CVE-2026-121438
Published:November 03, 2026
Updated:November 04, 2026
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
mistune (CONDA):
Affected version(s) >=2.0.0 <3.3.3 || >=0.5 <3.3.3 || >=3.0.0 <3.3.3Fix Suggestion:
Update to version 3.3.3https://github.com/lepture/mistune.git (GITHUB):
Affected version(s) >=v0.1.0 <v3.3.3 || >=v2.0.0a1 <v3.3.3 || >=v3.0.0a1 <v3.3.3Fix Suggestion:
Update to version v3.3.3mistune (PYTHON):
Affected version(s) >=2.0.0a1 <3.3.3 || >=0.1.0 <3.3.3 || >=3.0.0a1 <3.3.3Fix Suggestion:
Update to version 3.3.3Related Resources (5)
Do you need more information?
Contact UsCVSS v4
Base Score:
6.9
Attack Vector
NETWORK
Attack Complexity
LOW
Attack Requirements
NONE
Privileges Required
NONE
User Interaction
NONE
Vulnerable System Confidentiality
NONE
Vulnerable System Integrity
NONE
Vulnerable System Availability
LOW
Subsequent System Confidentiality
NONE
Subsequent System Integrity
NONE
Subsequent System Availability
NONE
CVSS v3
Base Score:
5.3
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
NONE
Integrity
NONE
Availability
LOW
Weakness Type (CWE)
Uncontrolled Resource Consumption
EPSS
Base Score:
0.29