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Vulnerabilities

Vulnerability · CVE-2026-121438

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

Published Nov 3, 2026 · catalog updated Nov 4, 2026 · Mistune before 3.3.3 contains

Last checked Nov 4, 2026. A connected source was read within the last 7 days.

Check if you run it

No confirmed exploitation is on record for CVE-2026-121438. EPSS is 0.29%. Act only if a listed version of Mistune before 3.3.3 contains is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (Mistune before 3.3.3 contains).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Medium5.3
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.29%
Remediation
Advisory link
Affected products
Debian:12 mistune · Debian:13 mistune · Debian:14 mistune

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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • Debian:12 mistune2.0.4-1, 2.0.4-2, 3.0.2-1
  • Debian:13 mistune3.1.3-1, 3.1.4-1, 3.3.4-1
  • Debian:14 mistune3.1.3-1, 3.1.4-1, 3.3.4-1

How to fix CVE-2026-121438

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-121438. EPSS is 0.29%. Act only if a listed version of Mistune before 3.3.3 contains is installed.

Which versions are affected?

Compare what you have installed with 2.0.4-1, 2.0.4-2, 3.0.2-1, 3.1.3-1, 3.1.4-1, 3.3.4-1. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

No weakness class is stored. EPSS 0.29%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Technology hubs

Debian security

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. Nov 3, 2026 · OpenSSF OSV / OSV Scanner

    Detected

    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.

  2. Nov 4, 2026 · OpenSSF OSV / OSV Scanner

    Updated

    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.

Sources

  • CVE.orgVulnerability recordCVE-2026-121438

    Official CVE identifier

    cve.org
  • OpenSSF OSV / OSV ScannerPackage advisoryDEBIAN-CVE-2026-121438
    osv.dev
  • Trivy Vulnerability DBPackage advisoryDEBIAN-CVE-2026-121438

    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 l

    osv.dev

References

Freshness

Last verified in this catalog Nov 4, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.

  • CVE.orgNov 4, 2026
  • Trivy Vulnerability DBNov 4, 2026
  • OpenSSF OSV / OSV ScannerNov 4, 2026