PostureCheck MCP

Domain security checks for MCP-capable assistants — SPF, DKIM, DMARC, MTA-STS, TLS certificates and protocol support, and HTTP security headers.

An assistant can explain what DMARC is perfectly well. What it can't do is tell you whether your domain has it — that needs a live DNS lookup. This fills that gap.

> Is example.com protected against email spoofing?

  [PASS] SPF: SPF record found
  [FAIL] SPF policy: ~all (softfail — consider upgrading to -all)
  [PASS] DKIM: DKIM found (selector: google), 2048-bit
  [PASS] DMARC: DMARC record found
  DMARC policy: p=none (monitoring only — provides no protection against spoofing)

Install

No installation needed — it runs via npx.

Claude Desktop

Add to claude_desktop_config.json:

{
  "mcpServers": {
    "posturecheck": {
      "command": "npx",
      "args": ["-y", "posturecheck-mcp"]
    }
  }
}

On macOS that file lives at ~/Library/Application Support/Claude/claude_desktop_config.json.

Cursor, Cline, Windsurf and others

Same shape, in whichever config file that client uses:

{
  "mcpServers": {
    "posturecheck": {
      "command": "npx",
      "args": ["-y", "posturecheck-mcp"]
    }
  }
}

Restart the client afterwards.

Tools

`scan_domain`

Full posture check in one call. Email authentication, TLS, and HTTP security headers, with an overall score.

Useful for: "Is acme.com configured securely?", "Can anyone spoof email from our domain?", "Review the security of these five domains."

`check_tls`

TLS detail. Certificate validity, expiry, issuer, key strength, the full chain, and — importantly — which protocol versions the server actually accepts, tested with a separate handshake per version.

That last point matters. Most tools report the protocol that got negotiated, which is always the best one both sides support. Whether TLS 1.0 is still enabled is a different question, and it's the one a PCI DSS assessor asks.

Useful for: "Which TLS versions does example.com accept?", "When does this certificate expire?", "Why does this site work in my browser but fail in curl?"

`analyse_spf`

Resolves the complete SPF include tree, counts DNS lookups against the RFC 7208 limit of 10, and names the services authorised to send mail.

SPF silently fails with a PermError above 10 lookups, and nested includes make the count hard to eyeball — a record with five includes can easily be at eleven.

Useful for: "Why is our SPF failing?", "How many DNS lookups is our SPF using?", "Which services can send mail as our domain?"

What it checks

Email — SPF presence and policy, DNS lookup count, DKIM key detection and strength, DMARC policy and reporting address, MTA-STS.

TLS — certificate validity, expiry, issuer, key type and size, hostname coverage, full chain with missing-intermediate detection, protocol support for TLS 1.0 through 1.3, forward secrecy, CAA records.

HTTP — HSTS, Content-Security-Policy, X-Frame-Options, X-Content-Type-Options, Referrer-Policy.

Notes

Checks read publicly published DNS records and make standard HTTPS connections — the same information any browser or mail server receives. Nothing is probed, exploited, or accessed beyond that. See the scanning policy.

Results are not stored. There is no account and no API key.

Requests are rate limited. If you're scanning a large number of domains you'll be throttled; space them out.

Results reflect externally observable configuration only. A good score doesn't mean a domain is secure — it means the things visible from the public internet are configured sensibly.

Configuration

Variable	Default	Purpose
`POSTURECHECK_API`	`https://posturecheck.io`	Point at a different instance

Licence

MIT