “game over.” Sometimes it just means, “try a little harder!”

403 bypass tricks are nothing new. So Viktor started testing the usual bypassing techniques:

• Swapping HTTP methods like GET to POST to HEAD.
• Adding spoofed headers like X-Forwarded-For or X-Original-IP.
• Messing with the URL, trying things like /admin/./index.html or /admin?.
• Even flipping HTTP versions to see if that would do the trick.

None of them worked. Security controls holding strong.

But then, he remembered something.

Some access control systems rely on case-sensitive URL matching—meaning they only block exact lowercase versions of a URL.

So Viktor tried something simple. He typed /admin in lowercase and he got a 403 Forbidden response. Then he typed /Admin with an uppercase A, and boom—he got a 200 OK response and just like that, the admin page loaded. The trick was to make the A in the /admin uppercase. Creative, right?

So why did this work? Simple—the system blocked only lowercase /admin but treated /Admin with an uppercase A like a totally different path. It should’ve been case-insensitive. URLs shouldn’t care about uppercase or lowercase letters. But it did. So now, Viktor had a login page right there in front of him. The only problem was that he still needed credentials to get past it.

Bypassing 403 error was one thing. But getting past the login was another challenge.

A properly secured admin panel should have one of the following controls in place:

• Username enumeration protection
• Rate limiting to stop brute-force attacks
• A Web Application Firewall (WAF) to detect rapid login attempts

But this admin panel had none of those controls in place.

Next, Viktor did what any good pentester would do in this situation—he fired up Burp Suite Intruder, loaded a huge list of leaked credentials, and let it run.

A few thousand tries later? A successful login response and just like that, he was inside the admin panel, with full admin privileges in his hands.

And here’s where things get really ugly.

This wasn’t just some basic dashboard with a few harmless settings. No, this was a fully unlocked admin system. Viktor could do whatever he wanted—modify user accounts, mess with the whole website configurations, or extract sensitive user data.

To prove the impact of the attack, he scraped the entire database. We’re talking about 39 thousand records containing names, emails, phone numbers, and addresses. All because of one simple access control mistake. One letter change. That’s all Viktor needed.

This attack should never have worked.

Security failed for two big reasons: the access control system relied on case-sensitive URL filtering, and then the admin login page had no brute-force protection, no rate limiting, nothing.

Here’s what should have been done differently to prevent this attack:

• Enforce proper server-side access control. Block all unauthorized paths to sensitive pages like admin panels, regardless of case sensitivity.
• Implement rate limiting on login pages to mitigate brute-force attacks. Cap it at something like 5 failed tries per minute, then lock the account or IP for a bit.
• Log and monitor bypass attempts.

If all three were in place? Viktor is stuck at 403. No admin access. No data. Nothing.

Fellow pentesters, here are your takeaways:

• Always test 403 restrictions—a blocked page doesn’t always mean no access.
• Try unusual bypass moves—flip those letters, try variations.
• Brute-force attacks still work—test for weak login protections.

Never assume a "Forbidden" error means you’re actually forbidden. One letter—that’s all it took.

FINDING #2 – Smuggling Requests, Hijacking Responses

The first attack was all about bypassing access controls. Just one letter change in a URL, and suddenly, an admin panel that should have been locked down was wide open.

But what if the next attack didn’t even need a login page? You send one HTTP request. Just one. No brute force. No authentication bypass. No direct vulnerability to exploit. But in seconds, you’ve hijacked user sessions, poisoned caches, and manipulated backend systems. 

That’s exactly what Albinowax uncovered and reported on HackerOne— an HTTP request smuggling attack so powerful it let him manipulate web infrastructure, compromising both commercial sites and high-security environments. Crazy, right? Let’s break it down.

Before we get into the attack, let’s quickly break down what HTTP request smuggling is—for anyone who’s not familiar with it. 

 Since HTTP/1.1, web apps can send multiple HTTP requests over one connection. usually, you got a reverse proxy sitting there in front of the backend server, passing requests back and forth between users and the internal systems. Everything should be synchronized.

But here’s the problem—if the frontend and backend don’t interpret HTTP headers the same way, things get messy. And when that happens, attackers are right there, ready to slip through the cracks.

By crafting a carefully manipulated HTTP request, an attacker can pull off some serious damage—injecting payloads into someone else’s request, altering responses, stealing credentials, even executing code on the server. And that’s exactly what happened here.

Alright, let’s get back to the story. Before Albinowax could exploit anything, he needed to find a weak spot—a setup where the frontend and backend didn’t parse requests the same way. So, he sent a simple test payload: he started with POST request  HTTP/1.1," then added the following headers: "Host: example.com," followed by "Transfer-Encoding: chunked," and "Content-Length: 6." After that, he included a line with just number "0" and finished it off with a single "X" 

Now, here’s what happens.

