Mistune: XSS via percent-encoded javascript URI bypass in safe_url()

Summary

An XSS vulnerability in Mistune allows bypassing of safe_url() protections via percent-encoded javascript URIs.

Details

The vulnerability exists in HTMLRenderer.safe_url() in Mistune.

The function is intended to block harmful URL schemes such as "javascript:" by checking the prefix of the provided URL:

 1_url = url.lower()
 2if _url.startswith(self.HARMFUL_PROTOCOLS):
 3    return "#harmful-link"

However, the input URL is not URL-decoded before this check. Because of this, an attacker can use percent-encoding to bypass the filter. For example:

 1javascript%3Aalert(1)

Since "%3A" is not decoded to ":", the check does not detect the "javascript:" scheme.

When rendered in a browser, the URL is decoded, resulting in execution of arbitrary JavaScript upon user interaction.

This effectively bypasses Mistune's built-in safe_url() protection mechanism.

PoC

1. Install vulnerable version:

   pip install mistune==3.2.0

2. Run the following code:

   import mistune

   markdown = mistune.create_markdown()
   html = markdown("j")

   print(html)

3. Output:

   <p><a href="javascript%3Aalert(1)">j</a></p>

4. Open the rendered HTML in a browser and click the link.

5. The browser decodes "%3A" into ":" and executes:

   javascript:alert(1)

Impact

This is a cross-site scripting (XSS) vulnerability.

An attacker can craft a malicious Markdown link that executes JavaScript in the victim's browser when clicked.

Impact includes:

• Session hijacking (e.g., cookie theft)
• Execution of arbitrary JavaScript in the victim's context
• Potential account takeover depending on the application

This affects any application that renders user-controlled Markdown using Mistune without additional URL sanitization.