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# lab-4

## Web Shell Upload via Extension Blacklist Bypass

**Difficulty:** Practitioner

### Lab Description

This lab contains a vulnerable image upload function. Certain file extensions are blacklisted, but the defence can be bypassed due to a fundamental flaw in the blacklist configuration.

**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 application blacklists common PHP extensions (e.g. `.php`) but still allows users to upload server configuration files such as `.htaccess`. By uploading a malicious `.htaccess` file, an attacker can instruct Apache to treat a custom file extension (e.g. `.evil`) as executable PHP. Once the mapping is in place, a script with that extension is uploaded and executed.

```
Application
    │
    ▼
Allows uploading .htaccess (configuration file)
    │
    ▼
Apache honours the uploaded configuration
    │
    ▼
Custom extension treated as PHP
    │
    ▼
Code execution
```

### Exploitation

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

   ```
   AddType application/x-httpd-php .evil
   ```

   This tells Apache to execute files with the `.evil` extension as PHP.
3. Upload `.htaccess` as the avatar.
4. Create a second file named `php.evil` with the following content:

   ```php
   <?php echo file_get_contents('/home/carlos/secret'); ?>
   ```
5. Upload `php.evil` as the avatar.
6. Open the avatar in a new tab — Apache executes the `.evil` file as PHP and the secret is returned in the response.
7. Submit the secret to complete the lab.

### Lesson Learned

Never rely on extension blacklisting alone. Attackers can use alternate or custom extensions, or upload server configuration files like `.htaccess` to modify execution rules and bypass the blacklist entirely. Secure file upload requires: strict extension allowlisting, content-based file type validation, and ensuring the server never executes files from the upload directory regardless of extension.


---

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