Mistune from 3.3.0 before 3.3.4 contains a denial of service vulnerability that allows remote attackers to exhaust CPU
Affects mistuneSource GCVE Database
Not known to be exploited · CVSS 8.7 High.
Data as of 3 Nov 2026, 16:27 UTC
Thin evidence
One scorer or none| CNA (VulnCheck) | CVSS 4.0 | 8.7 |
|---|---|---|
| CNA (VulnCheck) | CVSS 3.1 | 7.5 |
| CNA | CVSS 4.0 | 8.7 |
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:NExploitation evidence
Disclosed: evidence found.
CVE published 3 Nov 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors
A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 3 Nov 2026.
What to do
Affected software
| Product | Affected | Fixed in |
|---|---|---|
PyPImistunepkg:pypi/mistuneOther threats | Affected– | Fixed in– |
Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.
AI-suggested
Upgrade Mistune to version 3.3.4 or later where this vulnerability is fixed. Since the affected versions are before 3.3.4, applying this official fix will remediate the issue. No other mitigations are specified.
Description
Mistune versions from 3.3.0 up to but not including 3.3.4 contain a denial of service vulnerability. This flaw allows remote attackers to exhaust CPU resources by submitting specially crafted markdown documents that trigger quadratic-time processing in emphasis delimiter handling. The vulnerability causes an O(n²) index-fixup loop, which can stall workers for several seconds, impacting availability.
References
- github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9 (opens in a new tab)
- nvd.nist.gov/vuln/detail/CVE-2026-121437 (opens in a new tab)
- github.com/lepture/mistune/commit/b4dd3284e52ec741ed9a227db29ade9017828e90 (opens in a new tab)
- vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-process-emphasis-delimiters (opens in a new tab)