CVE-2026-59925 in Mistune
Summary
by MITRE • 07/08/2026
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, long sequences of well-formed double-asterisk or triple-asterisk emphasis pairs around a character cause quadratic work in src/mistune/inline_parser.py because the parser scans forward for matching close markers from every potential opening run, allowing denial of service in default Mistune parsing. This issue is fixed in version 3.3.0.
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Analysis
by VulDB Data Team • 07/08/2026
The mistune markdown parser vulnerability represents a classic example of a denial of service attack through algorithmic complexity exploitation. This flaw affects versions prior to 3.3.0 and specifically targets the inline parser component located in src/mistune/inline_parser.py. The vulnerability arises from how the parser handles emphasis markers, particularly when processing long sequences of well-formed double-asterisk or triple-asterisk emphasis pairs surrounding specific characters. The core technical issue stems from the parser's inefficient algorithmic approach where it scans forward for matching closing markers from every potential opening run, creating a quadratic time complexity scenario rather than the expected linear processing.
The operational impact of this vulnerability manifests as significant performance degradation and potential system unavailability during parsing operations. When maliciously crafted markdown content containing long sequences of emphasis markers is processed, the parser's computational requirements grow exponentially rather than linearly with input size. This behavior creates a denial of service condition where legitimate parsing operations can be disrupted or rendered unresponsive for extended periods. The vulnerability affects default parsing behavior without requiring any special configuration or advanced attack vectors, making it particularly concerning for applications that process user-generated content through mistune.
From a cybersecurity perspective, this vulnerability aligns with CWE-1333 which describes inefficient regular expression complexity and relates to the broader category of algorithmic complexity flaws. The issue also maps to ATT&CK technique T1499.004 which covers "Utilities: System Compromise" through resource exhaustion attacks. The quadratic behavior creates a predictable performance degradation pattern that can be exploited by attackers to consume excessive computational resources, potentially leading to service disruption across multiple applications utilizing mistune for markdown processing.
The fix implemented in version 3.3.0 addresses the root cause by optimizing the marker matching algorithm to avoid redundant scanning operations and reduce complexity from quadratic to linear time complexity. This remediation ensures that parsing operations maintain predictable performance characteristics regardless of input content structure. Organizations should prioritize upgrading to version 3.3.0 or later to protect against this vulnerability, while also implementing proper input validation and rate limiting measures for user-generated content processing. Additionally, monitoring systems should be configured to detect unusual parsing patterns that might indicate exploitation attempts, though the primary defense remains the software update addressing the underlying algorithmic flaw.
This vulnerability demonstrates the importance of careful algorithm design in security-critical components and highlights how seemingly innocuous parsing operations can become attack vectors when algorithmic complexity is not properly managed. The issue serves as a reminder that even well-established libraries require ongoing security scrutiny, particularly in components handling untrusted input data where performance characteristics directly impact system availability and user experience.