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Malik Abualzait
Malik Abualzait

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Micro‑MCP Security: Designing Least‑Privilege Context for LLMs

#security #llm #mcp #ai

By Malik Abualzait

Micro‑MCP applies microservice boundaries to the Model Context Protocol (MCP): a thin gateway composes many small MCP services, enforcing authentication, policy, and audit in one place while each service holds the minimal credentials it needs. The result is smaller blast radius, clearer ownership, and a safer path from demo to production.

Security and Policy Flow

Why security first for MCP

LLM apps need access to tools and data, but monolithic MCP servers concentrate power and permissions. One misconfiguration can expose everything. Micro‑MCP breaks capabilities into single‑purpose services (files, fetch, vector, SaaS) and adds a gateway that unifies discovery, authN/Z, and audit.

Threat model (practical)

  • Compromised or buggy capability implementation
  • Over‑permissive configuration (e.g., wildcard domains)
  • Prompt abuse leading to unintended tool execution
  • Network adversaries (tampering/replay) for remote transports

Security objectives

  • Least privilege: scope and time‑bound credentials
  • Isolation: per‑capability blast radius containment
  • Accountability: structured audit with request/response linkage
  • Transparency: explicit capability catalogs and policy decisions

Architecture security controls

Architecture security controls

  • Gateway (single client session):

    • Authenticate caller (static token in MVP; OIDC/mTLS in prod)
    • Authorize method with allowlists/scopes; log decision and reason
    • Namespacing (fs.*, http.fetchText) for clear boundaries
    • Correlate/restore request IDs; emit structured audit to stderr/logs
    • Optional: consent prompts, purpose binding for sensitive actions

  • Micro‑MCP services (N):

    • Minimal domain‑scoped credentials (e.g., read‑only FS, HTTP domain allowlist)
    • Input/output schema validation; output filtering/redaction where needed
    • Local logs/health; fail‑closed on policy or credential errors

Identity and authorization

  • Who: Human user, application, or agent identity
  • How: OIDC/OAuth2 bearer tokens or mutual TLS between client↔gateway
  • What: Scope‑based allowlists per capability; contextual ABAC for high‑risk methods
  • When: Short‑lived tokens; rotate credentials; no long‑lived static keys

Data protection and privacy

  • Redact PII/sensitive fields in logs
  • Classify outputs; tag structured audit with data categories
  • Prefer immutable resources by content hash; use signed URLs for large blobs

Deployment hardening

Deployment Hardening

  • Containerize gateway and services; use minimal images
  • SBOMs and signed artifacts (SPDX/SLSA); verify provenance
  • Network policy between gateway and services; least‑privilege egress
  • Health checks and autoscaling; circuit breakers for flaky backends

Common attack paths and mitigations

  • Over‑broad fetcher: enforce domain allowlists and content size limits
  • Path traversal in FS: canonicalize paths; enforce read‑only roots; deny symlinks
  • Prompt injection: policy guardrails + user consent for high‑impact tools
  • Token leakage: never log secrets; scope tokens narrowly; rotate

Why Micro‑MCP vs a single MCP server

  • Smaller blast radius: a compromise in http service doesn’t expose filesystem
  • Easier reviews: focused threat models per service; simpler SBOMs
  • Faster iteration: independent deploy/rollback of capabilities
  • Clearer SLOs: performance/error budgets per capability

Security checklist (starter) 

Gateway
- [ ] Require auth (OIDC/mTLS) and validate token audience/issuer
- [ ] Enforce method allowlists/scopes; log allow/deny decisions
- [ ] Emit structured audit: ts, principal, method, decision, latency
- [ ] Rate limits and request size caps per namespace

Services
- [ ] Use domain‑scoped, short‑lived credentials
- [ ] Validate inputs/outputs; sanitize outputs for logs
- [ ] Read‑only FS roots; deny symlinks; allowlist HTTP domains
- [ ] Health and minimal logs; fail‑closed on policy/secret errors

Platform
- [ ] Minimal base images; SBOM + signatures (SLSA)
- [ ] Network policies; no wildcard egress
- [ ] Secrets via manager; never in images or logs
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Roadmap for enterprise hardening

  • Externalize policy to OPA/Rego or Cedar; add consent/purpose prompts
  • Dynamic discovery and health with version negotiation; capability catalogs
  • End‑to‑end tracing with OpenTelemetry; redaction pipelines
  • Sandboxing (seccomp/Firecracker) for risky capabilities

Closing

Security is a property of boundaries. Micro‑MCP makes those boundaries explicit: a protocol‑aware gateway for cross‑cutting controls, and tiny services with least‑privilege credentials. That’s how “context access” becomes safe, observable, and ready for production.

Top comments (1)

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william_andersen profile image
William Andersen •

Excellent.
Thank you for sharing the good article.