CVE-2026-121438
Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing
CVSS Score
5.3
EPSS Score
0.0%
EPSS Percentile
0th
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.
| CWE | CWE-400 |
| Vendor | lepture |
| Product | mistune |
| Published | Nov 3, 2026 |
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CVSS v3 Breakdown
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low
Affected Versions
lepture / mistune
0 < 3.3.3
References
github.com: https://github.com/lepture/mistune/security/advisories/GHSA-47p6-w2rc-5qf7 github.com: https://github.com/lepture/mistune/commit/fe02f40faf091f86da9ed62c865f31e11da111d7 vulncheck.com: https://www.vulncheck.com/advisories/mistune-before-3.3.3-denial-of-service-via-balanced-bracket-link-parsing
Credits
🔍 Str1ckl4nd