CVE-2026-59928: mistune — Exploit & Fix

Description

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown document containing many repeated or distinct reference-link definitions causes quadratic work in src/mistune/block_parser.py and the ref_links environment dictionary handling, 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/2b04d7ba341c16ac78fe82d3076bdd5c3de87c69

Exploits

https://github.com/lepture/mistune/security/advisories/GHSA-ffq3-xpv3-j92q https://github.com/lepture/mistune/security/advisories/GHSA-ffq3-xpv3-j92q

Other References

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

Frequently Asked Questions

What is CVE-2026-59928? +

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown document containing many repeated or distinct reference-link definitions causes quadratic work in src/mistune/block_parser.py and the ref_links environment dictionary handling, 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-2026-59928? +

CVE-2026-59928 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-2026-59928? +

CVE-2026-59928 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-2026-59928? +

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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