MCP Security Checklist

Essential security checklist for MCP servers covering input validation, authentication, transport security, and data protection.

title: "MCP Security Checklist" description: "Essential security checklist for MCP servers covering input validation, authentication, transport security, and data protection." order: 4 keywords: ["MCP security", "MCP security checklist", "MCP input validation", "MCP authentication security"] date: "2026-04-01"

Why MCP Security Matters

AI models can be prompted to call tools in unexpected ways. An MCP server that trusts all inputs without validation is vulnerable to prompt injection, data exfiltration, and unauthorized access.

1. Input Validation

import { z } from "zod";

// Bad: overly permissive
const badSchema = {
  query: z.string(), // No length limit, no sanitization
  path: z.string(),  // Allows directory traversal
};

// Good: strict validation
const goodSchema = {
  query: z.string()
    .min(1, "Query cannot be empty")
    .max(500, "Query too long")
    .describe("Search query"),
  path: z.string()
    .regex(/^[a-zA-Z0-9_\-./]+$/, "Path contains invalid characters")
    .refine(p => !p.includes(".."), "Directory traversal not allowed")
    .describe("File path within the workspace"),
};

import path from "path";

const WORKSPACE_ROOT = "/home/user/workspace";

function validatePath(userPath: string): string {
  const resolved = path.resolve(WORKSPACE_ROOT, userPath);
  if (!resolved.startsWith(WORKSPACE_ROOT)) {
    throw new Error("Access denied: path is outside the workspace.");
  }
  return resolved;
}

2. Authentication & Authorization

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { McpError, ErrorCode } from "@modelcontextprotocol/sdk/types.js";

function requireAuth(token: string | undefined): void {
  if (!token || !isValidToken(token)) {
    throw new McpError(
      ErrorCode.InvalidParams,
      "Authentication required. Provide a valid API token."
    );
  }
}

// Middleware approach for HTTP transport
function authMiddleware(req: Request): string {
  const auth = req.headers.get("Authorization");
  if (!auth?.startsWith("Bearer ")) {
    throw new Error("Missing Bearer token");
  }
  const token = auth.slice(7);
  if (!isValidToken(token)) {
    throw new Error("Invalid or expired token");
  }
  return token;
}

3. Transport Security

import https from "https";
import fs from "fs";

const server = https.createServer({
  cert: fs.readFileSync("/etc/ssl/certs/server.crt"),
  key: fs.readFileSync("/etc/ssl/private/server.key"),
}, app);

4. Data Sanitization

function sanitizeUser(raw: any) {
  return {
    id: raw.id,
    name: raw.name,
    email: raw.email,
    role: raw.role,
    // Deliberately omit: password_hash, api_key, internal_notes
  };
}

server.tool("get-user", "Get user profile", {
  userId: z.string(),
}, async ({ userId }) => {
  const raw = await db.getUser(userId);
  const safe = sanitizeUser(raw);
  return { content: [{ type: "text", text: JSON.stringify(safe, null, 2) }] };
});

// Bad: SQL injection vulnerability
const result = await db.query(`SELECT * FROM users WHERE name = '${name}'`);

// Good: parameterized query
const result = await db.query("SELECT * FROM users WHERE name = $1", [name]);

5. Rate Limiting

class RateLimiter {
  private requests = new Map<string, { count: number; resetAt: number }>();

  check(clientId: string, limit: number, windowMs: number): boolean {
    const now = Date.now();
    const entry = this.requests.get(clientId);

    if (!entry || now > entry.resetAt) {
      this.requests.set(clientId, { count: 1, resetAt: now + windowMs });
      return true;
    }

    if (entry.count >= limit) return false;
    entry.count++;
    return true;
  }
}

const limiter = new RateLimiter();

// In tool handler
if (!limiter.check(clientId, 60, 60000)) {
  return {
    content: [{ type: "text", text: "Rate limit exceeded. Maximum 60 requests per minute." }],
    isError: true,
  };
}

6. Audit Logging

function auditLog(entry: {
  tool: string;
  params: Record<string, unknown>;
  clientId: string;
  status: "success" | "error";
  duration: number;
}) {
  const sanitized = { ...entry.params };
  for (const key of ["password", "token", "secret", "key"]) {
    if (key in sanitized) sanitized[key] = "[REDACTED]";
  }

  // Always log to stderr in MCP servers
  console.error(JSON.stringify({
    type: "audit",
    ...entry,
    params: sanitized,
    timestamp: new Date().toISOString(),
  }));
}

Security Checklist Summary