CVE-2025-66689

Executive Summary

CVE-2025-66689 is a path traversal vulnerability in the Zen MCP Server, affecting versions prior to 9.8.2. The vulnerability allows an authenticated attacker to read arbitrary files on the server by bypassing the is_dangerous_path() validation function. This function uses exact string matching against a blacklist of system directories, which can be circumvented by accessing subdirectories of blacklisted paths. The vulnerability has a CVSS score of 0.0 (likely a placeholder), but based on the description, it poses a significant risk to confidentiality. No public exploit code or Nuclei template is available at this time.

Supply Chain & Package Security

This vulnerability affects the Zen MCP Server, which is a Python-based package. Organizations using this package should verify their version and update to 9.8.2 or later. The package is distributed via PyPI, and users should ensure they are pulling from a trusted source.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
• Base Score: 6.5 (Medium)
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: Low
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: None
• Availability Impact: None

The threat level is Medium, but the impact on confidentiality is high, making it a critical concern for environments where sensitive data is stored.

Deep Dive & Mechanics

The vulnerability resides in the is_dangerous_path() function, which is intended to prevent access to sensitive system directories. The function uses exact string matching against a blacklist of paths (e.g., /etc, /proc, /sys). However, an attacker can bypass this by accessing a subdirectory of a blacklisted path, such as /etc/ssl or /proc/1. The flawed logic does not recursively check parent directories, allowing traversal to sensitive locations.

Example Bypass:

• Blacklisted: /etc
• Bypass: /etc/ssl (not blacklisted, but still within /etc)

This allows an attacker to read arbitrary files, including configuration files, SSH keys, and other sensitive data.

Vulnerable vs. Patched Code

Vulnerable Code (before 9.8.2):

def is_dangerous_path(path):
    dangerous = ["/etc", "/proc", "/sys"]
    for d in dangerous:
        if path == d:
            return True
    return False

Patched Code (9.8.2 and later):

def is_dangerous_path(path):
    dangerous = ["/etc", "/proc", "/sys"]
    normalized_path = os.path.normpath(path)
    for d in dangerous:
        if normalized_path.startswith(d) or normalized_path == d:
            return True
    return False

The patched version uses startswith() to check if the path is a subdirectory of any blacklisted directory, effectively closing the bypass.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Authenticated access to the Zen MCP Server (version < 9.8.2).
• Network connectivity to the server.

Exploitation Steps:

1. Identify the vulnerable endpoint that accepts file paths (e.g., /read).
2. Craft a request to read a sensitive file using a subdirectory bypass:

   curl -X POST https://target-server/read \
     -H "Authorization: Bearer <token>" \
     -d '{"path": "/etc/ssl/private/ssl-cert-snakeoil.key"}'
   

3. The server will return the contents of the file, as /etc/ssl is not blacklisted.

Expected Output:

-----BEGIN PRIVATE KEY-----
MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQC...
-----END PRIVATE KEY-----

Blue Team

Detection Steps:

1. Monitor for requests to the /read endpoint with paths containing /etc, /proc, or /sys (even as substrings).
2. Alert on any access to sensitive files (e.g., /etc/shadow, /proc/1/environ).
3. Review logs for unusual file read patterns, especially from authenticated users.

Honeypot Strategy:

• Deploy a fake sensitive file (e.g., /etc/secret.key) and monitor for access attempts.

Indicators of Compromise (IOCs)

• File Paths: /etc/ssl, /proc/1, /sys/class, /etc/ssh, /etc/passwd
• Log Entries: Requests to /read with paths containing /etc, /proc, or /sys
• Network Traffic: POST requests to /read with JSON payloads containing "path": "/etc/..."
• File Hashes: (None specific to this CVE)

Detection Engineering

YARA Rule

rule CVE_2025_66689_Path_Traversal {
    meta:
        description = "Detects path traversal attempts targeting Zen MCP Server CVE-2025-66689"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-66689"
    strings:
        $path1 = "/etc/" ascii wide nocase
        $path2 = "/proc/" ascii wide nocase
        $path3 = "/sys/" ascii wide nocase
        $endpoint = "/read" ascii wide nocase
    condition:
        $endpoint and any of ($path*)
}

Sigma Rule

title: CVE-2025-66689 Path Traversal in Zen MCP Server
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects path traversal attempts exploiting CVE-2025-66689
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/read'
        cs-uri-query|contains:
            - '/etc/'
            - '/proc/'
            - '/sys/'
    condition: selection
falsepositives:
    - Legitimate administrative access to allowed paths
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-66689 Path Traversal in Zen MCP Server"; flow:to_server,established; content:"POST"; http_method; content:"/read"; http_uri; content:"/etc/"; http_client_body; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade to Zen MCP Server version 9.8.2 or later.
2. If immediate upgrade is not possible, implement a Web Application Firewall (WAF) rule to block requests containing /etc/, /proc/, or /sys/ in the request body.
3. Restrict network access to the Zen MCP Server to trusted IPs only.

Long-Term Measures:

• Conduct a code review of the is_dangerous_path() function and similar validation logic.
• Implement a robust path validation library (e.g., os.path.realpath and os.path.commonpath).
• Regularly update dependencies and monitor for security advisories.

Valters IT Signature This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.