medium CVSS 6.1

CVE-2026-44898·Python vulnerability

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_toc_ul() builds a <ul> table-of-contents tree from a list of (level, id, text) tuples. Both the id value (used as href="#<id>") and the text value (used as the visible link label) are inserted into <a> tags via a plain Python format string — with no HTML escaping applied to either value. When heading IDs are derived from user-supplied heading text (the standard use-case for readable slug anchors), an attacker can craft a heading whose text breaks out of the href="#..." attribute context, injecting arbitrary HTML tags including <script> blocks directly into the rendered TOC. This vulnerability is fixed in 3.2.1.

Severity
medium
Software
Python
Fixed in
3.2.1
Published
2026-05-26

Affected versions

Until: 3.2.1

Fixed in: 3.2.1

How to fix this CVE

Update the Mistune Python markdown parser to version 3.2.1 or later to patch a cross-site scripting vulnerability in the table-of-contents renderer. This fix prevents attackers from injecting malicious HTML and JavaScript through specially crafted markdown headings. Verify the update is applied and test your markdown rendering pipeline to ensure normal functionality.

sudo dnf update python3-mistune

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST requests to markdown rendering endpoints containing heading text with escaped attribute delimiters (e.g., '# Test" onload="alert') or <script> tags in heading content that appear unescaped in the rendered HTML output

WAF mitigation (if patching is not yet possible)

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

Block or sanitize POST/PUT requests containing markdown with heading text that includes unescaped HTML special characters or script tags; implement strict output encoding on all markdown rendering endpoints to ensure TOC elements are properly HTML-escaped

How to check if you are affected

  1. Run 'python3 -m pip list | grep mistune' to check if Mistune is installed and identify the version
  2. If Mistune is installed, verify the version is 3.2.1 or higher by running 'python3 -c "import mistune; print(mistune.__version__)"'
  3. Search application logs for markdown rendering errors or unexpected HTML output, particularly logs from the render_toc_ul() function
  4. Confirm the patched version is active by running the version check command again after updating

FAQ

What is CVE-2026-44898?

This is an XSS vulnerability in Mistune's table-of-contents generation where heading IDs and text are not HTML-escaped before insertion into anchor tags, allowing attackers to inject arbitrary HTML and JavaScript through malicious markdown headings.

Is CVE-2026-44898 being actively exploited?

No, this vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits are currently available.

What versions of Mistune are affected by CVE-2026-44898?

All versions of Mistune prior to 3.2.1 are affected. Version 3.2.1 and later contain the fix.

How do I check if my server is vulnerable to CVE-2026-44898?

Run 'python3 -c "import mistune; print(mistune.__version__)"' and verify the version is 3.2.1 or higher. If it is lower, your installation is vulnerable.

Does Defensia detect CVE-2026-44898?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Mistune is installed on a monitored server, CVE-2026-44898 will appear in your dashboard with remediation steps.

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References

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