medium CVSS 6.1 EPSS <0.1%

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

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at <0.1% — higher than 9% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

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.

Related Python CVEs

CVE-2026-25632CVSS 10EPyT-Flow is a Python package designed for the easy generation of hydraulic and water quality scenario data of water distribution networks. Prior to 0.16.1, EPyT-Flow’s REST API parses attacker-controlled JSON request bodies using a custom deserializer (my_load_from_json) that supports a type field. When type is present, the deserializer dynamically imports an attacker-specified module/class and instantiates it with attacker-supplied arguments. This allows invoking dangerous classes such as subprocess.Popen, which can lead to OS command execution during JSON parsing. This also affects the loading of JSON files. This vulnerability is fixed in 0.16.1.
CVE-2026-34938CVSS 10PraisonAI is a multi-agent teams system. Prior to version 1.5.90, execute_code() in praisonai-agents runs attacker-controlled Python inside a three-layer sandbox that can be fully bypassed by passing a str subclass with an overridden startswith() method to the _safe_getattr wrapper, achieving arbitrary OS command execution on the host. This issue has been patched in version 1.5.90.
CVE-2026-33054CVSS 10Mesop is a Python-based UI framework that allows users to build web applications. Versions 1.2.2 and below contain a Path Traversal vulnerability that allows any user supplying an untrusted state_token through the UI stream payload to arbitrarily target files on the disk under the standard file-based runtime backend. This can result in application denial of service (via crash loops when reading non-msgpack target files as configurations), or arbitrary file manipulation. This vulnerability heavily exposes systems hosted utilizing FileStateSessionBackend. Unauthorized malicious actors could interact with arbitrary payloads overwriting or explicitly removing underlying service resources natively outside the application bounds. This issue has been fixed in version 1.2.3.
CVE-2026-28505CVSS 10Tautulli is a Python based monitoring and tracking tool for Plex Media Server. Prior to version 2.17.0, the str_eval() function in notification_handler.py implements a sandboxed eval() for notification text templates. The sandbox attempts to restrict callable names by inspecting code.co_names of the compiled code object. However, co_names only contains names from the outer code object. When a lambda expression is used, it creates a nested code object whose attribute accesses are stored in code.co_consts, NOT in code.co_names. The sandbox never inspects nested code objects. This issue has been patched in version 2.17.0.
CVE-2026-25115CVSS 9.9n8n is an open source workflow automation platform. Prior to version 2.4.8, a vulnerability in the Python Code node allows authenticated users to break out of the Python sandbox environment and execute code outside the intended security boundary. This issue has been patched in version 2.4.8.

References

Track CVEs across your fleet automatically

Defensia scans your Linux servers and tells you exactly which ones are running vulnerable versions — including CVE-2026-44898. Free for 1 server.

Get started free