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The tool [**recollapse**](https://github.com/0xacb/recollapse) can generate variations from a given input to try to bypass the used regex. Check [**this post**](https://0xacb.com/2022/11/21/recollapse/) also for more information.
It might be possible that the server is **filtering the original request** of a SSRF **but not** a possible **redirect** response to that request.\
For example, a server vulnerable to SSRF via: `url=https://www.google.com/` might be **filtering the url param**. But if you uses a [python server to respond with a 302](https://pastebin.com/raw/ywAUhFrv) to the place where you want to redirect, you might be able to **access filtered IP addresses** like 127.0.0.1 or even filtered **protocols** like gopher.\
[Check out this report.](https://sirleeroyjenkins.medium.com/just-gopher-it-escalating-a-blind-ssrf-to-rce-for-15k-f5329a974530)
```python
#!/usr/bin/env python3
#python3 ./redirector.py 8000 http://127.0.0.1/
import sys
from http.server import HTTPServer, BaseHTTPRequestHandler
In short, the _backslash-trick_ relies on exploiting a minor difference between two “URL” specifications: the [WHATWG URL Standard](https://url.spec.whatwg.org/#url-parsing), and [RFC3986](https://datatracker.ietf.org/doc/html/rfc3986#appendix-B). RFC3986 is a generic, multi-purpose specification for the syntax of _Uniform Resource Identifiers_, while the WHATWG URL Standard is specifically aimed at the Web, and at URLs (which are a subset of URIs). Modern browsers implement the WHATWG URL Standard.
Both of them describe a way of parsing URI/URLs, with one slight difference. The WHATWG specification describes [one extra character](https://url.spec.whatwg.org/#authority-state), the `\`, which behaves just like `/`: ends the hostname & authority and starts the path of the URL.
image from [https://claroty.com/2022/01/10/blog-research-exploiting-url-parsing-confusion/](https://claroty.com/2022/01/10/blog-research-exploiting-url-parsing-confusion/)
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* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).
