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82 lines
2.7 KiB
82 lines
2.7 KiB
---
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title: "The \"Quiz Your Friends\" XSS Exploit"
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layout: post
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---
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## TODO write intro
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## How I Found This Exploit
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While filling in one of my friend's surveys I thought it would be
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funny for them to know it is me without anyone else knowing.
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We were young and had `Inspect Element`ed a few things together,
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so it was a safe bet that an HTML joke would let them know.
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So I typed in my name like so:
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`BOLD_STEVE.JPG`
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Now in theory this should have shown in in the leaderboard as: "<b>Bold Steve</b>"
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However, to my horror and excitement, I saw this in the leaderboard:
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`BOLD_STEVE_LEADERBOARD.JPG`
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The text "Bold Steve" showed up **in bold** on the leaderboard.
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This told me all I needed to know. To add a `<script>` tag was next.
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#### Hacking:
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So I went on my merry way thinking about ways to use malicious javascript.
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Then, I thought that might be mean, so I decided to warn users instead.
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I filled in the name with a script tag and a call to `alert()` to warn the user about this site.
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`JAVASCRIPT_NAME.JPG`
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I ran out of room before I could finish it. Hmmm.
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What if I do "Inspect Element" and manually override the max-length attribute?
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The unfortunate truth is: this worked as well!
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Not only could I manually set the max-length by changing it in the HTML,
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but there were no client-side OR server-side checks to make sure the name I was sending was less than or equal to 20 characters.
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If Javascript checked it, it would have stopped me (although maybe not a professional).
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If the server checked it, it could have stopped almost anyone.
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##### Server-Side Validation
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As a side note, here is a great reason why you should do most of your validation on the server side.
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As a user, I can edit any of the HTML, CSS, or Javascript your server serves to me.
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Imagine trying to fit in a script tag doing anything useful with 20 characters?
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`<script src="http:">`
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There's your 20 characters! No more! Good luck trying to do anything useful with this!
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The Quiz My Friends server has *no such checks in place*.
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Therefore, I can send an almost arbitrary load to them.
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Being able to send something potentially very large (more than a few megabytes) is a vulnerability of its own.
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Imagine being able to send entire executable progrmas as your "name" in one of these quizes?
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## Executing An Exploit
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Suppose we're on a vulnerable site like [Quiz Your Friends](https://www.quizyourfriends.com/)
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and you decide you want to hack your friend's quiz!
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How can this be done?
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#### Creating A Quiz
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Here is my quiz below:
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`CREATING_QUIZ.IMG`
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#### Setting A Name With an HTML Tag
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Just like the image above, about how I found out about this vulnerability: go ahead and use an HTML tag in your name to test this out.
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`BOLD_ITALIC_STEVE.JPG`
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