CVE-2026-59923 in Mistune
Summary
by MITRE • 07/08/2026
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, HTMLRenderer.safe_url() does not block percent-encoded javascript URIs, allowing attacker-supplied Markdown links or images to bypass URL protections and execute script in rendered HTML. This issue is fixed in version 3.3.0.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 07/08/2026
The mistune Python markdown parser contains a critical security vulnerability in its HTML rendering functionality that enables attackers to bypass URL validation mechanisms through percent-encoded javascript URIs. This vulnerability affects versions prior to 3.3.0 and represents a significant weakness in the library's security posture. The flaw exists within the HTMLRenderer.safe_url() method which is responsible for sanitizing URLs in rendered HTML output, specifically failing to properly validate percent-encoded javascript protocols that could be used to execute malicious scripts.
The technical nature of this vulnerability stems from inadequate input validation within the safe_url() function, which processes user-supplied markdown links and images to generate HTML output. When attackers provide markdown content containing percent-encoded javascript URIs such as javascript:alert(1) encoded as javascript%3Aalert%281%29, the vulnerable version fails to block these potentially dangerous URLs. This oversight allows malicious code embedded in markdown documents to be executed when rendered into HTML, creating a server-side request forgery or cross-site scripting vulnerability depending on the context of use. The vulnerability directly relates to CWE-79 which describes improper neutralization of script-related HTML tags in web applications.
The operational impact of this vulnerability is substantial as it enables attackers to inject malicious scripts into rendered HTML content through seemingly benign markdown links or images. An attacker could craft markdown content that appears safe but when processed by affected versions of mistune would execute arbitrary javascript code in the context of users viewing the rendered output. This creates a vector for various attacks including session hijacking, data exfiltration, or redirection to malicious sites. The vulnerability affects web applications that use mistune to render user-provided markdown content, particularly those implementing user-generated content features where security is paramount.
Mitigation strategies should focus on immediate remediation through version upgrade to 3.3.0 or later where the vulnerability has been addressed. Organizations using affected versions must conduct thorough code reviews to identify any instances where user-supplied markdown is rendered without proper validation. Additional defensive measures include implementing Content Security Policy headers, sanitizing all user-generated content at multiple levels, and considering additional URL validation layers beyond what mistune provides. The fix implemented in version 3.3.0 likely includes enhanced URL parsing logic that properly decodes and validates percent-encoded URIs to prevent javascript protocol execution. This vulnerability demonstrates the importance of comprehensive input validation in web applications and aligns with ATT&CK technique T1203 which covers Exploitation for Client Execution through malicious content delivery mechanisms.
The security implications extend beyond simple XSS attacks as this vulnerability could enable more sophisticated attack chains where attackers leverage the rendered markdown to establish persistent access or perform privilege escalation within affected applications. Organizations should also consider implementing automated scanning tools that can detect vulnerable mistune versions in their dependency trees and ensure proper patch management processes are in place to prevent similar issues from arising in other components of their software stack. The vulnerability serves as a reminder of the critical importance of validating all user inputs, particularly in libraries that process untrusted content and generate executable output for web applications.