DoS via Deeply Nested Tokens in Mistune Markdown Parser

Executive Summary

This is a Denial of Service (DoS) vulnerability in the Mistune Python Markdown parser. It occurs when deeply nested emphasis tokens (like consecutive asterisks) are rendered into HTML. The recursive rendering process exceeds Python's recursion limit, causing a RecursionError that crashes the parsing process.

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Detection Guidance

Check Mistune version with pip show mistune. If version is between 3.3.0 and 3.3.2, the system is vulnerable. Test by parsing Markdown with 1000 consecutive asterisks using a Python script calling mistune.markdown() or mistune.html().

Monitor for RecursionError exceptions in application logs. Use Python's sys.getrecursionlimit() to check default recursion depth.

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Impact Analysis

An attacker can exploit this by submitting specially crafted Markdown with excessive nesting (e.g., 1,000 consecutive asterisks). This crashes the Python process handling the parsing, disrupting service availability for applications using vulnerable Mistune versions.

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Compliance Impact

This vulnerability primarily impacts service availability through DoS attacks, which may indirectly affect compliance with standards like GDPR or HIPAA that require data availability and system reliability. However, the CVE itself does not directly violate these regulations but could contribute to non-compliance if exploited to disrupt critical systems handling sensitive data.

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Mitigation Strategies

Upgrade Mistune to version 3.3.3 or later. Implement input validation to limit consecutive emphasis characters. Set max_emphasis_depth parameter to 20 or lower in InlineParser.

Apply request timeouts and higher recursion limits in deployed applications. Refactor HTML rendering to use iterative stack-based approach instead of recursion.

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