Mistune toc / TableOfContents directive: heading IDs use predictable toc_N numbering with no slugification, allowing collision with attacker-controlled id="toc_N" content

Exploitation

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

CVSS

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

EPSS

• CVE-2026-59930: 0.19% estimated exploitation activity in the next 30 days; percentile 8.90 (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 Type: Predictable identifier generation. The toc plugin and TableOfContents directive both default to generating heading IDs of the form toc1, toc2, toc3, ... with no input-derived component. An attacker who can place a heading anywhere in the document can predict which tocN ID it will receive, and can inject HTML elsewhere (in a non-heading context) that uses the same id="tocN" to either (a) shadow the legitimate heading anchor, breaking same-page navigation, or (b) collide with CSS or JavaScript that targets #tocN selectors, redirecting click handlers and styling to attacker-chosen content. File: src/mistune/toc.py line 36-37 (headingid = lambda token, index: "toc" + str(index + 1)); src/mistune/directives/toc.py line 33 (same default). Root cause: the default headingid callback ignores the heading's text content and uses only the headings's order in the document. Two documents rendered together (or one document with attacker-influenced headings spliced into trusted…

Published by the source

20 Jul 2026, 21:35 UTC

Last modified by the source

20 Jul 2026, 21:46 UTC

Source last checked

26 Oct 2026, 13:49 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-2hm2-hc3v-44h9

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-2hm2-hc3v-44h9

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# Patch Next advisory brief: CVE-2026-59930

Advisory: Mistune toc / TableOfContents directive: heading IDs use predictable `toc_N` numbering with no slugification, allowing collision with attacker-controlled `id="toc_N"` content
IDs: CVE-2026-59930, GHSA-2hm2-hc3v-44h9
Record: https://patchnext.com/advisories/cve-2026-59930 (revision 2, source checked 26 Oct 2026, 13:49 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mistune 3.3.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI mistune: affected < 3.3.0; fixed in 3.3.0

## 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-2hm2-hc3v-44h9 (OSV record GHSA-2hm2-hc3v-44h9 lists affected ranges: PyPI mistune >=0 <3.3.0 (fixed))
- Verified: https://github.com/advisories/GHSA-2hm2-hc3v-44h9 (GitHub-reviewed advisory GHSA-2hm2-hc3v-44h9 lists affected ranges: PyPI mistune >=0 <3.3.0)

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