CVE-2026-59927 Mistune directives/include: mutual `.. include::` recursion crashes the renderer with `RecursionError`, denial of service via two attacker-controlled markdown files
An attacker can crash the Mistune Markdown renderer by providing two files that mutually include each other, causing unbounded recursion.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-59927?
CVE-2026-59927 is a medium-severity denial of service vulnerability in lepture mistune, published on 8 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the Include directive in src/mistune/directives/include.py detects only direct self-includes and not indirect cycles, allowing two markdown files that include each other to trigger unbounded recursion, raise RecursionError, and crash the rendering request. This issue is fixed in version 3.3.0.
Which versions are affected by CVE-2026-59927?
3 open source packages ship an affected version, including mistune. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
mistune PyPI | All versions | 3.3.0 |
mistune PyPI | All versions | 3.3.0 |
mistune PyPI | All versions | 3.3.0 |
Products include Mistune Project Mistune.
How do I fix CVE-2026-59927?
Upgrade mistune to 3.3.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-59927 being exploited?
There is no known exploitation of CVE-2026-59927 in the wild and no public exploit code.
Other vulnerabilities in lepture mistune
- CVE-2026-59922 Mistune plugins/formatting: quadratic-time parsing on long runs of `~~x~~`, `==x==`, and `^^x^^` markers (strikethrough / mark / insert)
- CVE-2026-59925 inline_parser: quadratic-time parsing on long runs of `**x**` and `***x***` emphasis pairs
- CVE-2026-59928 Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions
- CVE-2026-59923 Mistune: XSS via percent-encoded javascript URI bypass in safe_url()
- CVE-2026-59929 Mistune renderers/html.safe_url: HARMFUL_PROTOCOLS list misses legacy and chained schemes that historically chain to `javascript:` execution
Sources
Also known as DEBIAN-CVE-2026-59927, ECHO-4fd8-a362-a349, GHSA-8mpj-m6qm-5qr8, PYSEC-2026-2215, UBUNTU-CVE-2026-59927.