This challenge I decided to go for a cool challenge, marcodowno, in which we had to find XSS vulnerability that pops alert(1) without the user interaction. After successfuly doing it in our client, we had to paste the working solution in the form of URL into a service that would check it, and upon correct exploitation, grant us a flag.

Upon visiting a website, we can see a service that converts markdown input into a html code and views it.


If we view the source of the website, the main part that converts the page is a simple regexp one liner searching for markdown components and replacing them with html.

input = decodeURIComponent([^&#]+)/)[1]);

function markdown(text){
  text = text.replace(/[<]/g, '').replace(/----/g,'<hr>').replace(/> ?([^\n]+)/g, '<blockquote>$1</blockquote>').replace(/\*\*([^*]+)\*\*/g, '<b>$1</b>').replace(/__([^_]+)__/g, '<b>$1</b>').replace(/\*([^\s][^*]+)\*/g, '<i>$1</i>').replace(/\* ([^*]+)/g, '<li>$1</li>').replace(/##### ([^#\n]+)/g, '<h5>$1</h5>').replace(/#### ([^#\n]+)/g, '<h4>$1</h4>').replace(/### ([^#\n]+)/g, '<h3>$1</h3>').replace(/## ([^#\n]+)/g, '<h2>$1</h2>').replace(/# ([^#\n]+)/g, '<h1>$1</h1>').replace(/(?<!\()(https?:\/\/[a-zA-Z0-9./?#-]+)/g, '<a href="$1">$1</a>').replace(/!\[([^\]]+)\]\((https?:\/\/[a-zA-Z0-9./?#]+)\)/g, '<img src="$2" alt="$1"/>').replace(/(?<!!)\[([^\]]+)\]\((https?:\/\/[a-zA-Z0-9./?#-]+)\)/g, '<a href="$2">$1</a>').replace(/`([^`]+)`/g, '<code>$1</code>').replace(/```([^`]+)```/g, '<code>$1</code>').replace(/\n/g, "<br>");
  return text;


I immedietally went for something which can spawn the XSS without the user interaction, which is img tag. Let’s take a look at the part converting []() notation into a <img>.

.replace(/!\[([^\]]+)\]\((https?:\/\/[a-zA-Z0-9./?#]+)\)/g, '<img src="$2" alt="$1"/>')

As we can see, we can put any characters in the first brackets, while in the source brackets we have to have something starting with https://. We can check that with this.


After regexp finds an exact match, it will place the content of the second brackets just where $2 is, while content of first brackets inside $1 in the img src="$2" alt="$1"/>. As you can probably see already, we can put inside the second brackets " character to break out of alt, the use onerror=alert(1) to spawn the XSS.



After that we can submit the URL into a service and get the flag.