# Neonify

## Enumeration

Checking the website, we are greeted with a service that allows our input to be "neonified".

<figure><img src="/files/Ee5urK5UKK1UoYFr3Anp" alt=""><figcaption></figcaption></figure>

Checking the Dockerfile, we see that we're using Ruby so this is most likely a Ruby on Rails web service.

<figure><img src="/files/IWglJILwqdL76J54dU7F" alt=""><figcaption></figcaption></figure>

## Finding the vulnerable sink

Checking the source code, we find that when we do a `POST` request, it gets the `neon` parameter from that request, and throws it into a regex which checks if its only alphanumeric. After that, it calls in ERB which is a templating language built for Ruby.

<figure><img src="/files/UdPLeQ0oGgebTSL9hxGt" alt=""><figcaption></figcaption></figure>

Looking for vulnerabilities on the source code, there's a[ StackOverflow post](https://stackoverflow.com/questions/577653/difference-between-a-z-and-in-ruby-regular-expressions) regarding the use of `^` and `$` in regex. Basically, these two characters only match up to a newline character so when the application receives an input such as `hello\n{malicious_input}`, the malicious input will not be inspected by the regex.&#x20;

What is instead recommended is to either use:

* &#x20;`\A` - which matches the beginning of the string.
* &#x20;`\z` which matches the end of a string.
* &#x20;`\Z` in some cases where you want to match the end of a string unless there's a newline character in which it'll try to match just before that.

We can see that in the code, it uses both `^` and `$`. Since this is being passed to an ERB function, we can then perform server-side template injection. [HackTricks](https://book.hacktricks.xyz/pentesting-web/ssti-server-side-template-injection#erb-ruby) has a good article on how we can exploit SSTI for ERB on Ruby.

## SSTI via neon parameter

As a proof-of-concept, we'll send this payload which tries to read `/etc/passwd`.

```
test\n<%= File.open('/etc/passwd').read %>
test%0A%3C%25%3D+File%2Eopen%28%27%2Fetc%2Fpasswd%27%29%2Eread+%25%3E
```

Testing it out, we see it works and we have code execution. It is now trivial to retrieve the flag.

<figure><img src="/files/9jHAUVjqalReG0PuEFDf" alt=""><figcaption></figcaption></figure>


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