MCP Security Top 10 - Part 2: Over-Privileged Access

This is the second article in a series about the top 10 security risks associated with the Model Context Protocol (MCP). This post focuses on Over-Privileged Access, a critical vulnerability where AI agents can accidentally or maliciously invoke powerful tools with excessive permissions.

Introduction

The Model Context Protocol (MCP) enables AI systems to interact with external tools and data sources in a standardized way (Introducing the Model Context Protocol \ Anthropic). While this offers significant benefits for automation and functionality, it also introduces substantial security challenges. One of the most severe risks is over-privileged access - when an AI agent has more permissions than necessary to perform its intended tasks.

MCP Security Top 10 Series

This article is part of a comprehensive series examining the top 10 security risks when using MCP with AI agents:

0. MCP Security Top 10 Series: Introduction & Index
1. MCP Overview
2. Over-Privileged Access (this article)
3. Prompt Injection Attacks
4. Malicious MCP Servers
5. Unvalidated Tool Responses
6. Command Injection
7. Resource Exhaustion
8. Cross-Context Data Leakage
9. MITM Attacks
10. Social Engineering
11. Overreliance on AI

What is Over-Privileged Access in MCP?

Over-privileged access occurs when an MCP server provides an AI agent with broader permissions than needed for its legitimate functions. This can happen through misconfigured MCP servers, inadequate access controls, insufficient sandboxing, or overly permissive tool designs. When an AI agent has excessive privileges, it can potentially:

1. Access sensitive files or data that should be restricted
2. Modify critical system components
3. Execute dangerous commands
4. Connect to unauthorized network resources
5. Escalate privileges further within a system

Real-World Example

Consider this example of an over-privileged MCP server:

// VULNERABLE: Overly permissive file system access
import { MCPServer, createTool } from 'mcp-sdk-ts';
import { execSync } from 'child_process';

const executeCommandTool = createTool({
  name: "execute_command",
  description: "Execute a shell command",
  inputSchema: {
    type: "object",
    properties: {
      command: { type: "string" }
    },
    required: ["command"]
  },
  outputSchema: {
    type: "object",
    properties: {
      output: { type: "string" }
    }
  },
  handler: async ({ command }) => {
    // DANGER: No restrictions on what commands can be executed
    const output = execSync(command).toString();
    return { output };
  }
});

async function main() {
  const server = new MCPServer();
  server.addTool(executeCommandTool);
  server.start();
}

main().catch(console.error);

This MCP server allows the AI agent to execute any shell command without restriction. If an attacker can influence the AI's behavior (e.g., through prompt injection), they could trick it into deleting files, stealing data, or installing malware.

Attack Vectors

1. Unrestricted File System Access

MCP servers with unrestricted file access can be manipulated to:

• Read sensitive configurations or credentials
• Modify system files
• Access files outside expected directories

2. Unfiltered Command Execution

Tools that allow arbitrary command execution are particularly dangerous:

• An AI could be tricked into running harmful commands
• Command execution might enable privilege escalation
• No limitation on what executables can be invoked

3. Excessive Network Permissions

Over-privileged network access can lead to:

• Connection to unauthorized internal services
• Exfiltration of sensitive data
• Access to cloud provider metadata services
• Lateral movement within network infrastructure

4. Database Over-Privilege

When MCP tools have excessive database permissions:

• Reading sensitive records beyond what's needed
• Modification or deletion of critical data
• Schema alterations that could break applications

Detection Methods

To identify over-privileged MCP servers in your environment:

1. Static Analysis: Review MCP server implementations for unrestricted file, command, or network operations
2. Sandboxing Controls: Verify whether servers run in proper isolated environments
3. Permission Auditing: Check effective permissions of the process running the MCP server
4. Runtime Monitoring: Observe actual resource accesses by MCP servers during operation
5. Security Testing: Attempt to abuse MCP server permissions in a controlled environment

Mitigation Strategies

1. Implement Least Privilege

Design MCP servers to operate with the minimum privileges required:

// IMPROVED: Restricted file access with explicit permissions
import { MCPServer, createTool } from 'mcp-sdk-ts';
import * as fs from 'fs';
import * as path from 'path';

const allowedDirectories = ['/data/public', '/tmp/workspace'];

const readFileTool = createTool({
  name: "read_file",
  description: "Read the content of an allowed file",
  inputSchema: {
    type: "object",
    properties: {
      filepath: { type: "string" }
    },
    required: ["filepath"]
  },
  outputSchema: {
    type: "object",
    properties: {
      content: { type: "string" }
    }
  },
  handler: async ({ filepath }) => {
    // Validate and normalize the path
    const normalizedPath = path.normalize(filepath);

    // Check if the path is within allowed directories
    const isAllowed = allowedDirectories.some(dir =>
      normalizedPath.startsWith(dir)
    );

    if (!isAllowed) {
      throw new Error(`Access denied: ${filepath} is outside allowed directories`);
    }

    const content = fs.readFileSync(normalizedPath, 'utf8');
    return { content };
  }
});

2. Sandbox MCP Servers

Run MCP servers in isolated environments:

• Use containerization (Docker, Podman)
• Leverage OS-level isolation mechanisms
• Apply seccomp/AppArmor/SELinux profiles
• For web-based MCP, use Web Workers or separate origins

3. Implement Allowlists

Restrict operations to explicit allowlists:

• For file operations, limit to specific directories
• For command execution, only allow specific commands
• For network access, restrict to specific hosts/ports
• For database operations, use parameterized queries with field restrictions

4. Apply Filtering and Validation

Add robust input validation to all tool inputs:

• Validate file paths against path traversal
• Check commands against injection patterns
• Apply rate limits to prevent abuse
• Filter inputs based on context-specific rules

5. Use Runtime Monitoring

Implement continuous runtime monitoring:

• Watch for suspicious process spawning
• Monitor file system access patterns
• Detect unusual network connections
• Log and alert on potential violations

Conclusion

Over-privileged access is a critical security risk when implementing MCP servers. By carefully restricting permissions, implementing proper sandboxing, and following security best practices, you can significantly reduce this risk while still preserving the functionality and convenience that MCP provides.

In subsequent articles in this series, we'll explore other top security risks for MCP implementations, including prompt injection, unvalidated tool responses, and more. Each of these risks requires specific mitigation strategies to ensure that AI agents can safely leverage the power of external tools.