Inefficient Algorithmic Complexity Affecting mistune package, versions [3.3.0,3.3.4)
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-PYTHON-MISTUNE-22190629
- published4 Nov 2026
- disclosed3 Nov 2026
- creditzx, manus-use
Introduced: 3 Nov 2026
NewCVE-2026-121437 (opens in a new tab)How to fix?
Upgrade mistune to version 3.3.4 or higher.
Overview
Affected versions of this package are vulnerable to Inefficient Algorithmic Complexity in the _process_emphasis_delimiters function in inline_parser.py. An attacker can cause quadratic CPU growth that pins a worker by submitting Markdown with a long sequence of emphasis delimiters, because each time a matched run is collapsed the parser rewrites the index of every remaining delimiter in a full array sweep, making total work grow with the square of the delimiter count. The impact is denial of service only, against any application that parses untrusted Markdown with the default parser.