CVE-2026-59923: Mistune: XSS via percent-encoded javascript URI bypass in safe_url()

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

Details The vulnerability exists in HTMLRenderer.safeurl() in Mistune.

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

url = url.lower() if url.startswith(self.HARMFULPROTOCOLS): 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:

javascript%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 safeurl() protection mechanism.

PoC 1. Install vulnerable version:

pip install mistune==3.2.0

2. Run the following code:

import mistune

markdown = mistune.createmarkdown() 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.