CVE-2021-47839: Marky 0.0.1 XSS Vulnerability

CVE-2021-47839 Overview

CVE-2021-47839 is a persistent cross-site scripting (XSS) vulnerability affecting Marky version 0.0.1, a markdown editor application. This vulnerability allows attackers to inject malicious scripts into markdown files that execute when the file is opened by a victim user. The persistent nature of this XSS means the malicious payload is stored within the markdown file itself, making it particularly dangerous as it can affect any user who subsequently opens the crafted file.

Critical Impact

Attackers can upload crafted markdown files with embedded JavaScript payloads that execute in the context of the victim's session, potentially enabling remote code execution and unauthorized access to sensitive data.

Affected Products

• Marky 0.0.1

Discovery Timeline

• 2026-01-16 - CVE-2021-47839 published to NVD
• 2026-01-16 - Last updated in NVD database

Technical Details for CVE-2021-47839

Vulnerability Analysis

This vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The flaw exists because Marky 0.0.1 fails to properly sanitize or escape user-supplied input within markdown files before rendering them in the application's context.

When a user opens a specially crafted markdown file containing embedded JavaScript, the application processes and renders the malicious script without adequate input validation. This allows the attacker's code to execute within the security context of the application, potentially compromising user sessions, stealing credentials, or performing actions on behalf of the victim.

The vulnerability is accessible over the network and requires user interaction (the victim must open the malicious markdown file). An attacker needs low privileges to exploit this vulnerability, typically just the ability to share or upload a markdown file that the victim will open.

Root Cause

The root cause of this vulnerability is the lack of proper input sanitization in Marky's markdown rendering pipeline. The application does not adequately filter or escape potentially dangerous HTML and JavaScript content embedded within markdown files before displaying them to the user. This allows arbitrary script injection through crafted markdown syntax or embedded HTML tags that contain executable JavaScript code.

Attack Vector

The attack vector for CVE-2021-47839 involves network-based exploitation where an attacker crafts a malicious markdown file containing JavaScript payloads. The attack flow typically proceeds as follows:

1. The attacker creates a markdown file with embedded malicious JavaScript, using techniques such as script tags within raw HTML blocks or event handlers in HTML elements
2. The attacker distributes the malicious file to potential victims through file sharing, email, or other distribution methods
3. When a victim opens the crafted markdown file in Marky 0.0.1, the application renders the content without proper sanitization
4. The embedded JavaScript executes in the victim's session context, allowing the attacker to steal session tokens, perform actions as the victim, or potentially achieve remote code execution depending on the application's privileges

Technical details and proof-of-concept information are available through the Exploit-DB entry #49831 and the VulnCheck Advisory for Marky.

Detection Methods for CVE-2021-47839

Indicators of Compromise

• Presence of markdown files containing suspicious HTML tags such as <script>, <iframe>, or event handlers like onerror, onload, onclick
• Unusual JavaScript execution or network requests originating from the Marky application
• Unexpected modifications to user session data or authentication tokens
• Log entries showing access to external domains or suspicious URLs when opening markdown files

Detection Strategies

• Implement content security policy (CSP) monitoring to detect unauthorized script execution within the application
• Deploy endpoint detection and response (EDR) solutions to monitor for anomalous behavior when markdown files are processed
• Use web application firewalls (WAF) configured with XSS detection rules to identify malicious payloads in uploaded files
• Scan incoming markdown files for known XSS patterns and suspicious HTML/JavaScript content before allowing them to be opened

Monitoring Recommendations

• Enable verbose logging for the Marky application to capture file open events and any script execution attempts
• Monitor network traffic for unexpected outbound connections originating from the Marky process
• Implement file integrity monitoring for markdown files in shared or collaborative environments
• Configure security information and event management (SIEM) rules to alert on patterns consistent with XSS exploitation

How to Mitigate CVE-2021-47839

Immediate Actions Required

• Discontinue use of Marky 0.0.1 until a patched version is available or implement strict workarounds
• Avoid opening markdown files from untrusted or unknown sources
• Implement network segmentation to limit the potential impact of exploitation
• Review any markdown files currently in use for signs of malicious content

Patch Information

As of the last NVD update, no official vendor patch information is available for this vulnerability. Users should monitor the Marky GitHub repository for security updates and newer releases that address this vulnerability. Consider transitioning to alternative markdown editors that implement proper input sanitization if no patch becomes available.

Workarounds

• Use a markdown sanitization library or pre-processor to strip potentially dangerous HTML and JavaScript from markdown files before opening them in Marky
• Configure the application environment to restrict script execution capabilities where possible
• Open untrusted markdown files in isolated environments or sandboxed containers to limit potential damage
• Implement organizational policies requiring review and approval of markdown files before distribution

# Example: Pre-scan markdown files for suspicious content before opening
# This is a basic check and should be combined with proper sanitization

grep -E "<script|javascript:|onerror=|onload=|onclick=" suspicious_file.md
# If any matches found, do not open the file in Marky