CVE-2026-121438

medium

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.

References

https://www.vulncheck.com/advisories/mistune-before-3.3.3-denial-of-service-via-balanced-bracket-link-parsing

https://github.com/lepture/mistune/security/advisories/GHSA-47p6-w2rc-5qf7

https://github.com/lepture/mistune/commit/fe02f40faf091f86da9ed62c865f31e11da111d7

https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-115860

Details

Source: Mitre, NVD

Published: 2026-11-03

Updated: 2026-11-03

Risk Information

CVSS v2

Base Score: 5

Vector: CVSS2#AV:N/AC:L/Au:N/C:N/I:N/A:P

Severity: Medium

CVSS v3

Base Score: 5.3

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Severity: Medium

CVSS v4

Base Score: 6.9

Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

Severity: Medium

EPSS

EPSS: 0.00291