CVE-2025-56406: mcp-neo4j RCE Vulnerability

CVE-2025-56406 Overview

A command injection vulnerability exists in mcp-neo4j version 0.3.0 that allows attackers to obtain sensitive information or execute arbitrary commands via the Server-Sent Events (SSE) service. The vulnerability stems from insufficient authentication controls on the MCP (Model Context Protocol) server, which exposes the SSE service to potential abuse when deployed in network-accessible environments.

Critical Impact

Unauthenticated attackers can exploit the SSE service to extract sensitive database information or execute arbitrary commands on the underlying system, potentially leading to full system compromise.

Affected Products

• mcp-neo4j version 0.3.0
• MCP server deployments without authentication middleware
• Neo4j integrations using the vulnerable mcp-neo4j package

Discovery Timeline

• 2025-09-10 - CVE-2025-56406 published to NVD
• 2025-09-16 - Last updated in NVD database

Technical Details for CVE-2025-56406

Vulnerability Analysis

This vulnerability is classified as CWE-77 (Command Injection), where insufficient input validation allows attackers to inject arbitrary commands through the SSE service endpoint. The mcp-neo4j package provides a Model Context Protocol server for Neo4j graph database interactions. The SSE service, designed to stream real-time updates, lacks mandatory authentication by default.

When the MCP server is exposed to network access—contrary to the supplier's intended local-only deployment model—attackers can leverage the unauthenticated SSE endpoint to craft malicious requests. These requests can bypass intended access controls and either exfiltrate sensitive Neo4j database contents or achieve command execution on the host system.

The supplier has noted that authentication is not mandatory for MCP servers by design, and the mcp-neo4j implementation is intended exclusively for local development environments where authentication would be unnecessary. However, in production deployments or scenarios where the server is inadvertently exposed to network access, this design decision creates a significant security risk.

Root Cause

The root cause of this vulnerability lies in the absence of mandatory authentication mechanisms for the SSE service endpoint in mcp-neo4j. The Model Context Protocol specification does not require authentication, and the mcp-neo4j implementation follows this permissive default. Combined with insufficient input validation on SSE service requests, this allows attackers to inject and execute arbitrary commands through specially crafted payloads.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker with network access to the mcp-neo4j SSE service can send malicious requests to the endpoint. The vulnerability can be exploited by crafting SSE requests that contain command injection payloads. These payloads bypass input sanitization and are executed in the context of the MCP server process.

The attack flow typically involves: (1) identifying an exposed mcp-neo4j SSE endpoint, (2) sending crafted requests containing command injection payloads, and (3) receiving either sensitive data exfiltration results or achieving command execution on the target system. For detailed technical information, see the GitHub issue discussion and the MCP authorization specification.

Detection Methods for CVE-2025-56406

Indicators of Compromise

• Unusual SSE connection requests from external IP addresses to the mcp-neo4j service
• Unexpected command execution patterns or shell spawning from the MCP server process
• Anomalous Neo4j query patterns indicating data exfiltration attempts
• Network traffic containing command injection payloads targeting SSE endpoints

Detection Strategies

• Monitor network traffic for unauthorized connections to MCP server ports from non-local addresses
• Implement application-level logging to capture all SSE service requests and responses
• Deploy intrusion detection rules to identify command injection patterns in HTTP/SSE traffic
• Audit process execution trees to detect child processes spawned by the MCP server

Monitoring Recommendations

• Enable verbose logging on the mcp-neo4j server to capture all incoming SSE requests
• Configure network monitoring to alert on any external connections to the MCP server endpoint
• Implement file integrity monitoring on the Neo4j and mcp-neo4j installation directories
• Monitor for unusual database query patterns that may indicate reconnaissance or data theft

How to Mitigate CVE-2025-56406

Immediate Actions Required

• Ensure mcp-neo4j deployments are restricted to local/localhost access only as intended by the supplier
• Deploy the supplier-provided middleware to isolate the MCP server from external network access
• Audit existing deployments to verify the SSE service is not exposed to untrusted networks
• Implement network-level access controls (firewall rules) to restrict access to the MCP server port

Patch Information

The supplier has indicated that authentication is not mandatory for MCP servers by design, and the package is intended for local development use only. For deployments requiring network exposure, the supplier provides middleware to help isolate the MCP server from external access. Organizations should evaluate whether this middleware meets their security requirements or consider implementing additional authentication controls. For more information, refer to the mcp-neo4j GitHub issue and the MCP authorization specification.

Workarounds

• Bind the mcp-neo4j service exclusively to 127.0.0.1 or localhost interfaces
• Deploy the service behind a reverse proxy with authentication enabled
• Use network segmentation to isolate the MCP server from untrusted networks
• Implement IP whitelisting at the firewall level to restrict access to known trusted sources

# Configuration example - Restrict mcp-neo4j to localhost only
# In your MCP server configuration or startup script:
export MCP_BIND_ADDRESS="127.0.0.1"
export MCP_BIND_PORT="8080"

# Alternatively, use iptables to restrict access
iptables -A INPUT -p tcp --dport 8080 -s 127.0.0.1 -j ACCEPT
iptables -A INPUT -p tcp --dport 8080 -j DROP