Mistune directives/include: mutual .. include:: recursion crashes the renderer with RecursionError, denial of service via two attacker-controlled markdown files

Exploitation

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

CVSS

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

EPSS

• CVE-2026-59927: 0.53% estimated exploitation activity in the next 30 days; percentile 44.81 (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: Uncontrolled recursion via mutual include. The Include directive checks for direct self-reference (a.md cannot include a.md), but does not detect indirect cycles. Two markdown files that include each other (a.md → includes b.md → includes a.md) cause unbounded recursion until Python's stack limit fires RecursionError. The exception propagates out of the renderer and crashes the calling code. File: src/mistune/directives/include.py, lines 33-37 (the self-include check is the only cycle-detection logic). Root cause: the include logic only compares os.path.abspath(dest) == os.path.abspath(sourcefile). There is no per-render set of "files already included" that would catch transitive cycles. When a.md includes b.md, the recursive block.parse(newstate) call uses dest (b.md) as the new file, which then includes a.md (passing the self-check, because the immediate parent file is b.md, not a.md), which then includes b.md, and so on. Each recursion level adds Python frames; the…

Published by the source

20 Jul 2026, 21:24 UTC

Last modified by the source

20 Jul 2026, 21:30 UTC

Source last checked

26 Oct 2026, 13:49 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-8mpj-m6qm-5qr8

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-8mpj-m6qm-5qr8

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# Patch Next advisory brief: CVE-2026-59927

Advisory: Mistune directives/include: mutual `.. include::` recursion crashes the renderer with `RecursionError`, denial of service via two attacker-controlled markdown files
IDs: CVE-2026-59927, GHSA-8mpj-m6qm-5qr8
Record: https://patchnext.com/advisories/cve-2026-59927 (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 5.3)
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-8mpj-m6qm-5qr8 (OSV record GHSA-8mpj-m6qm-5qr8 lists affected ranges: PyPI mistune >=0 <3.3.0 (fixed))
- Verified: https://github.com/advisories/GHSA-8mpj-m6qm-5qr8 (GitHub-reviewed advisory GHSA-8mpj-m6qm-5qr8 lists affected ranges: PyPI mistune >=0 <3.3.0)

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