# What is XSS?

#### Saturday, April 25 2020

I found a cross-site scripting (XSS) attack in a well-known quiz hosting website. I disclosed the vulnerability to them years ago, so I thought now might be a good time to write about it.

In this first article I will explain what XSS is.

In the next article I will explain how I found this attack.

## What is cross-site scripting (XSS)

Cross-site scripting, XSS for short, is a technique to execute arbitrary Javascript code on a user visiting a website by linking to Javascript code stored on another server.

So for example:

I have a file on my website called hacked.js. If I was able to run this javascript file on anybody visiting a certain website that is not mine, this would be called cross-site scripting.

Click the above `hacked.js` link to view the code I use to “hack” this website. It’s safe, I promise ;)

Now, how can we get this code to execute when a user visits this site? To explain, I will start with some of the underlying technologies.

### Escape Characters!

No, this is not a Sherlock Holmes novel!

If we suppose that a website is built with sequences like these (called “tags”): `<body>`, `<p>` (for paragraph), `<link>` and `<b>` for bold, then why can you see the left and right angle bracket characters? Don’t they mean something? Shouldn’t they be telling the browser: “Hey! Make me bold!”? Why doesn’t everything after me typing `<b>` turn bold?

The answer is:

There are special characters in HTML to type a visible left (<) and visible right angle bracket (>) in a website. If I use the left and right brackets on my keyboard however, things will indeed show up bold.

This is the code for the sentence I wrote above:

    There are special characters in HTML to type a visible left (&lt;)
    and visible right angle bracket (&gt;) in a website.
    If I use the left and right brackets on my keyboard however,
    things will indeed <b>show up bold</b>.

Notice how all visible left angle brackets use an `&lt;` to show them?

These are called escape characters. They tell a system, in this case your web browser: “Hello! Please show me off! I don’t want to be hidden.”

#### Sanitization

Most of the time XSS attacks are done using poorly sanitized HTML `<input>` elements.

Sanitization is when a program (usually on the server side), will remove characters like `<` and replace them with the aforementioned “escape characters”. Internally this would be something like `&lt;`, but they would show up to a user as `<`.

When inputs are not properly sanitized and the input is shown to the user in another part of the website, then a malicous user can type in HTML that will run whenever anybody tries to look at what they typed. For example: a name for a quiz website (input) and the leaderboard for said quiz (display).

HTML, by itself is not very dangerous. The worst thing you could do is probably put a link on your name, and then point it to a porn site. Make your name bold, italic. Maybe make the background a funny color. Although this may annoy your victim it is not dangerous security wise.

There is one tag however, that is scary…

## `<script>`

The `<script>` tag allows you to write code that can:

1. Change the page contents.
2. Redirect the user to a new page automatically.
3. Get a user’s location.
4. Open a user’s microphone/webcam.
5. With the `src` attribute you can also load a script from another site. (This is XSS)

Those last two will ask for permission from the user (if their browser isn’t insanely insecure).

In my next article I’ll talk about a website I found which is vulnerable to this attack. And, show you how you can run your own XSS attack.