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🗂️PortSwigger Lab Writeup: Web Shell Upload via Obfuscated File Extension

PortSwigger lab banner: Web shell upload via obfuscated file extension

🎯 Objective​

The objective of this lab is to exploit a file upload vulnerability where the app has a vulnerable image upload function which can be bypassed using obfuscation techniques. The goal is to upload a basic web shell and exfiltrate the contents of file /home/carlos/secret

  • Lab URL: https://portswigger.net/web-security/file-upload/lab-file-upload-web-shell-upload-via-obfuscated-file-extension
  • Category: File Upload
  • Difficulty: Practitioner

🧪 Exploitation Steps​

🕵️Step 1: Observe the Website​

  • Firstly open the lab URL in your browser, and observe what it is about and how it works. Blogging website homepage displaying blog posts and navigation menu
    Login page interface with username and password fields
  • At first glance, the website seems to be a blogging website with a login page. In the lab description, it is mentioned that we need to exfiltrate the secret of carlos.

📝Step 2: Upload the payload​

  • First login with the given credentials - wiener:peter to access file upload function. Login page with wiener credentials

  • Now, we can see the file upload function where we can upload an avatar of the user. User profile page with file upload form for avatar image

  • Make basic php web shell named payload.php with the given code:

    <?php echo file_get_contents('/home/carlos/secret'); ?>

  • Now, Upload this payload. File upload dialog showing payload.php file selected for upload

  • You will notice that the request was rejected as only jpg & png files are allowed to upload. Upload rejection message showing only image file extensions are permitted

  • Therefore, we will try to bypass this using Null Byte in filename.

  • Now, Open the HTTP History in Burpsuite and send the POST /my-account/avatar request to Repeater tab. Burp Repeater with Content-Type modified to image/png for MIME type bypass

  • Change the value of filename header from payload.php to payload.php%00.jpg and send the request. Burp Repeater with null byte obfuscation in filename for extension bypass

  • Hence, our web shell is successfully uploaded to the server.

🧑‍💼Step 3: Access the Secret​

  • Open the uploaded web shell at /files/avatars/payload.php Web shell executing and displaying carlos secret file content
  • Hence, our web shell executed successfully and returned the secret of carlos.
  • Now, Copy and submit it to complete the lab. Lab solved confirmation message after submitting the correct secret
  • And Finally, the Lab is solved.

🧠 Conclusion​

  • This lab demonstrated how null byte injection can obfuscate file extensions and bypass validation checks. By appending %00.jpg to the filename, the server processed it as a PHP file while validation checks only saw the jpg extension.
  • The impact is critical — attackers can bypass extension-based file validation using null bytes and other encoding tricks to upload executable code.
  • Fix: validate files based on content (magic bytes) not extensions, use strict parsing to reject encoded characters in filenames, and implement server-side execution restrictions.