Description
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 products
python-markdown: markdown
References10
- github.com/Python-Markdown/markdown/issues/1534 ExploitIssue Tracking
- github.com/Python-Markdown/markdown Product
- github.com/Python-Markdown/markdown/actions/runs/15736122892 Product
- www.openwall.com/lists/oss-security/2026/03/06/4 Mailing ListThird Party Advisory
- access.redhat.com/errata/RHSA-2026:10184
- access.redhat.com/errata/RHSA-2026:13508
- access.redhat.com/errata/RHSA-2026:13512
- access.redhat.com/errata/RHSA-2026:13826
- access.redhat.com/errata/RHSA-2026:14835
- access.redhat.com/errata/RHSA-2026:14873
Metrics
- CVSS 3.1
-
7.5
HIGH
Score source: NVD
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - EPSS — exploit probability
- 1% percentile: 52 · as of 12 November 2026
- CWE
- CWE-400CWE-617
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.