CVE-2026-59922

Mistune plugins/formatting: quadratic-time parsing on long runs of `~~x~~`, `==x==`, and `^^x^^` markers (strikethrough / mark / insert)

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.

Affected

lepture · mistune

CVSS v3.1 GITHUB

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

EPSS

0.64% chance of exploitation in the next 30 days, 50th percentile

Published

2026-07-08

Updated

2026-07-09

Proof-of-concept exploits (1)

• advisories/GHSA-c8j7-8cv4-2xmq

Security advisories

• lepture/mistune/security/advisories/GHSA-c8j7-8cv4-2xmq
• lepture/mistune/commit/96d0f57f8fe9eeb06bb4cff521962a27d7c402e7

References

• CVE Record
• NVD entry