Python-Markdown has an Uncaught Exception
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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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | markdown | < 3.8.1 | 3.8.1 |
References
- https://github.com/Python-Markdown/markdown/actions/runs/15736122892
- http://www.openwall.com/lists/oss-security/2026/03/06/4
- https://github.com/Python-Markdown/markdown/issues/1534
- https://github.com/Python-Markdown/markdown
- https://github.com/advisories/GHSA-5wmx-573v-2qwq
- https://nvd.nist.gov/vuln/detail/CVE-2025-69534
- https://github.com/Python-Markdown/markdown/pull/1535
- https://github.com/pypa/advisory-database/tree/main/vulns/markdown/PYSEC-2026-89.yaml
- 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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