> For the complete documentation index, see [llms.txt](https://ganesha-hk.gitbook.io/cybersecurity-writeups/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ganesha-hk.gitbook.io/cybersecurity-writeups/port-swigger-file-upload-labs/lab-3.md).

# lab-3

## Web Shell Upload via Path Traversal

**Difficulty:** Practitioner

### Lab Description

This lab contains a vulnerable image upload function. The server is configured to prevent execution of user-supplied files within the upload directory, but this restriction can be bypassed by exploiting a secondary vulnerability.

**Objective:** Upload a basic PHP web shell and use it to exfiltrate the contents of `/home/carlos/secret`. Submit the secret using the button in the lab banner.

**Credentials:** `wiener:peter`

### Root Cause

The server correctly prevents PHP scripts from executing inside the upload directory — any `.php` file stored there is served as plain text. However, the `filename` parameter in the multipart upload request is vulnerable to path traversal. By supplying `../php.php` as the filename, the attacker can cause the server to store the script one directory above the upload folder, where execution restrictions may not apply. The script is then directly accessible and executable, leading to arbitrary command execution and remote code execution.

### Exploitation

1. Log in as `wiener:peter` and navigate to the account page.
2. Create a file named `php.php` with the following content:

   ```php
   <?php echo file_get_contents('/home/carlos/secret'); ?>
   ```
3. Upload `php.php` as the avatar and capture the request in Burp Suite.
4. Find the `POST /my-account/avatar` request and send it to Repeater.
5. In the multipart body, change the `filename` parameter from `php.php` to `../php.php`.
6. Send the request — the file is stored in the parent directory where scripts can be executed.
7. Navigate to the account page, open the avatar in a new tab, and adjust the URL path to point to the uploaded file's location in the parent directory. The secret is returned in the response.
8. Submit the secret to complete the lab.

### Lesson Learned

A restricted upload directory is only effective if the `filename` parameter is also validated. User-controlled filenames must be sanitised to remove path traversal sequences before the file is stored. Even if the upload directory blocks execution, path traversal can move a malicious file to a directory where execution is permitted.


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