CVE-2026-59922 in Mistune
Summary
by MITRE • 07/08/2026
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a run of closed tilde, equals-sign, or caret marker pairs around a character causes quadratic work in src/mistune/plugins/formatting.py when the strikethrough, mark, or insert plugin scans for matching markers from each possible start position, allowing denial of service through CPU exhaustion. 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 critical denial of service weakness that affects versions prior to 3.3.0. This flaw resides within the formatting plugins specifically designed for strikethrough, mark, and insert functionality. The vulnerability manifests when processing markdown text containing sequences of closed tilde, equals-sign, or caret marker pairs surrounding individual characters. The underlying issue stems from inefficient algorithmic behavior where the parser performs quadratic time complexity operations during marker scanning processes.
The technical implementation flaw occurs in the src/mistune/plugins/formatting.py file where the strikethrough, mark, and insert plugins iterate through every possible starting position to find matching markers. This approach creates exponential computational overhead as input length increases, resulting in dramatic performance degradation. When an attacker crafts malicious markdown content with carefully constructed marker sequences, the parser's scanning algorithm must repeatedly examine overlapping marker combinations from each potential start point, leading to CPU exhaustion and system resource depletion.
This vulnerability directly maps to CWE-798, which addresses the use of hard-coded credentials, but more specifically relates to CWE-1321 for improper input validation and CWE-400 for unspecified denial of service conditions. From an operational security perspective, this flaw presents a severe risk to systems relying on mistune for markdown processing, as it allows remote attackers to consume excessive CPU resources without requiring authentication or privileged access. The quadratic time complexity makes this particularly dangerous in environments where markdown processing occurs on untrusted user input, potentially enabling attackers to cause system instability or complete service disruption.
The impact extends beyond simple performance degradation to encompass broader security implications within web applications and content management systems that utilize mistune for markdown rendering. Systems handling user-generated content become vulnerable to resource exhaustion attacks that can effectively render services unavailable to legitimate users. The ATT&CK framework categorizes this under T1499.004 for Network Denial of Service and T1583.001 for Acquisition of Infrastructure, as attackers can leverage this weakness to compromise system availability. Organizations using affected versions should immediately implement patch management protocols to upgrade to mistune version 3.3.0 or later, which includes algorithmic improvements that eliminate the quadratic scanning behavior.
Mitigation strategies involve not only applying the official patch but also implementing input validation measures such as maximum length restrictions on markdown content and rate limiting for processing requests. Additionally, organizations should consider deploying web application firewalls with signature-based detection capabilities specifically targeting this vulnerability pattern. The fix implemented in version 3.3.0 demonstrates proper algorithmic optimization by eliminating redundant marker scanning operations and implementing more efficient matching algorithms that maintain linear time complexity while preserving all intended formatting functionality.