{"id":"CVE-2026-121438","title":"Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing","summary":"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…","severity":"medium","cvss":5.3,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","cvssSource":"cna","cwe":["CWE-400"],"vendor":"lepture","product":"mistune","affected":["mistune < 3.3.3"],"ssvc":{"exploitation":"poc","automatable":"yes","technicalImpact":"partial","timestamp":"2026-11-07T03:01:54.408797Z"},"exploitAvailable":true,"published":"2026-11-03","updated":"2026-11-07","sourceUpdated":"2026-11-07T03:02:22.098Z","source":"CVEORG","sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-121438","references":[{"url":"https://github.com/lepture/mistune/security/advisories/GHSA-47p6-w2rc-5qf7","label":"GitHub Security Advisory (GHSA-47p6-w2rc-5qf7)"},{"url":"https://github.com/lepture/mistune/commit/fe02f40faf091f86da9ed62c865f31e11da111d7","label":"Patch Commit"},{"url":"https://www.vulncheck.com/advisories/mistune-before-3.3.3-denial-of-service-via-balanced-bracket-link-parsing","label":"VulnCheck Advisory: Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing"}],"tags":["cve.org","exploit-available"],"epss":0.00291,"epssPercentile":0.21842,"ingestedAt":"2026-11-05T19:30:42.970Z","slug":"CVE-2026-121438","body":"## Overview\n\nMistune 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.\n\n## Affected\n\n- `mistune < 3.3.3`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"twilight","depthScore":41,"depthScoreParts":{"impact":29.2,"likelihood":0.1,"exploitation":12,"ransomware":0},"changes":[{"seq":235725,"id":"CVE-2026-121438","ts":1794020953168,"field":"exploit_available","old":"false","new":"true"}]}