Infraviewer/MCP_SECURITY.md

MCP Read-Only Security Guide

Current Security Measures

The mssql-mcp-readonly.js file implements multiple layers of read-only enforcement:

1. Query Validation (Application Layer)

• ✅ Only SELECT queries allowed
• ✅ Blocks INSERT, UPDATE, DELETE, DROP, CREATE, ALTER, TRUNCATE
• ✅ Blocks EXEC/EXECUTE (prevents stored procedure execution)
• ✅ Blocks SP_/XP_ (prevents system stored procedures)
• ✅ Blocks MERGE, GRANT, REVOKE, DENY
• ✅ Prevents query chaining with semicolons

2. Recommended: Database User Permissions (Database Layer)

For maximum security, create a read-only database user:

-- Create a read-only user
CREATE LOGIN mcp_readonly WITH PASSWORD = 'YourSecurePassword123!';
USE Infra;
CREATE USER mcp_readonly FOR LOGIN mcp_readonly;

-- Grant only SELECT permission
GRANT SELECT ON SCHEMA::dbo TO mcp_readonly;

-- Or grant SELECT on specific tables only:
GRANT SELECT ON dbo.SDS_Teile TO mcp_readonly;
GRANT SELECT ON dbo.SDS_Teile_CA TO mcp_readonly;
GRANT SELECT ON dbo.SDS_Teile_CS TO mcp_readonly;
GRANT SELECT ON dbo.SDS_Teile_LC TO mcp_readonly;
GRANT SELECT ON dbo.SDS_Teile_ME TO mcp_readonly;
GRANT SELECT ON dbo.SDS_Teile_MK TO mcp_readonly;
GRANT SELECT ON dbo.SDS_Teile_MS TO mcp_readonly;
GRANT SELECT ON dbo.SDS_Teile_WT TO mcp_readonly;
GRANT SELECT ON dbo.SDS_Teile_EL TO mcp_readonly;

-- Explicitly deny write permissions (extra safety)
DENY INSERT, UPDATE, DELETE, ALTER ON SCHEMA::dbo TO mcp_readonly;

3. Update MCP Configuration

After creating the read-only user, update ~/.cursor/mcp.json:

{
  "mcpServers": {
    "mssql-mcp-readonly": {
      "command": "node ",
      "args": ["/home/sdsadmin/infraviewer/mssql-mcp-readonly.js"],
      "env": {
        "MSSQL_SERVER": "192.168.120.88",
        "MSSQL_PORT": "1433",
        "MSSQL_USER": "mcp_readonly",
        "MSSQL_PASSWORD": "YourSecurePassword123!",
        "MSSQL_DATABASE": "Infra",
        "MSSQL_ENCRYPT": "true",
        "MSSQL_TRUST_SERVER_CERTIFICATE": "true"
      }
    }
  }
}

Security Layers

Layer	Protection	Bypassed if...
Application Validation	Keyword blocking	Code has bugs
Database Permissions	SQL Server enforced	Wrong credentials used
Both Combined	Double protection	Nearly impossible ✅

Testing Read-Only Access

Test 1: Valid SELECT (should work)

SELECT TOP 10 * FROM SDS_Teile

Test 2: Invalid INSERT (should fail)

INSERT INTO SDS_Teile (Teil) VALUES ('TEST')

Expected: Error: "Forbidden keyword detected: INSERT"

Test 3: Query Chaining (should fail)

SELECT 1; DROP TABLE SDS_Teile

Expected: Error: "Multiple statements not allowed"

Test 4: Stored Procedure (should fail)

EXEC sp_helpdb

Expected: Error: "Forbidden keyword detected: EXEC"

Current vs Recommended Setup

Current Setup (Application Layer Only)

[AI] → [mssql-mcp-readonly.js] → [SQL Server]
         ↑ Validates queries          ↑ Full permissions

Risk: If validation code has a bug, data could be modified

Recommended Setup (Defense in Depth)

[AI] → [mssql-mcp-readonly.js] → [SQL Server with read-only user]
         ↑ Validates queries          ↑ SELECT-only permissions

Benefit: Even if validation fails, SQL Server will block write operations

Monitoring

Check MCP server logs for blocked attempts:

# If running as systemd service
journalctl -u mcp-readonly -f

# If running manually
tail -f /var/log/mcp-readonly.log

Best Practices

1. ✅ Use dedicated read-only database user
2. ✅ Keep validation keywords up to date
3. ✅ Monitor for failed query attempts
4. ✅ Regularly audit database permissions
5. ✅ Use strong passwords for database users
6. ✅ Enable SQL Server audit logging
7. ✅ Limit network access to database server
8. ✅ Keep mssql package updated (npm update mssql)

Additional Hardening

1. Add Query Size Limits

Prevent resource exhaustion:

function validateQuery(query) {
  if (query.length > 10000) {
    throw new Error("Query too long!");
  }
  // ... rest of validation
}

2. Add Rate Limiting

Prevent abuse:

const rateLimit = require('express-rate-limit');
// Limit to 100 queries per minute

3. Add Query Logging

Track all queries:

console.log(`[${new Date().toISOString()}] Query: ${query.substring(0, 100)}...`);