CVE-2026-59928

highCVSS 7.5covered by 2 sourcesfirst seen 2026-07-02

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.

CSIRTS triage

What: This vulnerability causes quadratic-time parsing issues on long lists of repeated reference-link definitions.
Who is affected: Deployments of Mistune are affected.
Urgency: Remediation is urgent due to the high severity of the vulnerability.
Action: Update to the latest version of Mistune to address this issue.

AI-assisted analysis generated from the source advisory — verify against the original.

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Exploitation outlook

Low exploitation risk0.37% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 29% of all EPSS-scored CVEs.

Advisory coverage (2)

highCVE-2026-59928: Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown doc…nvd · 2026-07-08
highCVE-2026-59928: Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definiti…msrc · 2026-07-02

External references

NVD record for CVE-2026-59928

CVE.org record

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