If the frontend trusts the Content-Length header, but the backend relies on Transfer-Encoding, the backend never sees the termination chunk (0).

That means the backend thinks the request is incomplete. But instead of closing the connection, it waits— holding the attacker’s payload in memory, waiting for more data.

And that’s exactly what happened.

The backend server timed out, just sitting there waiting for that missing chunk. But when the next user sent a request?

Their request got processed with the attacker’s injected payload. 

Now that he could control how requests were processed, the next step was to pick an attack vector and one of the most dangerous ones is Session hijacking.

By injecting malicious headers into another user’s request, Albinowax tricked the system into sending authentication tokens and cookies to a domain he controlled. And we all know what that means, right?—full access to victim accounts.

But he didn’t stop there. He took it further—using request smuggling to poison web caches, injecting malicious JavaScript into the site’s CDN.

Now, every user who visited a certain page got that infected script, which executed directly inside their browser.

So how did Albinowax actually inject those malicious headers into someone else’s request? That’s probably what you’re wondering right now.

It’s all about that smuggling trick he used. He sent a request with both Content-Length and Transfer-Encoding headers— It’s like having two bosses giving different instructions on how big the request should be—but they don’t agree. The frontend looks at Content-Length and says, “Cool, it’s 6 bytes, done.” But the backend? It’s all about Transfer-Encoding, waiting for that “0” chunk to finish it off. They’re not on the same page. When the frontend and backend don’t sync up, the backend sits there confused, holding that connection open with Albinowax’s sneaky payload just sitting there, waiting to be processed. Then, an unsuspecting user sends a normal request, it gets tangled with that leftover payload.

Let’s break it down with an example—here’s what he sent first:
 He started with "POST / HTTP/1.1," added "Host: example.com," "Transfer-Encoding: chunked," and "Content-Length: 6." After that, he put a line with number "0," and then "X" on the next line.

Now, the frontend saw Content-Length: 6, counted the “0” line and stopped—6 bytes, done. But the backend? It saw Transfer-Encoding: chunked and waited for more chunks after that “0,” so it kept that "X" hanging around, not processed yet. Next user comes in, sends something normal—like this:
 They sent "GET /page HTTP/1.1," with "Host: example.com," and maybe "Cookie: user-session=abc123."

Backend grabs that leftover "X" from Albinowax, and appends it to the user’s request as a header.

that "X" could be anything nasty. When the server processed the request, it sent that injected data (which we call it X here) right back in the response. And the user’s browser? It executed it—no questions asked.

Just assume that “X” is a header like "Evil-Script: sendCookiesToattackerdomain()."
 The victim sends a normal GET request. But the backend, still holding that injected payload, appends the malicious script header to the response. 

The user’s browser executes sendCookiesToattackerdomain script, and just like that—cookies are stolen, session hijacked, and full account take over.

That’s how he did it—slipping his headers into someone else’s request by tricking the backend into holding his payload. Simple, sneaky, and brutal.

This attack should never have worked. Security failed for two big reasons: the frontend and backend servers parsed requests all different, and the system accepted malformed, conflicting requests instead of rejecting them.

Here’s what should have been done differently to prevent this attack:

•   Standardize request parsing—frontend and backend servers must process headers the same way.
•  Flag and reject requests with both Content-Length and Transfer-Encoding headers unless frontend and backend parse them the same way.
• Enforce strict header validation—only allow known, valid headers and block anything suspicious.
•  Protect user sessions—secure cookies with HttpOnly, Secure, and SameSite attributes to prevent session hijacking.
• Monitor for anomalies— security tools should detect and flag unusual request patterns, conflicting headers, or unexpected payloads.

Even one of these controls could have blocked the attack—or at least made it a lot harder.

Alright, fellow pentesters, here’s how you find vulnerabilities like this in your daily engagements:

• Test for request parsing inconsistencies—send requests with conflicting headers and see how different layers handle them.
•  Look for multi-layered architectures—if a site uses a reverse proxy, CDN, or load balancer in the mix, request smuggling may be possible.
•  Play with header obfuscation—use unusual spacing, encoding tricks, or malformed headers to evade detection.

 A web app or a proxy might be secure on their own. But when you put them together? That’s where vulnerabilities emerge. Real-world security isn’t just about finding vulnerabilities— it’s about understanding how layers interact and testing the assumptions developers believe are solid. 

OUTRO

Always remember—finding flaws is just the start. Responsibly disclosing them and getting them fixed is what makes the internet safer.

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Let’s make cybersecurity knowledge accessible to all. See you in the next episode.