CVE-2025-66689 Overview
A path traversal vulnerability exists in Zen MCP Server before version 9.8.2 that allows authenticated attackers to read arbitrary files on the system. The vulnerability is caused by flawed logic in the is_dangerous_path() validation function that uses exact string matching against a blacklist of system directories. Attackers can bypass these restrictions by accessing subdirectories of blacklisted paths.
Critical Impact
Authenticated attackers can bypass path validation controls to read sensitive files outside intended directories, potentially exposing configuration files, credentials, and other confidential system data.
Affected Products
- Zen MCP Server versions prior to 9.8.2
Discovery Timeline
- 2026-01-12 - CVE-2025-66689 published to NVD
- 2026-01-13 - Last updated in NVD database
Technical Details for CVE-2025-66689
Vulnerability Analysis
This path traversal vulnerability (CWE-22) stems from an incomplete validation mechanism within the Zen MCP Server's file access controls. The is_dangerous_path() function is designed to prevent access to sensitive system directories by comparing requested paths against a blacklist. However, the implementation uses exact string matching, which creates a critical security gap.
When an attacker requests access to a subdirectory of a blacklisted path, the exact match comparison fails to recognize the dangerous access attempt. For example, if /etc is blacklisted, accessing /etc/passwd would not trigger the security check because the strings are not identical. This allows authenticated users to read arbitrary files on the system, including sensitive configuration files, private keys, and other confidential data that should be protected.
The vulnerability requires network access and valid authentication credentials, but once authenticated, exploitation is straightforward and requires no user interaction.
Root Cause
The root cause is the use of exact string matching in the is_dangerous_path() validation function. Instead of checking whether a requested path starts with or contains a blacklisted directory, the function only blocks access when the path exactly matches an entry in the blacklist. This implementation flaw fails to account for subdirectory traversal, allowing attackers to access any file within a blacklisted directory hierarchy by simply specifying a more complete path.
Attack Vector
The attack is executed over the network by authenticated users. An attacker with valid credentials can craft file access requests that specify paths to sensitive files located within subdirectories of protected system directories. Since the validation only checks for exact matches, requests for specific files within these directories bypass the security controls entirely.
The exploitation flow involves:
- Authenticating to the Zen MCP Server with valid credentials
- Identifying blacklisted directories through testing or documentation
- Requesting files within subdirectories of those blacklisted paths
- Successfully retrieving sensitive file contents that should be protected
For detailed technical analysis of this vulnerability, refer to the GitHub CVE-2025-66689 Analysis and the original GitHub Issue #293.
Detection Methods for CVE-2025-66689
Indicators of Compromise
- File access requests targeting sensitive system paths such as /etc/passwd, /etc/shadow, or configuration directories
- Unusual file read operations from authenticated users attempting to access files outside designated application directories
- Log entries showing sequential probing attempts against various system file paths
Detection Strategies
- Monitor file access logs for requests containing path traversal sequences or references to sensitive system directories
- Implement anomaly detection rules for authenticated users accessing files outside their normal operational scope
- Configure alerts for any access attempts to critical system configuration files through the MCP Server interface
Monitoring Recommendations
- Enable comprehensive audit logging for all file access operations within Zen MCP Server
- Deploy file integrity monitoring on sensitive system directories to detect unauthorized read access
- Review authentication logs for unusual patterns that may indicate credential compromise preceding exploitation
How to Mitigate CVE-2025-66689
Immediate Actions Required
- Upgrade Zen MCP Server to version 9.8.2 or later immediately
- Audit access logs for any suspicious file read operations that may indicate prior exploitation
- Review and restrict user permissions to limit the blast radius of authenticated access
- Consider implementing additional network-level access controls for the MCP Server
Patch Information
The vulnerability has been addressed in Zen MCP Server version 9.8.2. Organizations should update to this version or later to remediate the vulnerability. For more information, see the GitHub Issue #293 for patch details and discussion.
Workarounds
- Implement network segmentation to restrict access to the Zen MCP Server to trusted networks only
- Apply strict firewall rules limiting which hosts can authenticate to the MCP Server
- Deploy a web application firewall (WAF) with path traversal detection rules in front of the server
- Temporarily disable file access functionality if not critical to operations until patching is complete
# Example: Restrict network access to Zen MCP Server using iptables
iptables -A INPUT -p tcp --dport 8080 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 8080 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
