8.7

CVE-2026-121437: Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters

Deferred
Published 03-11-2026Last modified 03-11-2026

Vulnerability summary

CVE-2026-121437 is a high (CVSS 8.7) Inefficient Algorithmic Complexity vulnerability in lepture mistune.

Priority
Prioritisesevere

Severe, CVSS 8.7 (CVE.org); no exploitation reported by the 5 checked sources; EPSS not yet published

Source freshness unavailable

How is this decided?
CVSS score
8.7/10
010
High · v4.0 (CVE.org)
EPSS scoreNot scoredRisk metrics not yet published.
KEVNot listedNot listed by any tracked catalog.
RansomwareNot flaggedNo ransomware use recorded.
Public exploits0No public exploits recorded

Timeline

  1. 03-11-2026
    PublishedVulnerability first disclosed.
  2. 03-11-2026
    Last modifiedVulnerability information updated.

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.

CVSS metrics

4 scores · 2 sources · 2 versions

EPSS trend

No EPSS score has been published for this record yet.

Known exploited

Compare KEV catalogs
CISA KEV

Not in the CISA KEV catalog.

ENISA CNW

Not in the ENISA CNW catalog.

CIRCL

Not in the CIRCL catalog.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Version check

Known conditions: network-reachable, no authentication, no user interaction (CVSS vector). Other conditions: not stated in public data.

Lepture1 product

1 vendor · 1 product

References

3 links