CVE-2017-16876 in Mistune
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in the _keyify function in mistune.py in Mistune before 0.8.1 allows remote attackers to inject arbitrary web script or HTML by leveraging failure to escape the "key" argument.
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Analysis
by VulDB Data Team • 01/19/2023
The CVE-2017-16876 vulnerability represents a critical cross-site scripting flaw in the Mistune Python markdown parser library, specifically within the _keyify function. This vulnerability affects versions prior to 0.8.1 and demonstrates a classic input sanitization failure that enables remote code execution through web script injection. The flaw occurs when the library processes markdown content that contains unescaped key arguments, creating an avenue for malicious actors to inject arbitrary HTML or JavaScript code into web applications that utilize this parsing functionality.
The technical implementation of this vulnerability stems from inadequate output escaping within the _keyify function, which is responsible for processing and sanitizing key identifiers during markdown parsing operations. When user-supplied content contains special characters or HTML entities within key arguments, the function fails to properly escape these inputs before rendering them in the final output. This failure directly violates established security principles for input validation and output encoding, creating a persistent XSS vector that can be exploited across multiple web applications using the vulnerable Mistune library. The vulnerability operates at the application layer and requires no special privileges to exploit, making it particularly dangerous in web environments where user input is processed without proper sanitization.
The operational impact of CVE-2017-16876 extends beyond simple script injection, as it enables attackers to perform a wide range of malicious activities including session hijacking, credential theft, and data exfiltration. When exploited, this vulnerability can allow attackers to execute arbitrary JavaScript code within the context of a victim's browser, potentially leading to full compromise of user sessions and sensitive data exposure. The vulnerability is particularly concerning because it affects the core parsing functionality of the Mistune library, meaning that any web application or service using this library for markdown processing becomes vulnerable to XSS attacks. The attack surface is broad since Mistune is widely used in various Python web frameworks and content management systems, amplifying the potential impact of this flaw.
Organizations and developers should immediately upgrade to Mistune version 0.8.1 or later to remediate this vulnerability, as no effective workarounds exist for the underlying issue. The fix implemented in version 0.8.1 addresses the root cause by properly escaping key arguments within the _keyify function, ensuring that special characters and HTML entities are appropriately encoded before being rendered in the final output. Security practitioners should also implement additional defensive measures including content security policy headers, input validation at multiple layers, and regular dependency scanning to prevent similar vulnerabilities from being introduced into web applications. This vulnerability aligns with CWE-79, which specifically addresses cross-site scripting flaws, and represents a clear violation of ATT&CK technique T1059.007 for script injection, demonstrating the critical importance of proper input sanitization in web application security.