Medium

Python-Markdown has an Uncaught Exception

  • Disclosed: 05.03.2026
  • Updated: 11.10.2026 15:17

Python-Markdown version 3.8 contain a vulnerability where malformed HTML-like sequences can cause html.parser.HTMLParser to raise an unhandled AssertionError during Markdown parsing. Because Python-Markdown does not catch this exception, any application that processes attacker-controlled Markdown may crash. This enables remote, unauthenticated Denial of Service in web applications, documentation systems, CI/CD pipelines, and any service that renders untrusted Markdown. The issue was acknowledged by the vendor and fixed in version 3.8.1. This issue causes a remote Denial of Service in any application parsing untrusted Markdown, and can lead to Information Disclosure through uncaught exceptions.

Affected software

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PlatformPackageVulnerable versionsFixed in
Python (PyPI) markdown < 3.8.1 3.8.1

References

CVSS score 7.5/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness (CWE)
CWE-248, CWE-400
Platform
Python (PyPI)
Package
markdown
Disclosed
05.03.2026
Updated
11.10.2026 15:17

Sources and licenses

OSV / GitHub Advisory Database - PYSEC-2026-89 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-5wmx-573v-2qwq - CC BY 4.0

Vulnerability history: markdown

All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.

This is the only known vulnerability of this package.

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