Unhandled AssertionError in Python-Markdown 3.8 Causes Remote DoS

Executive Summary

[{'type': 'paragraph', 'content': 'CVE-2025-69534 is a vulnerability in Python-Markdown version 3.8 where malformed HTML-like sequences, specifically those containing the sequence "<![", cause the html.parser.HTMLParser to raise an unhandled AssertionError during Markdown parsing.'}, {'type': 'paragraph', 'content': 'Because Python-Markdown does not catch this exception, any application processing attacker-controlled Markdown may crash. This happens when the parser encounters malformed inputs that disrupt its handling of XML-style marked sections, leading to uncaught exceptions and crashes.'}, {'type': 'paragraph', 'content': 'The vulnerability was identified through fuzz testing and affects applications using the "extra" extension in Python-Markdown 3.8. It was fixed in version 3.8.1.'}] [1]

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Impact Analysis

This vulnerability can cause remote Denial of Service (DoS) in any application that parses untrusted Markdown, such as web applications, documentation systems, and CI/CD pipelines.

An attacker can send specially crafted Markdown inputs that trigger uncaught exceptions, causing the application to crash and become unavailable.

Additionally, there is a risk of information disclosure through stack traces if error details are exposed, potentially leaking sensitive information.

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Compliance Impact

I don't know

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Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability can be detected by testing if your Python-Markdown 3.8 installation with the "extra" extension enabled crashes or raises uncaught exceptions when parsing malformed Markdown inputs containing the sequence "<![".'}, {'type': 'paragraph', 'content': 'A practical detection method is to run a test script that feeds inputs like "<![", "<![>og))/uw_ f{tv+pAr$Ss+[6;^{=<:>g2oV|.pdTMu(Q-E#", or "<![ g\' \\" 7z5r7cojSO;2LAo0(1Vv5G>,-P" to the Markdown parser and observe if it crashes or throws exceptions.'}, {'type': 'paragraph', 'content': 'For example, you can use a Python script that imports markdown with the extra extension enabled and attempts to parse these inputs. If the parser crashes or raises an unhandled AssertionError, the vulnerability is present.'}, {'type': 'paragraph', 'content': 'No specific network commands are provided, but fuzz testing or targeted input testing on your Markdown processing service is recommended.'}] [1]

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Mitigation Strategies

[{'type': 'paragraph', 'content': 'The immediate mitigation step is to upgrade Python-Markdown from version 3.8 to version 3.8.1 or later, where this vulnerability has been fixed.'}, {'type': 'paragraph', 'content': 'Until the upgrade is applied, avoid processing untrusted Markdown inputs that may contain malformed HTML-like sequences such as "<![" to reduce the risk of Denial of Service.'}, {'type': 'paragraph', 'content': 'Additionally, consider implementing input validation or sanitization to filter out or escape suspicious sequences before parsing.'}] [1, 2]

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