high CVSS 7.5 EPSS 0.41%

CVE-2026-59928·Python vulnerability

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown document containing many repeated or distinct reference-link definitions causes quadratic work in src/mistune/block_parser.py and the ref_links environment dictionary handling, allowing denial of service through CPU exhaustion. This issue is fixed in version 3.3.0.

Severity
high
Software
Python
Fixed in
3.3.0
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 0.41% — higher than 34% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2026-59928 scores 7.5 on CVSS, so a successful attack would be serious — but at 0.41%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

Until: 3.3.0

Fixed in: 3.3.0

How to fix this CVE

Update Python's Mistune Markdown parser to version 3.3.0 or later to resolve a denial-of-service vulnerability caused by inefficient reference-link definition processing. The vulnerability allows attackers to exhaust server CPU resources by submitting maliciously crafted Markdown documents with many repeated or distinct reference links. Apply this patch immediately if your application processes untrusted Markdown input.

sudo dnf update python3-mistune

Defensia detects this vulnerability

How to check if you are affected

  1. Check installed Mistune version: python3 -c "import mistune; print(mistune.__version__)"
  2. Verify if your application imports or uses mistune in the codebase: grep -r "import mistune" /path/to/app /usr/lib/python*/dist-packages/
  3. Search application logs for evidence of CPU spikes or timeouts coinciding with Markdown processing: grep -i "timeout\|cpu\|denial" /var/log/app.log
  4. Confirm the patch is applied by re-running: python3 -c "import mistune; print(mistune.__version__)" and verifying version is 3.3.0 or higher

FAQ

What is CVE-2026-59928?

CVE-2026-59928 is a denial-of-service vulnerability in Mistune versions prior to 3.3.0 where the block parser inefficiently handles reference-link definitions, causing quadratic algorithmic complexity that exhausts CPU when processing Markdown documents with many repeated or distinct reference links.

Is CVE-2026-59928 being actively exploited?

No, CVE-2026-59928 is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits have been disclosed, though the vulnerability is straightforward to exploit and should be treated with urgency.

What versions of Python are affected by CVE-2026-59928?

This vulnerability affects Mistune (the Markdown parser library) versions prior to 3.3.0, not Python itself. Any Python 2.x or Python 3.x installation with a vulnerable Mistune version installed is affected.

How do I check if my server is vulnerable to CVE-2026-59928?

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

Does Defensia detect CVE-2026-59928?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Mistune is installed on a monitored server, CVE-2026-59928 will appear in your dashboard with remediation steps.

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References

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