Securing Your MCP Server with Firebase Auth: A Production Walkthrough

Model Context Protocol (MCP) servers let AI assistants interact with real user data. That means auth isn't optional — it's the difference between a useful tool and a data breach. This post walks through exactly how Can Tax Pro secures its Python MCP server with Firebase Authentication, supporting both Firebase ID tokens (for direct access) and a custom OAuth 2.0 flow (for third-party clients like Claude.ai).

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Architecture Overview

The system has three moving parts:

Browser / Claude.ai Client
       │
       │  Authorization: Bearer <token>
       ▼
  MCP Server (Python/FastMCP on Cloud Run)
       │
       │  Firebase Admin SDK
       ▼
  Firestore (data isolated by userId)

The MCP server accepts two token types:

1. Firebase ID tokens — issued by Firebase Authentication, verified cryptographically
2. Custom OAuth tokens (ctpo_*) — issued by the web app's OAuth server, stored as hashes in Firestore

The web app itself acts as the OAuth authorization server for third-party integrations.

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Step 1: Initialize Firebase Admin SDK

The server initializes Firebase Admin once at startup, with environment-aware credential resolution:

# main.py
import firebase_admin
from firebase_admin import credentials, auth as firebase_auth, firestore

if not firebase_admin._apps:
    sa_json = os.environ.get("FIREBASE_SERVICE_ACCOUNT")
    project_id = os.environ.get("FIREBASE_PROJECT_ID")
    if sa_json:
        cred = credentials.Certificate(json.loads(sa_json))
        firebase_admin.initialize_app(cred, {"projectId": project_id})
    else:
        firebase_admin.initialize_app(options={"projectId": project_id})

Locally: set FIREBASE_SERVICE_ACCOUNT to your service account JSON. On Cloud Run: omit it entirely — the SDK picks up Application Default Credentials (ADC) automatically via Workload Identity.

This means no secrets in production. Your Cloud Run service account just needs the Firebase Admin SDK Administrator Service Agent IAM role.

---

Step 2: Token Resolution

Two resolver functions handle each token type:

OAuth Tokens (ctpo_*)

Custom tokens are never stored in plaintext. The server hashes them with SHA-256 and looks up the hash in Firestore:

def resolve_oauth_token(bearer_token: str) -> str:
    token_hash = hashlib.sha256(bearer_token.encode()).hexdigest()
    doc = db.collection("oauthTokens").document(token_hash).get()
    if not doc.exists:
        raise ValueError("Invalid or revoked OAuth token")
    data = doc.to_dict()
    expires_at = data.get("expiresAt")
    if expires_at and expires_at < datetime.now(timezone.utc):
        raise ValueError("OAuth token expired")
    return data["userId"]

The Firestore document stores userId, expiresAt, clientId, and a refreshTokenHash. Revocation is instant — delete the document and the token stops working on the next request.

Firebase ID Tokens

Firebase handles the hard part:

def resolve_id_token(id_token: str) -> str:
    decoded = firebase_auth.verify_id_token(id_token)
    return decoded["uid"]

verify_id_token checks the signature against Google's public keys and validates claims (expiry, issuer, audience). It caches the public keys locally so it doesn't make a network call on every request.

---

Step 3: The Auth Middleware

A Starlette BaseHTTPMiddleware wraps every request. It tries the OAuth path first, then falls back to Firebase ID tokens:

class AuthMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        # Skip auth for public endpoints
        if request.url.path in _PUBLIC_PATHS:
            return await call_next(request)

        auth_header = request.headers.get("Authorization", "")
        if not auth_header.startswith("Bearer "):
            return Response(
                status_code=401,
                headers={"WWW-Authenticate": 'Bearer realm="tax-mcp"'},
            )

        token = auth_header.removeprefix("Bearer ").strip()
        token_set = _user_id_var.set("")

        try:
            # Try OAuth token first
            if token.startswith("ctpo_"):
                user_id = resolve_oauth_token(token)
            else:
                user_id = resolve_id_token(token)
            _user_id_var.set(user_id)
            return await call_next(request)
        except Exception as e:
            return Response(
                status_code=401,
                content=str(e),
                headers={"WWW-Authenticate": 'Bearer realm="tax-mcp"'},
            )
        finally:
            _user_id_var.reset(token_set)

Public paths (/health, /.well-known/oauth-protected-resource) bypass auth — necessary for health checks and OAuth discovery.

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Step 4: Thread-Safe User Context

Tools shouldn't take a user_id parameter — that would pollute every signature and make testing awkward. Instead, use a ContextVar to propagate identity through the async call stack:

# _context.py
from contextvars import ContextVar

_user_id_var: ContextVar[str] = ContextVar("user_id")

def get_user_id() -> str:
    try:
        return _user_id_var.get()
    except LookupError:
        raise RuntimeError("User context not set — request did not pass through auth middleware.")

Every tool calls get_user_id() to scope its Firestore queries:

# tools/income.py
from .._context import get_user_id

@mcp.tool()
def list_income(tax_year_id: str) -> list[dict]:
    docs = (
        db.collection("users")
          .document(get_user_id())
          .collection("taxYears")
          .document(tax_year_id)
          .collection("incomeEntries")
          .stream()
    )
    return [{"id": d.id, **d.to_dict()} for d in docs]

ContextVar is async-safe — each concurrent request gets its own context, so there's no cross-contamination between users even under high concurrency.

