# CVE-2026-59922: lepture Mistune denial of service in formatting plugins

Source: https://junglewise.ai/threats/cve-2026-59922-lepture-mistune-denial-of-service-in-formatting-plugins  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-08. Updated: 2026-07-20  
License: CC BY 4.0 with a link back

CVE-2026-59922 · Severity: high · CVSS 7.5 · Published 2026-07-08

Mistune is a Python library used to convert Markdown text into HTML. A vulnerability in its formatting plugins (strikethrough, mark, and insert) allows an attacker to submit specially crafted text that causes the server to consume excessive CPU resources. This can lead to a denial-of-service (DoS) condition where the application becomes unresponsive to legitimate users.

Technologies: [mistune (PyPI)](https://junglewise.ai/threats/technologies/mistune). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-59922 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-59922 being exploited in the wild.

**How severe is CVE-2026-59922?** High severity, with a CVSS score of 7.5, published 8 July 2026.

**Which products and versions are affected?** mistune (PyPI) < 3.3.0, ECOSYSTEM:introduced=0,fixed=3.3.0.

**Is there a fix for CVE-2026-59922?** Yes. mistune (PyPI): upgrade to 3.3.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-59922 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-59922?** Check whether you run mistune (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- mistune (PyPI): upgrade to 3.3.0 or later

  ```sh
  pip install "mistune>=3.3.0"
  ```


## Technical details

The vulnerability is an algorithmic complexity flaw (CWE-407) in `src/mistune/plugins/formatting.py`. When the strikethrough, mark, or insert plugins are enabled, the parser performs a forward scan for matching closing delimiters (`~~`, `==`, or `^^`) for every opening delimiter encountered. In inputs with many repeated markers, this results in O(N²) complexity. An attacker can exploit this by sending a relatively small payload (e.g., 8-16 KB) that exhausts CPU resources. The issue is resolved in version 3.3.0 by implementing a cap on tracked markers or using a more efficient delimiter-stack algorithm.

## Affected products

- lepture mistune < 3.3.0

## Identifiers

- CVE-2026-59922
- GHSA-c8j7-8cv4-2xmq
- PYSEC-2026-2210

## Timeline

- 2026-06-21: disclosed
- 2026-07-08: advisory: NVD publication
- 2026-07-20: patched: GitHub Advisory reviewed and updated

## References

- [https://github.com/lepture/mistune/security/advisories/GHSA-c8j7-8cv4-2xmq](https://github.com/lepture/mistune/security/advisories/GHSA-c8j7-8cv4-2xmq)
- [https://github.com/lepture/mistune/commit/96d0f57f8fe9eeb06bb4cff521962a27d7c402e7](https://github.com/lepture/mistune/commit/96d0f57f8fe9eeb06bb4cff521962a27d7c402e7)
- [https://github.com/lepture/mistune/releases/tag/v3.3.0](https://github.com/lepture/mistune/releases/tag/v3.3.0)
- [https://github.com/pypa/advisory-database/tree/main/vulns/mistune/PYSEC-2026-2210.yaml](https://github.com/pypa/advisory-database/tree/main/vulns/mistune/PYSEC-2026-2210.yaml)
- [https://api.github.com/repos/lepture/mistune/security-advisories/GHSA-c8j7-8cv4-2xmq](https://api.github.com/repos/lepture/mistune/security-advisories/GHSA-c8j7-8cv4-2xmq)

## Related threats

- [Mistune path traversal and XSS in Include directive](https://junglewise.ai/threats/duplicate-advisory-mistune-path-traversal-and-xss-in-include-directive-bf25c187)
- [Mistune path traversal and XSS in Include directive](https://junglewise.ai/threats/cve-2026-115355-mistune-before-3-3-0-contains-a-path-traversal-vulnerability-in)
- [Mistune uncontrolled recursion in HTML rendering](https://junglewise.ai/threats/cve-2026-76098-mistune-uncontrolled-recursion-in-html-rendering)
- [lepture Mistune predictable ID generation in TOC plugin](https://junglewise.ai/threats/cve-2026-59930-lepture-mistune-predictable-id-generation-in-toc-plugin)
- [lepture mistune XSS via incomplete URL scheme filtering in HTML renderer](https://junglewise.ai/threats/cve-2026-59929-lepture-mistune-xss-via-incomplete-url-scheme-filtering-in-html)