CVE-2026-59930 in Mistune
Summary
by MITRE • 07/08/2026
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the toc plugin and TableOfContents directive generate heading IDs as predictable toc_N values without slugifying the heading text, allowing attacker-controlled id="toc_N" content to collide with generated anchors and redirect same-page navigation, CSS selectors, or JavaScript handlers. This issue is fixed in version 3.3.0.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 07/08/2026
The Mistune Python Markdown parser vulnerability stems from insufficient input sanitization within its table of contents functionality, specifically affecting versions prior to 3.3.0. The core flaw exists in how the toc plugin and TableOfContents directive handle heading identification, where generated anchor IDs follow a predictable sequential pattern toc_1, toc_2, toc_3, and so forth without proper slugification of the actual heading text content. This predictable ID generation creates a deterministic vulnerability that allows attackers to craft malicious Markdown content with headings that will produce identical ID values, leading to collision scenarios within the rendered HTML output.
The technical implementation of this vulnerability occurs at the rendering layer where the parser fails to properly sanitize or transform user-provided heading text before generating anchor identifiers. When an attacker supplies specially crafted heading content, the system generates anchor IDs that match the predictable toc_N pattern, enabling malicious actors to manipulate navigation behavior by creating conflicting DOM elements with identical attribute values. This vulnerability directly maps to CWE-776, which addresses improper handling of special characters in identifiers, and represents a classic case of predictable identifier generation that undermines web application security.
The operational impact of this vulnerability extends beyond simple navigation disruption to encompass potential cross-site scripting attacks and CSS selector conflicts. An attacker could exploit this weakness to redirect same-page navigation to unintended locations by crafting headings that generate matching anchor IDs, potentially causing JavaScript event handlers to target incorrect DOM elements or CSS styling rules to apply incorrectly. The vulnerability creates opportunities for malicious code execution through DOM manipulation attacks where the predictable ID collisions interfere with legitimate page functionality and security controls.
Security mitigation requires immediate upgrade to Mistune version 3.3.0 or later, which implements proper slugification of heading text before generating anchor identifiers. Additional defensive measures include implementing Content Security Policy headers to restrict script execution, validating all user-provided Markdown content through sanitization libraries, and monitoring for suspicious heading patterns in rendered content. Organizations should also consider implementing automated testing routines that verify anchor ID uniqueness and consistency across generated HTML output to prevent similar vulnerabilities from reoccurring in other components of their security infrastructure. The fix addresses the underlying ATT&CK technique T1211 by preventing predictable identifier generation that could be exploited for privilege escalation or navigation manipulation attacks, while also aligning with secure coding practices outlined in OWASP Top Ten category A03:2021 - Injection vulnerabilities through proper input validation and sanitization.