Medium severity5.3NVD Advisory· Published Nov 3, 2026· Updated Nov 3, 2026

CVE-2026-121438

Description

Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that allows attackers to cause quadratic CPU growth by supplying balanced nested brackets. Attackers can submit crafted Markdown with thousands of nested bracket pairs not forming a valid link, triggering repeated _contains_nested_link scans that exhaust CPU in parsing applications.

Affected products

Authlib/Mistunellm-create
Range: <3.3.3

Patches

Vulnerability mechanics

References

github.com/lepture/mistune/commit/fe02f40faf091f86da9ed62c865f31e11da111d7nvd
github.com/lepture/mistune/security/advisories/GHSA-47p6-w2rc-5qf7nvd
www.vulncheck.com/advisories/mistune-before-3.3.3-denial-of-service-via-balanced-bracket-link-parsingnvd

News mentions

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cvss	0.345
epss	0.000
exploit	0.000
kev	0.000
patch	-0.070
ransomware	0.000