Vulnerability · Published Nov 3, 2026 · Assigned by VulnCheck
CVE-2026-121437Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters
High 7.5
In plain English
A high-severity security flaw in mistune from lepture. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 50% of all known vulnerabilities).
Severity (CVSS 3.1)7.5High
Exploit odds (30 days)0.6%top 50.2% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known
Official description
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:14 | mistune | no fix listed | DEBIAN-CVE-2026-121437 |
| Ubuntu:22.04:LTS | mistune | no fix listed | UBUNTU-CVE-2026-121437 |
| Ubuntu:24.04:LTS | mistune | no fix listed | UBUNTU-CVE-2026-121437 |
| Ubuntu:26.04:LTS | mistune | no fix listed | UBUNTU-CVE-2026-121437 |
| Ubuntu:26.10 | mistune | no fix listed | UBUNTU-CVE-2026-121437 |
Affected products (as reported)
Weakness: CWE-407 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
References
- GitHub Security Advisory (GHSA-qp42-r9j6-83g9) vendor-advisory
- Patch Commit patch
- VulnCheck Advisory: Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters third-party-advisory