Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing

Advisories

Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing

Severity: medium
Date: 11/3/2026
Affecting: mistune >= 0, < 3.3.3
CVE: CVE-2026-121438
CWE: CWE-400 Uncontrolled Resource Consumption
CVSS: 6.9
CVSS V4 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
References: GitHub Security Advisory (GHSA-47p6-w2rc-5qf7)
Patch Commit
Credit: Str1ckl4nd
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.

Ready to get Started?

Explore VulnCheck, a next-generation Cyber Threat Intelligence platform, which provides exploit and vulnerability intelligence to help you prioritize and remediate vulnerabilities that matter.

Vulnerability Prioritization
Prioritize vulnerabilities that matter based on the threat landscape and defer vulnerabilities that don't.
Early Warning System
Real-time alerting of changes in the vulnerability landscape so that you can take action before the attacks start.

Schedule a Demo