high CVSS 7.5

CVE-2026-59925·Python vulnerability

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, long sequences of well-formed double-asterisk or triple-asterisk emphasis pairs around a character cause quadratic work in src/mistune/inline_parser.py because the parser scans forward for matching close markers from every potential opening run, allowing denial of service in default Mistune parsing. This issue is fixed in version 3.3.0.

Severity: high
Software: Python
Fixed in: 3.3.0
Published: 2026-07-08

Affected versions

Until: 3.3.0

Fixed in: 3.3.0

How to fix this CVE

Update Python's Mistune Markdown parser library to version 3.3.0 or later to resolve a denial-of-service vulnerability triggered by malformed emphasis syntax. This patch optimizes the inline parser to prevent quadratic time complexity when processing certain markdown patterns. Apply the update immediately if Mistune is used in production environments that accept user-supplied markdown content.

sudo dnf update python3-mistune

Defensia detects this vulnerability

How to check if you are affected

Run `pip list | grep mistune` or `python3 -c "import mistune; print(mistune.__version__)"` to identify the installed Mistune version on your system.
Search your application codebase for `import mistune` or `from mistune import` statements to confirm Mistune is a direct dependency.
Review application logs for repeated timeout errors, CPU spikes, or unresponsive markdown rendering that coincide with unusual input patterns containing multiple asterisks.
After updating to version 3.3.0+, re-run the version check command to confirm the patch is installed: `python3 -c "import mistune; print(mistune.__version__)"` should return 3.3.0 or higher.

FAQ

What is CVE-2026-59925?

CVE-2026-59925 is a denial-of-service vulnerability in Mistune, a Python Markdown parser, where deeply nested or long sequences of emphasis markers (double or triple asterisks) cause the parser to perform redundant scans, consuming excessive CPU resources and potentially hanging the application.

Is CVE-2026-59925 being actively exploited?

No, this vulnerability is not listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, and no public exploit code is currently available. However, the vulnerability can still be triggered by attackers or exploited in targeted scenarios.

What versions of Python are affected by CVE-2026-59925?

All versions of Mistune prior to 3.3.0 are affected. The vulnerability exists in the inline parser component regardless of the underlying Python version used to run Mistune.

How do I check if my server is vulnerable to CVE-2026-59925?

Execute `python3 -c "import mistune; print(mistune.__version__)"` to retrieve the installed version. If the version is below 3.3.0, your installation is vulnerable.

Does Defensia detect CVE-2026-59925?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Mistune is installed on a monitored server, CVE-2026-59925 will appear in your dashboard with remediation steps.

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References

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