Securing MCP Servers: Auditing for Overprivileged Tools and Prompt Injection

Your MCP Server Is Probably Overprivileged - Here’s a Scanner For It

David McHale has released @hailbytes/mcp-security-scanner to address critical vulnerabilities in MCP server deployments. The scanner targets three recurring issues: overscoped filesystem/shell tools, missing authentication on transports, and unsanitized tool descriptions.

Why This Matters

In ideal models, MCP tools operate with least-privilege access; however, technical reality shows that local-dev SSE configurations are often promoted to production without authentication. This gap creates significant security risks where tool descriptions become prompt-injection surfaces and shell access is granted to entire filesystems rather than specific directories.

Key Insights

• Overprivileged tool scoping occurs when ‘shell’ or ‘fs’ tools are granted access to the entire filesystem instead of a single required directory (McHale, 2026).
• Prompt injection surfaces are created when tool descriptions are echoed verbatim into prompts without sanitization (McHale, 2026).
• @hailbytes/mcp-security-scanner provides SARIF output for integration into GitHub Code Scanning to alert developers on PRs.

Working Examples

CLI usage for scanning local configs and live endpoints with optional SARIF output.

# Scan a local config
npx @hailbytes/mcp-security-scanner ./mcp-config.json
# Scan a live endpoint
npx @hailbytes/mcp-security-scanner https://my-mcp-server.example.com
# SARIF output + fail the build
npx @hailbytes/mcp-security-scanner ./config.json --output=sarif --exit-code

Programmatic implementation of the security scanner.

import { scan } from "@hailbytes/mcp-security-scanner";
const report = await scan({ configPath: "./mcp-config.json" });
if (!report.passed) {
console.error(report.findings);
process.exit(1);
}

Practical Applications

References:

• https://dev.to/david_dev_sec/your-mcp-server-is-probably-overprivileged-heres-a-scanner-for-it-3cmb
• github.com/hailbytes/mcp-security-scanner