Inefficient Algorithmic Complexity Affecting mistune package, versions [3.0.0,3.3.3)
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.
Do your applications use this vulnerable package?
In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes.
Test your applications- Snyk IDSNYK-PYTHON-MISTUNE-22190621
- published4 Nov 2026
- disclosed3 Nov 2026
- creditLiyi, Ziyue, Str1ckl4nd, Maurice, Chenchen
Introduced: 3 Nov 2026
NewCVE-2026-121438 (opens in a new tab)How to fix?
Upgrade mistune to version 3.3.3 or higher.
Overview
Affected versions of this package are vulnerable to Inefficient Algorithmic Complexity in the parse_link method in inline_parser.py, via the _parse_link_text and _contains_nested_link helpers. An attacker can cause quadratic CPU growth by submitting Markdown containing a long run of balanced nested brackets (for example [[[...x...]]]), where each outer bracket drives a full re-scan of the inner text because the parser does not record positions already known not to start a link. The impact is denial of service only, and it affects applications that parse untrusted Markdown with the default parser.
Note: This is a bypass of the fix for the vulnerability described in CVE-2026-115347.