ReDoS in Mistune Markdown Parser via Quadratic Parsing

Executive Summary

CVE-2026-59925 is a denial-of-service vulnerability in the Mistune Python Markdown parser versions 3.2.1 and earlier. The issue arises because the parser processes long sequences of double-asterisk or triple-asterisk emphasis pairs (like **x** or ***x***) in a way that causes quadratic-time complexity. Specifically, the parser scans forward from every potential opening emphasis marker to find matching closing markers, resulting in excessive computational work.

This inefficient scanning leads to significant CPU consumption when processing specially crafted input with many repeated emphasis pairs, allowing an attacker to cause a denial of service by making the parser spend a long time processing the input.

The vulnerability is fixed in Mistune version 3.3.0 by changing the parsing approach to avoid this quadratic behavior.

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Compliance Impact

The provided information does not specify any direct impact of this vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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Impact Analysis

This vulnerability can impact you by allowing an attacker to cause a denial of service (DoS) on systems using vulnerable versions of the Mistune Markdown parser. By sending specially crafted Markdown input with long sequences of emphasis markers, the attacker can cause the parser to consume excessive CPU resources.

This can lead to service disruption, slowdowns, or crashes in applications that rely on Mistune for Markdown parsing, potentially affecting availability.

The vulnerability requires no authentication and affects the default configuration, making it easier for attackers to exploit remotely.

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Detection Guidance

This vulnerability can be detected by identifying if your system is running Mistune versions 3.2.1 or earlier, which are affected by the denial-of-service issue caused by quadratic-time parsing of long sequences of emphasis pairs.

To detect exploitation attempts or presence of the vulnerability, you can monitor for unusually high CPU usage when processing Markdown content, especially when the input contains long sequences of double-asterisk or triple-asterisk emphasis pairs.

There are no specific commands provided in the resources, but general detection steps include:

• Check the installed Mistune version with a Python command such as: `pip show mistune` or `pip freeze | grep mistune`.
• Monitor CPU usage during Markdown parsing to detect spikes that could indicate exploitation.
• Use network or application logs to identify requests containing long sequences of '**' or '***' repeated many times.

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Mitigation Strategies

The immediate mitigation step is to upgrade Mistune to version 3.3.0 or later, where this vulnerability has been fixed.

If upgrading is not immediately possible, consider limiting or sanitizing Markdown input to prevent long sequences of double-asterisk or triple-asterisk emphasis pairs that trigger the quadratic parsing behavior.

Additionally, monitoring and rate-limiting requests that contain suspicious Markdown payloads can help reduce the risk of denial-of-service attacks exploiting this vulnerability.

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