medium CVSS 6.1

CVE-2026-59923·Python vulnerability

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.

Severity
medium
Software
Python
Fixed in
3.3.0
Published
2026-07-08

Affected versions

Until: 3.3.0

Fixed in: 3.3.0

How to fix this CVE

Update Python's Mistune markdown parser to version 3.3.0 or later to patch a URL sanitization bypass that allows percent-encoded JavaScript URIs to execute in rendered HTML. This vulnerability affects any application using Mistune to parse user-supplied Markdown content for web display. Verify your Mistune installation and apply the security update immediately if you're processing untrusted markdown.

sudo dnf update python3-mistune

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST requests containing markdown with percent-encoded javascript URIs in link/image syntax (e.g., `[text](%6A%61%76%61%73%63%72%69%70%74:alert(...))` or `![img](%6A%61%76%61%73%63%72%69%70%74:alert(...))`) followed by reflected HTML output containing decoded javascript: scheme

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement WAF rules to block HTTP requests containing markdown syntax with percent-encoded protocol handlers (%6a, %6A, %3a) in link/image attributes. Alternatively, enforce strict Content-Security-Policy headers with script-src 'none' on pages rendering untrusted markdown.

How to check if you are affected

  1. Check installed Mistune version: python3 -c "import mistune; print(mistune.__version__)"
  2. Identify applications using Mistune: grep -r "import mistune" /opt /home /srv --include="*.py" 2>/dev/null
  3. Search application logs for markdown rendering errors or XSS-related warnings in the past 90 days: grep -i "javascript\|xss\|unsafe" /var/log/application*.log | grep -i "url\|href\|src"
  4. Verify remediation by re-running: python3 -c "import mistune; print(mistune.__version__)" and confirming version >= 3.3.0

FAQ

What is CVE-2026-59923?

CVE-2026-59923 is a URL validation bypass in Mistune's HTMLRenderer that fails to block percent-encoded JavaScript URIs (e.g., %6A%61%76%61%73%63%72%69%70%74:). An attacker can craft malicious markdown links or images that decode to executable JavaScript when rendered in a browser.

Is CVE-2026-59923 being actively exploited?

No, this vulnerability is not currently listed on CISA's Known Exploited Vulnerabilities catalog and no public exploits are available, though the attack method is straightforward.

What versions of Python are affected by CVE-2026-59923?

CVE-2026-59923 affects Mistune versions prior to 3.3.0. The vulnerability exists in Mistune's URL sanitization function, not Python itself, so all Python versions running vulnerable Mistune releases are affected.

How do I check if my server is vulnerable to CVE-2026-59923?

Run `python3 -c "import mistune; print(mistune.__version__)"` and check if the version is below 3.3.0. If Mistune is not installed, the vulnerability does not apply to your environment.

Does Defensia detect CVE-2026-59923?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Mistune is installed on a monitored server, CVE-2026-59923 will appear in your dashboard with remediation steps.

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References

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