Answer in brief
CVE-2026-121437 records a High severity (CVSS 8.7) vulnerability in Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters. The current sources do not mark it as known exploited. The current feed maps lepture/mistune (generic). Check affected ranges and fixed versions before updating.
Analysis pending evidence review
HOL Guard separates source facts from reviewed analysis. See the methodology.
CVSS is 8.7. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.
Analysis status
Analysis pending evidence review
Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.
The current feed maps lepture/mistune (generic). Check affected ranges and fixed versions before updating.
| Package | Affected range | Fixed version |
|---|---|---|
| lepture/mistunegeneric | >=3.3.0 <3.3.4 | 3.3.4 |
Published upstream
Nov 3, 2026
Evidence: source:cvelist:source_dates:source-dates:recordSource modified
Nov 3, 2026
Evidence: source:cvelist:source_dates:source-dates:recordFirst seen by HOL
Nov 3, 2026
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.
Quoted source text, attributed separately from HOL analysis.