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TI-2026-84319 Aliases CVE-2026-121437GHSA-xr4w-hw52-gjv9

Quadratic-time denial of service in Mistune emphasis processing

Priority HIGH 71CWE-407Published · Updated

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 · High
Fix status
Partial Red Hat: 1 without a fix

Attack profile from CVSS 4.0 · CNA. Fix status from CNA and Red Hat. Impact type from CWE-407.

Summary

Mistune from 3.3.0 before 3.3.4 contains a denial of service flaw in its emphasis delimiter processing. Inefficient algorithmic complexity (CWE-407) makes index fixing quadratic, so CPU use grows sharply with input size. A remote unauthenticated attacker can submit small markdown documents with repeated patterns to exhaust CPU and stall workers for seconds.

Written with Claude Haiku 5.5 from the sources below · 2026-11-04

Description from the CNA

Mistune from 3.3.0 before 3.3.4 contains a denial of service vulnerability that allows remote attackers to exhaust CPU through quadratic-time emphasis delimiter processing. Attackers can submit small markdown documents with repeated patterns like "*a_b" to trigger an O(n²) index-fixup loop, stalling workers for seconds.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00619 49th percentileScored 2026-11-07
Fix Partial mistune (lepture): 3.3.4Red Hat: 1 without a fix

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
No
Technical impact
Partial

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

Affected products

VendorProductAffected versionsFixed inStatusSource
lepturemistune3.3.0 to before 3.3.43.3.4Patch availableCNA
  • mistune Vendorlepture Affected 3.3.0 to before 3.3.4 Fixed in 3.3.4 Patch available CNA

CPE names Threatist (inferred) · 1

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

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

Remediation

Upgrade lepture mistune to 3.3.4, which fixes this issue. Red Hat rates this Important and lists Red Hat OpenShift AI (RHOAI) as affected with no fix plan available.

Written with Claude Haiku 5.5 from the sources below · 2026-11-04

  • Fix · CNA mistune (lepture) Fixed in 3.3.4

Vendor assessments

Red Hat

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

ProductStatusPlan or fix
Red Hat OpenShift AI (RHOAI)AffectedNo fix yet
Not affected: 2 products
  • Red Hat OpenShift AI (RHOAI)
  • Red Hat OpenShift Container Platform 4

Timeline

  1. 2026-11-03 Threatist record created from CVE-2026-121437
  2. 2026-11-03 Alias added: GHSA-xr4w-hw52-gjv9
  3. 2026-11-03 CISA SSVC decision points changed
  4. 2026-11-03 Priority changed: MEDIUM → HIGH
  5. 2026-11-04 Affected versions changed · Red Hat
  6. 2026-11-04 CVSS score added · Red Hat
  7. 2026-11-04 Affected versions changed · Red Hat
  8. 2026-11-04 Affected versions changed · Red Hat

References

Patch 1

Advisory 1

  • 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