Quick answers
- Is CVE-2026-49851 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-49851 being exploited in the wild.
- How severe is CVE-2026-49851?
- High severity, with a CVSS score of 7.5, published 24 June 2026.
- Which products and versions are affected?
- mistune (PyPI) < 3.3.0, ECOSYSTEM:introduced=0,fixed=3.3.0.
- Is there a fix for CVE-2026-49851?
- Yes. mistune (PyPI): upgrade to 3.3.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is CVE-2026-49851 attacked?
- It is attacked over the network, in a software library.
- Am I affected by CVE-2026-49851?
- Check whether you run mistune (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
How to fix
- mistune (PyPI): upgrade to 3.3.0 or later
pip install "mistune>=3.3.0"
Technical details
A denial-of-service vulnerability exists in Mistune's inline parser due to inefficient handling of link text. The root cause is a nested loop interaction between `InlineParser.parse()` and `parse_link_text()`: the outer loop advances only one character at a time when a link match fails, while the inner loop performs a full linear scan for a closing bracket. When processing a string with many consecutive '[' characters, this results in O(n²) quadratic-time complexity. An unauthenticated remote attacker can exploit this by submitting a small (~6 KB) Markdown payload to trigger high CPU utilization. The issue is fixed in version 3.3.0 by ensuring the parser skips ahead to the furthest scanned position after a failed match.
Affected products
- lepture mistune < 3.3.0
Identifiers
- CVE-2026-49851
- GHSA-qcq2-496w-v96p
- PYSEC-2026-2652
Timeline
- 2026-06-21: disclosed
- 2026-06-24: advisory: NVD publication
- 2026-07-09: patched: GitHub Advisory reviewed and updated with patch version 3.3.0