CVE-2019-9844 in simple-markdown
Summary
by MITRE
simple-markdown.js in Khan Academy simple-markdown before 0.4.4 allows XSS via a data: or vbscript: URI.
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Analysis
by VulDB Data Team • 08/27/2023
The vulnerability CVE-2019-9844 affects the simple-markdown.js library used by Khan Academy, representing a cross-site scripting vulnerability that emerges from improper handling of data and vbscript URIs within markdown parsing operations. This issue exists in versions prior to 0.4.4 of the simple-markdown library, which is a lightweight markdown parser designed for client-side processing. The vulnerability stems from the library's insufficient sanitization of URI schemes, particularly those that can execute code or trigger unintended behavior when processed through markdown rendering mechanisms. When a user inputs markdown content containing data: or vbscript: URIs, the parser fails to properly validate or escape these potentially dangerous schemes, allowing malicious code execution in the context of the victim's browser session.
The technical flaw manifests in the markdown parser's URI validation logic, which does not adequately filter or sanitize URI schemes that could lead to code execution. The data: URI scheme can contain arbitrary data including executable scripts, while vbscript: URIs can execute visual basic scripts within the browser context. These URI schemes bypass normal security controls because the simple-markdown library does not implement proper URI scheme validation or sanitization. The vulnerability operates through a classic XSS attack vector where an attacker crafts malicious markdown content containing these URI schemes, which then gets processed and rendered in the victim's browser, executing the embedded code in the context of the vulnerable application.
The operational impact of this vulnerability is significant as it allows attackers to execute arbitrary JavaScript code in the context of Khan Academy's web application, potentially leading to session hijacking, data theft, or further exploitation of the user's browser. The vulnerability affects any user who views markdown content processed through the vulnerable library, making it particularly dangerous in collaborative environments where users can submit content that gets rendered for others. Attackers could leverage this vulnerability to steal user credentials, access personal data, or perform actions on behalf of authenticated users. The risk is amplified because the vulnerability exists in a widely used library that processes user-generated content, creating a persistent threat vector that affects the entire Khan Academy platform and potentially other applications using the same vulnerable version.
Mitigation strategies for CVE-2019-9844 involve immediate upgrading to simple-markdown.js version 0.4.4 or later, which contains the necessary fixes for URI scheme validation and sanitization. Organizations should implement comprehensive input validation and output encoding for all user-generated content, particularly when processing markdown or HTML content. The fix should include implementing a whitelist approach for URI schemes, rejecting data: and vbscript: URIs, and ensuring proper sanitization of all user input before rendering. Additionally, organizations should consider implementing content security policies that restrict the execution of inline scripts and limit the sources from which scripts can be loaded. This vulnerability aligns with CWE-79 Cross-site Scripting and follows ATT&CK technique T1203 Exploitation for Client Execution, emphasizing the need for robust input validation and sanitization controls in web applications processing user content.