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CVE-2026-121437VulnerabilityPublished

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

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Authoritative record:CVE-2026-121437· High 8.7

Not known to be exploited · CVSS 8.7 High.

Data as of 3 Nov 2026, 16:27 UTC

CVSS 4.0 base score
8.7High
How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none
CVSS scores by source
CNA (VulnCheck)CVSS 4.08.7
CNA (VulnCheck)CVSS 3.17.5
CNACVSS 4.08.7
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 3 Nov 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq 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

ProductAffectedFixed in
PyPImistunepkg:pypi/mistuneOther threatsAffected–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

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