CVE-2026-59922: mistune — Exploit & Fix

Description

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a run of closed tilde, equals-sign, or caret marker pairs around a character causes quadratic work in src/mistune/plugins/formatting.py when the strikethrough, mark, or insert plugin scans for matching markers from each possible start position, allowing denial of service through CPU exhaustion. This issue is fixed in version 3.3.0.

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CVSS v3.1 Score

7.5

HIGH

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

EPSS — Exploit Prediction

0.0037

Probability of exploitation

0.29%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-407 CWE-407

CWE-1333 CWE-1333

Affected Products

Vendor	Product	Versions
mistune_project	mistune	< 3.3.0

References

Advisories & Patches

https://github.com/lepture/mistune/commit/96d0f57f8fe9eeb06bb4cff521962a27d7c402e7 https://github.com/lepture/mistune/security/advisories/GHSA-c8j7-8cv4-2xmq https://github.com/lepture/mistune/security/advisories/GHSA-c8j7-8cv4-2xmq

Exploits

https://github.com/lepture/mistune/security/advisories/GHSA-c8j7-8cv4-2xmq https://github.com/lepture/mistune/security/advisories/GHSA-c8j7-8cv4-2xmq

Other References

https://github.com/lepture/mistune/releases/tag/v3.3.0

Frequently Asked Questions

What is CVE-2026-59922? +

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a run of closed tilde, equals-sign, or caret marker pairs around a character causes quadratic work in src/mistune/plugins/formatting.py when the strikethrough, mark, or insert plugin scans for matching markers from each possible start position, allowing denial of service through CPU exhaustion. This issue is fixed in version 3.3.0. It has a CVSS v3.1 base score of 7.5 (HIGH).

How severe is CVE-2026-59922? +

CVE-2026-59922 has a CVSS v3.1 score of 7.5 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0037, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-59922? +

CVE-2026-59922 affects products from mistune_project, specifically: mistune. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-59922? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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