CVE-2026-59927 in mistune
Summary
by MITRE • 07/08/2026
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the Include directive in src/mistune/directives/include.py detects only direct self-includes and not indirect cycles, allowing two markdown files that include each other to trigger unbounded recursion, raise RecursionError, and crash the rendering request. This issue is fixed in version 3.3.0.
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Analysis
by VulDB Data Team • 07/08/2026
The mistune python markdown parser vulnerability represents a classic recursive denial of service condition that can be exploited through improper cycle detection in its include directive implementation. This flaw exists specifically within the src/mistune/directives/include.py module where the system fails to properly identify indirect inclusion cycles between markdown files. The vulnerability manifests when two or more markdown files contain include directives that reference each other in a circular dependency pattern, creating an unbounded recursion scenario that ultimately results in a python RecursionError and complete rendering failure.
The technical implementation of this vulnerability stems from insufficient cycle detection logic within the include directive processing mechanism. Prior to version 3.3.0, the system only checked for direct self-includes where a file attempts to include itself, but completely overlooked the more complex scenario of indirect cycles where file A includes file B and file B includes file A. This oversight creates an infinite recursion loop during the markdown parsing process as each included file triggers further inclusion processing without proper termination conditions. The vulnerability directly maps to CWE-674 - Uncontrolled Recursion which is categorized under the broader family of software weaknesses related to improper resource management and control flow handling.
From an operational perspective this vulnerability presents a significant risk to any application or service that relies on mistune for markdown processing, particularly those accepting user-generated content or allowing arbitrary file inclusion. An attacker could craft malicious markdown documents containing circular include directives to crash the rendering process, effectively creating a denial of service condition that impacts legitimate users and potentially disrupts entire applications. The impact extends beyond simple service disruption as this vulnerability can be exploited in web applications where markdown processing occurs server-side, making it particularly dangerous in multi-user environments where one malicious user could affect others.
The fix implemented in version 3.3.0 addresses this issue by enhancing the cycle detection mechanism to properly identify both direct and indirect inclusion cycles. This improvement ensures that when a recursive inclusion pattern is detected during processing, the system terminates the parsing operation gracefully rather than allowing infinite recursion to consume system resources. Security practitioners should note that this vulnerability highlights the importance of proper resource management in parsing libraries and demonstrates how seemingly innocuous features like file inclusion can become attack vectors when cycle detection is inadequate. The mitigation strategy involves updating to version 3.3.0 or later, which provides comprehensive protection against both direct and indirect inclusion cycles while maintaining backward compatibility with existing functionality.
This vulnerability also aligns with ATT&CK technique T1499.004 - Network Denial of Service by demonstrating how improper input validation in parsing components can lead to resource exhaustion through recursive operations. The issue serves as a reminder that even well-established libraries require continuous security review, particularly when handling user-provided content or implementing features that involve file system operations and dynamic content inclusion. Organizations using mistune should implement proper dependency management practices to ensure timely updates and maintain security posture against similar recursion-based vulnerabilities in other parsing or processing components.