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CVE-2026-44898 — Mistune TOC Anchor Injection XSS

Affected: lepture mistune

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.

GHSA-6269-cqxg-mhhv — Mistune TOC Anchor Injection XSS

Affected: PyPI: mistune

## Summary `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 closely related to H2 (unescaped `id=` in `heading()`): the same `heading_id` callback pattern that triggers H2 also populates the `toc_items` list that `render_toc_ul()` consumes, meaning both vulnerabilities fire simultaneously in a typical documentation setup.

## Details **File:** `src/mistune/toc.py`

```python def render_toc_ul(toc): ... for level, k, text in toc: # k = heading id (used verbatim as href fragment) # text = heading text (used verbatim as link label) item = '<a href="#{}">{}</a>'.format(k, text) # Neither k nor text is passed through escape() at any point ```

The `k` and `text` values come directly from the `toc_items` list accumulated during parsing. If `k` contains `"` or `>`, the `href` attribute is broken. If `text` contains `<`, raw tags are injected as the visible link content.…

PYSEC-2026-2208

Affected: PyPI: mistune