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# lab-4

## File Path Traversal, Traversal Sequences Stripped with Superfluous URL-Decode

**Difficulty:** Practitioner

### Lab Description

This lab contains a path traversal vulnerability in the display of product images. The application blocks input containing path traversal sequences, then performs a URL-decode of the input before using it.

**Objective:** Retrieve the contents of the `/etc/passwd` file.

### Root Cause

The application (or a proxy layer in front of it) blocks path traversal sequences such as `../` and `....//`. However, the blocking check is performed on the raw, encoded input, and the application then URL-decodes the input before passing it to the filesystem.

By double URL-encoding the traversal sequence, the encoded form passes the filter unrecognised. The application then decodes one layer, and if a second decode occurs before the file is opened, the traversal sequence is fully reconstructed and the attack succeeds.

For example:

* `../` → single-encoded: `%2e%2e%2f` → double-encoded: `%252e%252e%252f`

The filter sees `%252e%252e%252f` and finds no traversal sequence. The application decodes it to `%2e%2e%2f`, then to `../` before opening the file.

### Exploitation

1. Access the lab, open any product page, and open the image in a new tab.
2. Capture the request in Burp Suite.
3. Find the URL endpoint: `GET /image?filename=43.jpg`
4. Replace the filename value with the double URL-encoded traversal payload:

   ```
   ..%252f..%252f..%252f..%252fetc%252fpasswd
   ```
5. Send the request — the contents of `/etc/passwd` are returned in the response.

### Lesson Learned

Security checks must be applied to the fully decoded, normalised input — not to the raw encoded form. Any input that will be decoded before use must first be fully decoded, then validated. A proxy or WAF that inspects the encoded request but forwards a decoded version to the application creates exactly this gap.


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