Vulnerability · Published Nov 3, 2026 · Assigned by VulnCheck

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

Medium 5.3

In plain English

A medium-severity uncontrolled resource consumption in mistune from lepture. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 22% of all known vulnerabilities).

Severity (CVSS 3.1)5.3Medium

Exploit odds (30 days)0.3%top 78.2% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official 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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	mistune	no fix listed	DEBIAN-CVE-2026-121438
Debian:13	mistune	no fix listed	DEBIAN-CVE-2026-121438
Debian:14	mistune	no fix listed	DEBIAN-CVE-2026-121438
Ubuntu:22.04:LTS	mistune	no fix listed	UBUNTU-CVE-2026-121438
Ubuntu:24.04:LTS	mistune	no fix listed	UBUNTU-CVE-2026-121438
Ubuntu:26.04:LTS	mistune	no fix listed	UBUNTU-CVE-2026-121438
Ubuntu:26.10	mistune	no fix listed	UBUNTU-CVE-2026-121438

Affected products (as reported)

Vendor	Product	Versions
lepture	mistune	0 – before 3.3.3

Weakness: CWE-400 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

References

GitHub Security Advisory (GHSA-47p6-w2rc-5qf7) vendor-advisory
Patch Commit patch
VulnCheck Advisory: Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing third-party-advisory