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<title>The "Quiz Your Friends" XSS Exploit | tait.tech</title>
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<h1 class="post-title">The "Quiz Your Friends" XSS Exploit</h1>
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<time datetime="" class="post-date"></time>
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</header>
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<hr>
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<h2 id="todo-write-intro">TODO write intro</h2>
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<h2 id="how-i-found-this-exploit">How I Found This Exploit</h2>
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<p>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 <code class="language-plaintext highlighter-rouge">Inspect Element</code>ed a few things together,
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so it was a safe bet that an HTML joke would let them know.</p>
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<p>I decided to write my name like so: <code class="language-plaintext highlighter-rouge"><b>Steve</b></code>.
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Steve is in reference to the <a href="https://minecraft.gamepedia.com/Player">main character</a> in the video game Minecraft.</p>
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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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<p>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:</p>
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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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<p>The text “Steve” showed up <strong>in bold</strong> 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.</p>
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<h3 id="server-side-validation">Server-Side Validation</h3>
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<p>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.</p>
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<p>Quiz your friends uses the <code class="language-plaintext highlighter-rouge">maxlength=20</code> 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 <code class="language-plaintext highlighter-rouge"><script></code> 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 <a href="https://code.golf/">code golf</a> that, I know I couldn’t.</p>
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<p>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 <code class="language-plaintext highlighter-rouge">maxlength</code> attribute to anything I want.
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Let’s change it to 100!</p>
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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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<p>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 <em>could</em> 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 <em>no such checks in place</em>.
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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?</p>
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<h4 id="hacking">Hacking:</h4>
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<p>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 <code class="language-plaintext highlighter-rouge">alert()</code> to warn the user about this site.</p>
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<p><code class="language-plaintext highlighter-rouge">JAVASCRIPT_NAME.JPG</code></p>
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<p>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?</p>
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<p>The unfortunate truth is: this worked as well!</p>
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<p>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.</p>
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<p>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.</p>
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<h2 id="executing-an-exploit">Executing An Exploit</h2>
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<p>Suppose we’re on a vulnerable site like <a href="https://www.quizyourfriends.com/">Quiz Your Friends</a>
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and you decide you want to hack your friend’s quiz!
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How can this be done?</p>
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<h4 id="creating-a-quiz">Creating A Quiz</h4>
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<p>Here is my quiz below:</p>
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<p><code class="language-plaintext highlighter-rouge">CREATING_QUIZ.IMG</code></p>
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<h4 id="setting-a-name-with-an-html-tag">Setting A Name With an HTML Tag</h4>
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<p>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.</p>
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<p><code class="language-plaintext highlighter-rouge">BOLD_ITALIC_STEVE.JPG</code></p>
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</article>
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This page is mirrored on <a href="https://beta.tait.tech/2020-04-27-quiz-your-friends-xss/">beta.tait.tech</a>.
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