Mistune Heading ID Attribute has 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-2026-44897: 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 HTMLRenderer.heading() builds the opening <hN tag by string-concatenating the id attribute value directly into the HTML — with no call to escape(), safeentity(), 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. The default TOC hook assigns safe auto-incremented IDs (toc1, toc2, …) that never contain user text. However, the addtochook() API accepts a caller-supplied headingid callback. Deriving heading IDs from the heading text itself — to produce human-readable slug anchors like #installation or #getting-started — is by far the most common real-world usage of this callback (every major documentation generator does this). When the callback returns raw heading text, an attacker who controls heading content can break out of the id= attribute. Details File: src/mistune/renderers/html.py The text body…

Published by the source

9 May 2026, 00:13 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:45 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-v87v-83h2-53w7

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-v87v-83h2-53w7

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# Patch Next advisory brief: CVE-2026-44897

Advisory: Mistune Heading ID Attribute has Injection XSS
IDs: CVE-2026-44897, GHSA-v87v-83h2-53w7
Record: https://patchnext.com/advisories/cve-2026-44897 (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; 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-v87v-83h2-53w7 (OSV record GHSA-v87v-83h2-53w7 lists affected ranges: PyPI mistune >=0 <3.2.1 (fixed))
- Verified: https://github.com/advisories/GHSA-v87v-83h2-53w7 (GitHub-reviewed advisory GHSA-v87v-83h2-53w7 lists affected ranges: PyPI mistune >=0 <=3.2.0)

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