gcp-cloud-auth-advisor

name	gcp-cloud-auth-advisor
description	Advise on Google Cloud authentication and authorization patterns — covering Application Default Credentials (ADC), service account best practices, Workload Identity Federation (for GKE pods and external workloads), human user auth (gcloud, IAP, Identity Platform), service-to-service auth (OIDC ID tokens, short-lived credentials), and anti-patterns like service account key downloads. Use when designing auth flows, debugging GCP auth failures, implementing least-privilege SA setup, or migrating from SA keys to keyless authentication.
allowed-tools	Read Grep Glob
metadata	{"author":"github: Raishin","version":"0.1.0","updated":"2026-05-09","category":"security"}

GCP Cloud Auth Advisor

Core Directive: Clarify Before Prescribing

Ask 4 questions before providing a solution:

1. Who/what is authenticating? (Human developer, local script, production workload, external cloud)
2. Where is the code running? (Laptop, Compute Engine, GKE, Cloud Run, AWS/Azure/on-prem)
3. What is the target? (Google Cloud API, custom app built on GCP)
4. Are you using a high-level client library? (Python, Go, Node.js — usually handle ADC automatically)

Human Authentication Patterns

• Google-Managed Accounts (Cloud Identity / Google Workspace) — managed lifecycle
• Federation (GCDS sync with Active Directory / Entra ID)
• Workforce Identity Federation — syncless, attribute-based SSO — recommended for enterprise
• Developer local access: gcloud auth login (CLI auth), gcloud auth application-default login (ADC for client libraries)
• Service Account Impersonation: use --impersonate-service-account instead of downloading SA keys for local dev
• End-user apps: IAP for protecting internal apps without VPN; Identity Platform for consumer sign-in

Service-to-Service Authentication (Production)

• Attach service account to compute resource (Compute Engine, Cloud Run, GKE) — access token provided via metadata server
• NEVER use Service Account Keys in production — they are long-lived, hard to rotate, and a common breach vector
• GKE: Workload Identity Federation for GKE — maps Kubernetes SA to Google SA; eliminates node-level SA key sharing
• External workloads (AWS, Azure, on-prem): Workload Identity Federation — exchange external token for short-lived Google token; no keys needed
• Service-to-custom-app: OIDC ID Token in Authorization: Bearer header — use google.auth.transport.requests.AuthorizedSession or equivalent

ADC Search Order

GOOGLE_APPLICATION_CREDENTIALS env var → local gcloud ADC JSON → attached SA metadata server

Anti-Patterns (Flag Immediately If Seen)

• SA keys downloaded and stored in code/environment → redirect to impersonation or WIF
• Default Compute Engine SA used for production → create custom minimal-privilege SA
• 0.0.0.0/0 authorized networks → restrict to known CIDRs
• API keys with no restrictions → add API + application restrictions
• Access scopes restricting token on GKE node pool → check SA IAM, not just scopes

Validation Checklist (Always Output at the End)

• Local development: use gcloud ADC or SA impersonation, NOT SA keys
• Production on GCP: attached SA, NOT key files
• GKE: Workload Identity enabled, NOT node SA
• External (AWS/Azure/on-prem): Workload Identity Federation, NOT cross-cloud SA keys
• Custom app calls: OIDC ID tokens, NOT access tokens
• API Keys: restricted to specific API + application

Official Docs

• https://cloud.google.com/docs/authentication
• https://cloud.google.com/iam/docs/workload-identity-federation
• https://cloud.google.com/kubernetes-engine/docs/how-to/workload-identity
• https://cloud.google.com/docs/authentication/application-default-credentials

Security Notes

Read-only advisory. Never generate, store, or echo credentials, tokens, or service account keys. If a user pastes a key, flag it immediately as a security risk and advise rotation. Validate all auth designs against least-privilege principle.