# CVE-2026-121438: Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing

Status: published. Published: 2026-11-03. Updated: 2026-11-03.

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.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/lepture/mistune/commit/fe02f40faf091f86da9ed62c865f31e11da111d7

## Affected products and versions

- lepture / mistune (mistune): ≥ 0; < 3.3.3 (affected, semver); 3.3.3 (unaffected, semver). Source: CNA: VulnCheck

## Severity and prioritization

- CVSS 4.0: 6.9 (medium); 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; source: CNA: VulnCheck

- CVSS 3.1: 5.3 (medium); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L; source: CNA: VulnCheck

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-400

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/lepture/mistune/security/advisories/GHSA-47p6-w2rc-5qf7 (CNA: VulnCheck; vendor-advisory)

- https://github.com/lepture/mistune/commit/fe02f40faf091f86da9ed62c865f31e11da111d7 (CNA: VulnCheck; patch)

- https://www.vulncheck.com/advisories/mistune-before-3.3.3-denial-of-service-via-balanced-bracket-link-parsing (CNA: VulnCheck; third-party-advisory)

## Provenance

- CNA: VulnCheck: https://www.cve.org/CVERecord?id=CVE-2026-121438; retrieved 2026-11-03; source updated 2026-11-03

Revision: c4ce2689a6d4d5f75dd0098f3244b8519d02e28fcf908cb1f7549902efae5656

Canonical: https://y2.dev/cve/CVE-2026-121438

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