TI-2026-84320 Aliases CVE-2026-121438 GHSA-296h-qmjr-9mv2

Denial of service in Mistune via nested bracket link parsing

HIGH 70CWE-400Published 2026-11-03 · Updated 2026-11-07

Key facts

Attack vector: Network
Privileges required: None
User interaction: None
Attack complexity: Low
Attack requirements: None
Impact type: Denial of service
Confidentiality · integrity · availability: None · None · Low
Fix status: Partial Red Hat: 2 without a fix

Attack profile from CVSS 4.0 · CNA. Fix status from CNA and Red Hat. Impact type from CWE-400.

Summary

Mistune before 3.3.3 contains a denial of service flaw in InlineParser.parse_link. Balanced nested brackets that do not form a valid link cause CPU use to grow quadratically, a resource consumption problem (CWE-400). A remote unauthenticated attacker who submits crafted Markdown can exhaust CPU in applications that parse it, affecting availability.

Written with Claude Haiku 5.5 from the sources below · 2026-11-07

Description from the CNA

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.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV

EPSS 0.00291 21st percentileScored 2026-11-07

Fix Partial mistune (lepture): 3.3.3Red Hat: 2 without a fix

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: Poc
Automatable: Yes
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
lepture	mistune	before 3.3.3	3.3.3	Patch available	CNA

mistune Vendorlepture Affected before 3.3.3 Fixed in 3.3.3 Patch available CNA

CPE names Threatist (inferred) · 1

Inferred from how NVD named this product before; not given by a source.

CPE	Versions	Role	Vendor
cpe:2.3:a:mistune_project:mistune::::::::	—	—	lepture

cpe:2.3:a:mistune_project:mistune:*:*:*:*:*:*:*:* Versions — — ·lepture

Remediation

Update mistune to 3.3.3 or later. Red Hat OpenShift AI (RHOAI) and Red Hat OpenShift Container Platform 4 are affected with no fix plan available.

Written with Claude Haiku 5.5 from the sources below · 2026-11-07

Fix · CNA mistune (lepture) Fixed in 3.3.3

Vendor assessments

Red Hat

Red Hat rating: Moderate. Red Hat's page for this CVE →

Product	Status	Plan or fix
Red Hat OpenShift AI (RHOAI)	Affected	No fix yet: Fix deferred
Red Hat OpenShift Container Platform 4	Affected	No fix yet: Fix deferred

Timeline

2026-11-03 Threatist record created from CVE-2026-121438
2026-11-03 Alias added: GHSA-296h-qmjr-9mv2
2026-11-04 Affected versions changed · Red Hat
2026-11-04 CVSS score added · Red Hat
2026-11-05 Priority changed: MEDIUM → LOW
2026-11-07 CISA SSVC decision points changed
2026-11-07 Priority changed: LOW → HIGH

References

Patch 1

Patch Commit

Advisory 1

GitHub Security Advisory (GHSA-47p6-w2rc-5qf7)

TI-2026-79440 Path traversal in Mistune include directive lets attackers read files CVSS 4.0CRITICAL 9.2
TI-2026-36755 Mistune: Potential DoS via quadratic-time parsing in parse_link_text CVSS 4.0HIGH 8.7
TI-2026-25521 Mistune ReDoS in LINK_TITLE_RE allows denial of service with crafted Markdown titles CVSS 4.0HIGH 8.7
TI-2026-53257 Mistune has Denial of Service — RecursionError via Excessive Emphasis Markers in Markdown CVSS 3.1HIGH 7.5
TI-2026-42736 Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions CVSS 3.1HIGH 7.5
TI-2026-42730 Regular expression denial of service in lepture mistune CVSS 3.1HIGH 7.5

See all 30 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences