CVE-2026-121437

    Dashboard / Vulnerabilities / CVE-2026-121437

    CVE-2026-121437

    Published: 3 Nov 2026Last Modified: 5 Nov 2026

    Summary: Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters

    Details: 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.

    References: https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/121xxx/CVE-2026-121437.json, https://github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9, https://nvd.nist.gov/vuln/detail/CVE-2026-121437, https://www.vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-process-emphasis-delimiters, https://github.com/lepture/mistune/commit/b4dd3284e52ec741ed9a227db29ade9017828e90

    Affected packages

    Package

    Name:

    Purl:

    Affected ranges

    Type: GIT

    Events:

    Introduced- 15c3b79325125272263d7d42492ec8f757447191
    Fixed -69ec2b8a6d5b37ecb3c56d96df4b40b9d3004657

    Affected versions

    v3.3.3
    v3.3.2
    v3.3.1
    v3.3.0

    Common Vulnerability Scoring System

    Attack Vector
    Network
    Adjacent
    Local
    Physical
    Privileges Required
    None
    Low
    High
    User Interaction
    None
    Required
    Scope
    Unchanged
    Changed
    Confidentiality
    None
    Low
    High
    Integrity
    None
    Low
    High
    Availability
    None
    Low
    High
    CVE-2026-121437 | CVE-DB