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# lab-4

## SSRF with Whitelist-Based Input Filter

**Difficulty:** Expert

### Lab Description

This lab has a stock check feature which fetches data from an internal system. The developer has deployed an anti-SSRF whitelist defence that must be bypassed.

**Objective:** Change the stock check URL to access the admin interface at `http://localhost/admin` and delete the user `carlos`.

### Root Cause

The application performs improper URL validation by using ad hoc string matching instead of parsing the URL with a standards-compliant parser. Different components — such as the validation logic and the HTTP client — interpret the same URL differently, allowing a malicious URL to pass the whitelist check while the outbound request is made to an unintended destination.

### Exploitation

1. Identify the URL parameter that the server uses to make outbound requests.
2. Confirm that the application enforces a hostname whitelist.
3. Craft a URL that appears to match the whitelist but exploits URL parsing ambiguities — such as the userinfo component (`user@host`), double URL encoding, or parser discrepancies between the validation layer and the HTTP client.
4. The whitelist check accepts the crafted URL.
5. The HTTP client interprets the URL differently and sends the request to the intended internal destination (e.g. `http://localhost/admin`).
6. Access the admin panel from the response, retrieve the delete endpoint, and send a final request to delete `carlos`.

### Lesson Learned

* Never validate URLs using simple string operations such as `contains()`, `startsWith()`, or `endsWith()`.
* Parse the URL once using a standards-compliant parser, then validate the actual hostname and resolved IP address.
* Ensure the same parsing and decoding logic is used for both validation and the outbound request — discrepancies between the two are what make this attack possible.
* Validate the final destination after any redirects.
* Prefer strict allowlists combined with network-level egress restrictions over relying solely on application-layer URL validation.


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