CVE-2026-121438
Deferred Deferred - Pending Action

Mistune Denial of Service via Nested Bracket Parsing

Publication date: 2026-11-03

Last updated on: 2026-11-03

Assigner: VulnCheck

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.

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Meta Information

Published
2026-11-03
Last Modified
2026-11-03
Generated
2026-11-03
AI Q&A
2026-11-03
EPSS Evaluated
N/A
NVD
CVE-2026-121438
EUVD
EUVD-2026-115860

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
lepture mistune 0

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-400 The product does not properly control the allocation and maintenance of a limited resource.

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Executive Summary

Mistune before 3.3.3 has a denial of service vulnerability in InlineParser.parse_link where attackers can cause quadratic CPU growth by supplying Markdown with thousands of nested but unbalanced brackets. The parser inefficiently scans for nested links in these inputs, exhausting CPU resources in applications that process untrusted Markdown.

Detection Guidance

Check Mistune version with pip show mistune. If version is below 3.3.3, the system is vulnerable. Test by parsing crafted Markdown with nested brackets to observe CPU spikes.

Impact Analysis

This vulnerability can cause delays or complete failures in applications that parse untrusted Markdown input. Attackers can exploit it to consume excessive CPU resources, leading to degraded performance or denial of service in systems using affected Mistune versions.

Mitigation Strategies

Upgrade Mistune to version 3.3.3 or later immediately. Review applications using Mistune to ensure they do not pass untrusted Markdown input to the parser.

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