CVE-2026-121438 PUBLISHED

Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing

Assigner: VulnCheck
Reserved: 03.11.2026 Published: 03.11.2026 Updated: 03.11.2026

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.

CVSS 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
CVSS Score: 6.9

Exploitability Metrics		Vulnerable System Impact Metrics		Subsequent System Impact Metrics
Attack Vector	Network	Confidentiality	None	Confidentiality	None
Attack Complexity	Low	Integrity	None	Integrity	None
Attack Requirements	None	Availability	Low	Availability	None
Privileges Required	None
User Interaction	None

CVSS 4.0

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
CVSS Score: 5.3

Attack Vector	Network	Scope	Unchanged
Attack Complexity	Low	Confidentiality Impact	None
Privileges Required	None	Integrity Impact	None
User Interaction	None	Availability Impact	Low

CVSS 3.1

Product Status

Vendor	lepture
Product	mistune
Versions	Default: unaffected affected from 0 to 3.3.3 (excl.) Version 3.3.3 is unaffected

CVE-2026-121438 PUBLISHED

Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing

Metrics

Product Status

Credits

References

Problem Types