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🗂️PortSwigger Lab Writeup: Remote Code Execution via Web Shell Upload

PortSwigger lab banner: Remote code execution via web shell upload

🎯 Objective​

The objective of this lab is to exploit a file upload vulnerability where the app has a vulnerable image upload function which doesn't perform any validation before storing them. 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-remote-code-execution-via-web-shell-upload
  • Category: File Upload
  • Difficulty: Apprentice

🧪 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
    Upload confirmation message indicating successful file upload to server
  • 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 unvalidated file uploads can lead to remote code execution. By uploading a PHP web shell without any validation, we executed arbitrary code on the server and exfiltrated sensitive files.
  • The impact is critical — unrestricted file uploads allow attackers to execute arbitrary code and completely compromise the server.
  • Fix: implement file type validation (both extension and MIME type), store uploads outside web root, disable script execution in upload directories, and validate file contents server-side.