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# lab-3

## File Path Traversal, Traversal Sequences Stripped Non-Recursively

**Difficulty:** Practitioner

### Lab Description

This lab contains a path traversal vulnerability in the display of product images. The application strips path traversal sequences from the user-supplied filename before using it.

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

### Root Cause

The application implements a blacklist-based defence that strips `../` from the input — for example:

```php
$file = str_replace("../", "", $file);
```

However, this replacement is performed only once and non-recursively. An attacker can bypass it by nesting the traversal sequence: `....//`. When `../` is stripped from the middle of `....//`, the remaining characters collapse back into `../`, reconstructing the traversal sequence after the filter has run.

### 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 nested traversal payload:

   ```
   ....//....//....//....//etc/passwd
   ```

   After the non-recursive strip removes each `../`, this resolves to `../../../../etc/passwd`.
5. Send the request — the contents of `/etc/passwd` are returned in the response.

### Lesson Learned

Non-recursive stripping of traversal sequences is trivially bypassed using nested sequences. The fix must either apply the strip recursively until no further matches are found, or — better — validate the fully resolved canonical path after any sanitisation to confirm it remains within the intended directory.


---

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