Mistune Markdown Parser Denial of Service Vulnerability

Executive Summary

This is a denial of service vulnerability in the Mistune markdown parser affecting versions 3.3.0 to 3.3.3. It allows remote attackers to exhaust CPU resources by submitting specially crafted markdown documents with repeated patterns like '*a_b'. The vulnerability stems from an inefficient algorithm in the _process_emphasis_delimiters function that triggers an O(n²) index-fixup loop, causing significant delays.

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Detection Guidance

To detect this vulnerability, check if Mistune versions 3.3.0 to 3.3.3 are installed. Use commands like 'pip show mistune' or 'pip list | grep mistune' to verify the version. If vulnerable, monitor for unusual CPU usage spikes when processing markdown files.

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Impact Analysis

This vulnerability can cause severe performance degradation or complete service disruption by consuming excessive CPU resources. Attackers can stall systems for seconds with relatively small inputs (e.g., 64KB input takes ~8 seconds). It may lead to degraded user experience, system unavailability, or increased operational costs due to resource exhaustion.

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Mitigation Strategies

Upgrade Mistune to version 3.3.4 or later immediately. If upgrading is not possible, restrict untrusted input processing or implement rate limiting for markdown parsing requests to reduce attack surface.

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