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

Affected: Homebrew: adr-viewer

### 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:

_url = url.lower() if _url.startswith(self.HARMFUL_PROTOCOLS): 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 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](javascript%3Aalert(1))")

print(html)

3. Output:

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.…

EUVD-2026-334015

fkie_cve-2026-59923

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, HTMLRenderer.safe_url() does not block percent-encoded javascript URIs, allowing attacker-supplied Markdown links or images to bypass URL protections and execute script in rendered HTML. This issue is fixed in version 3.3.0.

GHSA-8c25-4j27-2rv3 — Mistune: XSS via percent-encoded javascript URI bypass in safe_url()

Affected: PyPI: mistune