Mistune TOC Anchor Injection XSS

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS

• CVE-2026-44898: 0.23% estimated exploitation activity in the next 30 days; percentile 13.65 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary rendertocul() 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 headingid callback pattern that triggers H2 also populates the tocitems list that rendertocul() consumes, meaning both vulnerabilities fire simultaneously in a typical documentation setup. Details File: src/mistune/toc.py The k and text values come directly from the tocitems list accumulated during parsing. If k contains "…

Published by the source

14 May 2026, 16:36 UTC

Last modified by the source

13 Jul 2026, 07:26 UTC

Source last checked

26 Oct 2026, 13:45 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-6269-cqxg-mhhv

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-6269-cqxg-mhhv

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# Patch Next advisory brief: CVE-2026-44898

Advisory: Mistune TOC Anchor Injection XSS
IDs: CVE-2026-44898, GHSA-6269-cqxg-mhhv
Record: https://patchnext.com/advisories/cve-2026-44898 (revision 2, source checked 26 Oct 2026, 13:45 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mistune 3.2.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI mistune: affected >= 3.2.0, <= 3.2.0; fixed in 3.2.1

## Suggested inspection (read-only)
- `python -m pip show mistune`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mistune.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-6269-cqxg-mhhv (OSV record GHSA-6269-cqxg-mhhv lists affected ranges: PyPI mistune >=3.2.0 <3.2.1 (fixed))
- Verified: https://github.com/advisories/GHSA-6269-cqxg-mhhv (GitHub-reviewed advisory GHSA-6269-cqxg-mhhv lists affected ranges: PyPI mistune >=3.2.0 <=3.2.0)

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