CVE-2026-59928 in Mistune
Summary
by MITRE • 07/08/2026
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown document containing many repeated or distinct reference-link definitions causes quadratic work in src/mistune/block_parser.py and the ref_links environment dictionary handling, 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 significant denial of service risk that affects versions prior to 3.3.0. This flaw manifests when processing markdown documents containing numerous repeated or distinct reference-link definitions, creating a computational complexity issue that can exhaust system resources. The vulnerability specifically targets the block_parser.py module and the ref_links environment dictionary handling mechanism within the parser's architecture.
The technical root cause of this vulnerability lies in how mistune processes reference-link definitions during markdown parsing operations. When multiple identical or different reference links are present in a single document, the parser enters a quadratic time complexity scenario where processing time increases exponentially with the number of reference links. This occurs because the ref_links dictionary handling logic does not properly optimize for duplicate entries or efficiently manage the lookup operations required for resolving reference links. The issue stems from inadequate input validation and resource management within the parsing engine's core algorithms.
From an operational perspective, this vulnerability enables malicious actors to perform denial of service attacks against systems using mistune by crafting specially formatted markdown documents that trigger the quadratic processing behavior. An attacker could submit a document containing hundreds or thousands of reference links, causing the parser to consume excessive CPU resources and potentially leading to system unresponsiveness or complete service failure. This makes the vulnerability particularly dangerous in web applications or APIs that accept user-provided markdown content without proper sanitization.
The vulnerability maps to CWE-400, which covers "Uncontrolled Resource Consumption" in software systems, specifically manifesting as a denial of service condition through excessive computational requirements. From an ATT&CK framework perspective, this represents a resource exhaustion technique that can be classified under T1499.004 - "Utilities: File and Directory Permissions Modification" or more appropriately T1499.001 - "Utilities: Network Denial of Service" depending on the attack vector. The impact extends beyond simple service disruption as it can potentially cause cascading failures in applications that rely on markdown processing for content management.
Mitigation strategies for this vulnerability include upgrading to mistune version 3.3.0 or later, which contains the necessary fixes for proper reference-link dictionary handling and quadratic complexity prevention. Organizations should also implement input validation measures such as limiting the maximum number of reference links allowed in a single document, implementing rate limiting on markdown processing requests, and establishing resource quotas for parsing operations. Additionally, deploying defensive coding practices that include early termination conditions for excessive link counts and implementing monitoring systems to detect unusual CPU consumption patterns during markdown processing can provide additional layers of protection against exploitation attempts.