CVE-2026-115349: Mistune 3.0.0 before 3.3.0 Denial of Service via Nested Bracket Parsing

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-22 · updated 2026-10-26
Weakness: CWE-1333

Description

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.

Affected products

lepture mistune: from 3.0.0 before 3.3.0 — fixed in 3.3.0

How to fix

Upgrade to a fixed version: lepture mistune 3.3.0. Check the vendor advisory for mitigations.

References

https://github.com/lepture/mistune/security/advisories/GHSA-3q64-rw38-243v
https://www.vulncheck.com/advisories/mistune-3.0.0-before-3.3.0-denial-of-service-via-nested-bracket-parsing

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