The middleware resets the var in a finally block, which is critical: without cleanup, the context leaks to the next request on a reused coroutine.

---

Step 5: The OAuth 2.0 Server (Web App Side)

For third-party clients (Claude.ai, etc.), the web app implements an OAuth 2.0 authorization server. Here's the flow:

1. Client → GET /oauth/authorize?client_id=...&code_challenge=...
2. User logs in (Firebase Auth)
3. Server → redirect with authorization_code
4. Client → POST /oauth/token with code + code_verifier
5. Server → { access_token: "ctpo_...", refresh_token: "ctpr_..." }

Token generation (TypeScript, server-side):

// server/routes/oauth/token.post.ts
import { createHash, randomBytes } from "crypto";

const accessToken = "ctpo_" + randomBytes(32).toString("hex");
const tokenHash = createHash("sha256").update(accessToken).digest("hex");

await db.collection("oauthTokens").doc(tokenHash).set({
  userId: session.userId,
  clientId: session.clientId,
  expiresAt: new Date(Date.now() + 60 * 60 * 1000), // 1 hour
  createdAt: new Date(),
});

return { access_token: accessToken, token_type: "bearer", expires_in: 3600 };

The ctpo_ prefix lets the MCP server route to the right resolver without trying Firebase first. Only the hash ever touches the database.

PKCE (Proof Key for Code Exchange) protects the authorization code flow. The client sends a code_challenge (SHA-256 of a random code_verifier) in step 1, then proves ownership by sending the raw code_verifier in step 4. The server hashes it and compares:

const verifierHash = createHash("sha256")
  .update(body.code_verifier)
  .digest("base64url");

if (verifierHash !== session.codeChallenge) {
  throw createError({ statusCode: 400, message: "Invalid code_verifier" });
}

---

Step 6: Client-Side (Browser)

The Angular/Analog web app attaches Firebase ID tokens to outbound API requests via an HTTP interceptor:

// auth.interceptor.ts
export const authInterceptor: HttpInterceptorFn = (req, next) => {
  const auth = inject(Auth);
  return from(auth.currentUser?.getIdToken() ?? Promise.resolve(null)).pipe(
    switchMap((token) => {
      if (!token) return next(req);
      return next(req.clone({
        setHeaders: { Authorization: `Bearer ${token}` },
      }));
    }),
  );
};

getIdToken() auto-refreshes the token when it's close to expiry, so you never send a stale JWT.

---

Step 7: Firestore Security Rules

The MCP server uses the Admin SDK, which bypasses all Firestore security rules. The get_user_id() context is your authorization layer. But rules provide defense-in-depth for direct client SDK access:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId}/{document=**} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
    match /oauthTokens/{tokenId} {
      allow read, write: if false; // Server-only
    }
  }
}

OAuth token documents are locked to server-only access. Users can never read or write them directly.

---

Step 8: OAuth Discovery Endpoint

Well-behaved OAuth clients (including Claude.ai) auto-discover server capabilities. Expose the RFC 9728 metadata endpoint:

@app.get("/.well-known/oauth-protected-resource")
async def oauth_protected_resource():
    return JSONResponse({
        "resource": "https://your-mcp-server.run.app",
        "authorization_servers": ["https://your-web-app.com"],
        "bearer_methods_supported": ["header"],
    })

This tells clients where to find the authorization server and how to present tokens. It's a public endpoint (no auth required) — make sure it's in _PUBLIC_PATHS.

---

Step 9: Local Testing

Don't rely on a real Firebase project for unit tests. Seed a test token directly:

# test_auth.py
import hashlib
from firebase_admin import firestore
from datetime import datetime, timezone, timedelta

TEST_TOKEN = "ctpo_localtest_" + "a" * 48
TEST_TOKEN_HASH = hashlib.sha256(TEST_TOKEN.encode()).hexdigest()

def seed_test_token(db, user_id: str):
    db.collection("oauthTokens").document(TEST_TOKEN_HASH).set({
        "userId": user_id,
        "clientId": "test-client",
        "expiresAt": datetime.now(timezone.utc) + timedelta(hours=1),
    })

def cleanup(db):
    db.collection("oauthTokens").document(TEST_TOKEN_HASH).delete()

Then test the three critical paths: no token → 401, invalid token → 401, valid token → 200.

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Security Properties

Property	How it's achieved
Token confidentiality	Only SHA-256 hashes stored; raw tokens never persist
Revocation	Delete Firestore doc; effective immediately
Expiry	Enforced server-side on every request
Multi-user isolation	ContextVar scopes every Firestore query to userId
No key management in prod	Cloud Run Workload Identity + ADC
Code injection protection	PKCE on OAuth code exchange
Defense in depth	Firestore rules lock down client SDK access

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Summary

Securing an MCP server with Firebase Auth comes down to four things:

1. Middleware that validates tokens before any tool runs
2. ContextVar to propagate user identity without polluting tool signatures
3. Hash-only storage for custom OAuth tokens
4. Workload Identity to eliminate service account key management in production

The dual-token design (Firebase ID tokens for direct use, custom OAuth tokens for third-party clients) keeps the server flexible while maintaining a single, auditable auth path. Every request either has a valid, unexpired token mapping to a real user, or it gets a 401.