CVE-2026-59923: lepture Mistune XSS via percent-encoded bypass in safe_url

CVE-2026-59923 · Severity: medium · CVSS 6.1 · Published 2026-07-08

Mistune is a popular Python library used to convert Markdown text into HTML. A security flaw in its URL filtering mechanism allows attackers to bypass safety checks by using specially encoded links. If a user clicks on one of these malicious links, an attacker could execute arbitrary code in the user's browser, potentially leading to account theft or unauthorized access to sensitive data.

Technologies: mistune (PyPI). Vendors: PyPI.

Quick answers

Is CVE-2026-59923 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-59923 being exploited in the wild.
How severe is CVE-2026-59923?
Medium severity, with a CVSS score of 6.1, published 8 July 2026.
Which products and versions are affected?
mistune (PyPI) < 3.3.0, ECOSYSTEM:introduced=0,fixed=3.3.0.
Is there a fix for CVE-2026-59923?
Yes. mistune (PyPI): upgrade to 3.3.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-59923 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-59923?
Check whether you run mistune (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • mistune (PyPI): upgrade to 3.3.0 or later
    pip install "mistune>=3.3.0"

Technical details

A cross-site scripting (XSS) vulnerability exists in Mistune's HTMLRenderer.safe_url() function. The component attempts to block harmful protocols like 'javascript:' by checking the start of the URL string, but it fails to URL-decode the input before performing this check. An attacker can bypass the filter by using percent-encoding (e.g., 'javascript%3A') which is ignored by the startswith() check but later decoded and executed by the victim's browser. This allows for arbitrary JavaScript execution in the context of the application. The vulnerability is addressed in version 3.3.0.

Affected products

  • lepture mistune < 3.3.0

Identifiers

  • CVE-2026-59923
  • GHSA-8c25-4j27-2rv3
  • PYSEC-2026-2211

Timeline

  • 2026-06-21: disclosed
  • 2026-07-08: advisory: NVD publication date
  • 2026-07-20: advisory: GitHub Advisory published
  • 2026-07-20: patched: Version 3.3.0 released

References

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.