6.1

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-44898

CVE-2026-44898: Mistune TOC Anchor Injection XSS

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.

(GitHub Advisory)

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CVE-2026-44898

CVE-2026-44898:

6.1

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

SSVC / BOD 26-04 ↗

Exploitation pocAutomatable NoTech Impact Partial

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Package Name	Ecosystem	Vulnerable Versions	First Patched Version
mistune	pip	= 3.2.0	3.2.1

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability lies within the render_toc_ul function in src/mistune/toc.py. This function generates an HTML table of contents from a list of heading items. The core of the vulnerability is in the line item = '<a href="#{}">{}</a>'.format(k, text), where k is the ID derived from the heading text and text is the visible label. The k variable was directly embedded into the href attribute without any HTML escaping. An attacker could craft a markdown heading where the text, when converted to an ID, contains malicious HTML. For example, a heading like ## x"><script>alert(1)</script> would result in the ID x"><script>alert(1)</script>. When render_toc_ul processes this, it generates a malformed <a> tag: <a href="#x"><script>alert(1)</script>">...</a>, which injects and executes the script in the browser. The patch 04880a0 addresses this by applying an escape() function to the k variable before it's inserted into the format string, ensuring that any special HTML characters are properly neutralized.

Vulnerable functions

render_toc_ul

src/mistune/toc.py

The function `render_toc_ul` constructs HTML for a table of contents. It uses a Python format string to create `<a>` tags. The `k` variable, which represents the heading ID and is used in the `href` attribute, was not escaped. This allows an attacker to provide a crafted heading ID that breaks out of the `href` attribute and injects arbitrary HTML, including script tags, leading to a cross-site scripting (XSS) vulnerability.

Vulnerability Intelligence
Miggo AI

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WAF Protection Rules

WAF Rule

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Reasoning

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