> 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-ssrf-lab/lab-3.md).

# lab-3

## SSRF with Blacklist-Based Input Filter

**Difficulty:** Practitioner

### Lab Description

This lab has a stock check feature which fetches data from an internal system. The developer has deployed two weak anti-SSRF defences 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

Some applications defend against SSRF by blacklisting known-bad values such as `127.0.0.1`, `localhost`, or `/admin`. However, these defences are fundamentally unreliable because the same destination can be represented in many valid forms that are not in the blacklist. For example, if `127.0.0.1` is blocked, alternatives such as `127.1` or `2130706433` (the decimal representation) may not be. Similarly, if the backend is case-insensitive, using `ADMIN` instead of `admin` may bypass a string-match filter, ultimately allowing access to the same internal infrastructure.

### Exploitation

1. Access any product page and find the **stock check** functionality. Click it and capture the request in Burp.
2. Find the `POST /product/stock` endpoint and send it to Repeater.
3. Change the `stockApi` parameter to `http://127.1/ADMIN` and send the request. This bypasses both the `localhost`/`127.0.0.1` block (using the shortened notation `127.1`) and the `/admin` block (using uppercase `ADMIN` on a case-insensitive server).
4. Open the response in the browser — the admin panel is accessible.
5. Click **Delete carlos** and note the URL path: `/admin/delete?username=carlos`.
6. Back in Repeater, set the `stockApi` parameter to:

   ```
   http://127.1/ADMIN/delete?username=carlos
   ```
7. Send the request — `carlos` is deleted and the lab is complete.

### Lesson Learned

* Blacklists are unreliable because the same destination can be represented in multiple valid ways — different IP notations, encoded inputs, hostnames, or redirects.
* Simply blocking strings like `127.0.0.1`, `localhost`, or `/admin` does not guarantee SSRF protection.
* Attackers can exploit differences in URL parsing, normalisation, DNS resolution, or redirect handling to reach blocked destinations.
* Allowlisting trusted destinations is far more effective than trying to enumerate every dangerous one.
* Applications should: normalise and validate the URL before applying security checks; resolve hostnames and verify the final IP is not loopback, private, or link-local; re-validate after following redirects or disable redirects where unnecessary; and restrict outbound network access at the infrastructure level so that even a successful SSRF cannot reach sensitive internal resources.


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