--- title: "What is XSS?" layout: post --- 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](/assets/js/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"): ``, `

` (for paragraph), `` and `` 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 `` 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 `<` to show them? These are called [escape characters](https://en.wikipedia.org/wiki/Escape_character). 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 `` 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 `<`, 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... ## `