6.1

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-44897

CVE-2026-44897: Mistune Heading ID Attribute Injection XSS

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, HTMLRenderer.heading() builds the opening <hN> tag by string-concatenating the id attribute value directly into the HTML — with no call to escape(), safe_entity(), or any other sanitisation function. A double-quote character " in the id value terminates the attribute, allowing an attacker to inject arbitrary additional attributes (event handlers, src=, href=, etc.) into the heading element. This vulnerability is fixed in 3.2.1.

(GitHub Advisory)

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CVE-2026-44897

CVE-2026-44897:

6.1

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

SSVC / BOD 26-04 ↗

Exploitation NoneAutomatable 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 exists in the mistune library because it fails to properly sanitize user-controllable input when generating HTML headings and table of contents. Specifically, the id attribute of a heading and the href attribute in the table of contents were constructed using string concatenation without proper escaping.

The analysis of the commits between the vulnerable version 3.2.0 and the patched version 3.2.1 revealed two key fixes.

In src/mistune/renderers/html.py, the heading method of the HTMLRenderer class was modified to use escape_text() on the _id variable before including it in the id attribute of the generated heading tag. This prevents attackers from breaking out of the id attribute and injecting malicious attributes.
In src/mistune/toc.py, the render_toc_ul function was updated to use escape() on the k variable (the heading ID) before inserting it into the href attribute of the anchor tag in the table of contents. This mitigates the risk of XSS through the table of contents links.

By exploiting this vulnerability, an attacker could inject malicious JavaScript code that would be executed in the context of the user's browser, leading to session hijacking, DOM manipulation, or other client-side attacks. The vulnerable functions HTMLRenderer.heading and render_toc_ul would appear in a runtime profile when the vulnerability is triggered.

Vulnerable functions

HTMLRenderer.heading

src/mistune/renderers/html.py

The 'id' attribute value is directly concatenated into the HTML string without proper escaping. An attacker can provide a malicious 'id' to inject arbitrary HTML attributes, leading to Cross-Site Scripting (XSS).

render_toc_ul

src/mistune/toc.py

The 'k' variable, representing the heading ID, is used to construct a 'href' attribute without escaping. This allows an attacker to inject malicious content into the 'href' attribute, potentially leading to XSS.

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Miggo AI

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WAF Protection Rules

WAF Rule

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Reasoning

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