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oauth2-oidc.md

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OAuth2 and OpenID Connect

Comprehensive reference for OAuth2 grant types, OIDC, provider integration, and social login.

OAuth2 Core Concepts

Roles

Role Description Example
Resource Owner The user who owns the data End user
Client The application requesting access Your web/mobile app
Authorization Server Issues tokens after authentication Auth0, Keycloak, your auth service
Resource Server Hosts the protected API Your API server

Key Terms

Term Description
Scope Permission level requested (e.g., read:users, write:posts)
Grant Type The flow used to obtain tokens
Authorization Code Temporary code exchanged for tokens
Access Token Token used to call the API
Refresh Token Token used to get new access tokens
Redirect URI Where the authorization server sends the user back
State CSRF protection parameter (random, unguessable)
PKCE Proof Key for Code Exchange (prevents code interception)

Authorization Code + PKCE

The recommended flow for web applications, mobile apps, and SPAs. PKCE (Proof Key for Code Exchange) protects against authorization code interception.

Flow

┌──────┐          ┌───────────────┐          ┌──────────────┐
│Client│          │ Authorization │          │   Resource   │
│      │          │    Server     │          │    Server    │
└──┬───┘          └───────┬───────┘          └──────┬───────┘
   │                      │                         │
   │ 1. Generate code_verifier (random)             │
   │    code_challenge = SHA256(code_verifier)       │
   │                      │                         │
   │ 2. Redirect to /authorize                      │
   │    ?response_type=code                         │
   │    &client_id=xxx                              │
   │    &redirect_uri=https://app/callback          │
   │    &scope=openid profile email                 │
   │    &state=random_csrf_value                    │
   │    &code_challenge=xxx                         │
   │    &code_challenge_method=S256                 │
   │──────────────>│                                │
   │               │                                │
   │ 3. User authenticates and consents             │
   │               │                                │
   │ 4. Redirect to callback                        │
   │    ?code=authorization_code                    │
   │    &state=random_csrf_value                    │
   │<──────────────│                                │
   │                                                │
   │ 5. POST /token                                 │
   │    grant_type=authorization_code               │
   │    &code=authorization_code                    │
   │    &redirect_uri=https://app/callback          │
   │    &client_id=xxx                              │
   │    &code_verifier=original_random_value        │
   │──────────────>│                                │
   │               │                                │
   │ 6. Response: access_token, refresh_token,      │
   │    id_token (if OIDC)                          │
   │<──────────────│                                │
   │                                                │
   │ 7. GET /api/resource                           │
   │    Authorization: Bearer access_token          │
   │────────────────────────────────────────────────>│
   │                                                │
   │ 8. Response: protected resource                │
   │<────────────────────────────────────────────────│

Implementation: Node.js

import crypto from 'crypto';

// Step 1: Generate PKCE values
function generatePKCE() {
  const verifier = crypto.randomBytes(32).toString('base64url');
  const challenge = crypto
    .createHash('sha256')
    .update(verifier)
    .digest('base64url');
  return { verifier, challenge };
}

// Step 2: Build authorization URL
function getAuthorizationUrl(config) {
  const { verifier, challenge } = generatePKCE();
  const state = crypto.randomBytes(16).toString('hex');

  // Store verifier and state in session
  // req.session.pkceVerifier = verifier;
  // req.session.oauthState = state;

  const params = new URLSearchParams({
    response_type: 'code',
    client_id: config.clientId,
    redirect_uri: config.redirectUri,
    scope: 'openid profile email',
    state,
    code_challenge: challenge,
    code_challenge_method: 'S256',
  });

  return `${config.authorizationEndpoint}?${params}`;
}

// Step 5: Exchange code for tokens
async function exchangeCode(code, verifier, config) {
  const response = await fetch(config.tokenEndpoint, {
    method: 'POST',
    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
    body: new URLSearchParams({
      grant_type: 'authorization_code',
      code,
      redirect_uri: config.redirectUri,
      client_id: config.clientId,
      client_secret: config.clientSecret, // Confidential clients only
      code_verifier: verifier,
    }),
  });

  if (!response.ok) {
    throw new Error(`Token exchange failed: ${response.status}`);
  }

  return response.json();
  // Returns: { access_token, refresh_token, id_token, token_type, expires_in }
}

Implementation: Python

import hashlib
import secrets
import base64
from urllib.parse import urlencode
import httpx

def generate_pkce():
    verifier = secrets.token_urlsafe(32)
    challenge = base64.urlsafe_b64encode(
        hashlib.sha256(verifier.encode()).digest()
    ).rstrip(b"=").decode()
    return verifier, challenge

def get_authorization_url(config: dict) -> tuple[str, str, str]:
    verifier, challenge = generate_pkce()
    state = secrets.token_hex(16)

    params = urlencode({
        "response_type": "code",
        "client_id": config["client_id"],
        "redirect_uri": config["redirect_uri"],
        "scope": "openid profile email",
        "state": state,
        "code_challenge": challenge,
        "code_challenge_method": "S256",
    })

    url = f"{config['authorization_endpoint']}?{params}"
    return url, verifier, state

async def exchange_code(code: str, verifier: str, config: dict) -> dict:
    async with httpx.AsyncClient() as client:
        response = await client.post(
            config["token_endpoint"],
            data={
                "grant_type": "authorization_code",
                "code": code,
                "redirect_uri": config["redirect_uri"],
                "client_id": config["client_id"],
                "client_secret": config["client_secret"],
                "code_verifier": verifier,
            },
        )
        response.raise_for_status()
        return response.json()

Redirect URI Validation

Critical security requirement: The authorization server must validate redirect URIs exactly.

Rule Why
Exact match required Prevents open redirect attacks
No wildcards in production Attacker could register matching subdomain
HTTPS required Prevent code interception on HTTP
No fragments (#) Fragment not sent to server
Pre-register all URIs Only allow known, trusted redirect targets

State Parameter

The state parameter prevents CSRF attacks on the OAuth2 flow:

// Before redirect: generate and store
const state = crypto.randomBytes(16).toString('hex');
req.session.oauthState = state;

// In callback: validate
if (req.query.state !== req.session.oauthState) {
  throw new Error('State mismatch - possible CSRF attack');
}
delete req.session.oauthState;

Client Credentials Grant

Server-to-server authentication with no user context.

┌──────────┐                    ┌───────────────┐
│  Service  │                    │ Authorization │
│  Client   │                    │    Server     │
└─────┬─────┘                    └───────┬───────┘
      │                                  │
      │  POST /token                     │
      │  grant_type=client_credentials   │
      │  &client_id=xxx                  │
      │  &client_secret=yyy             │
      │  &scope=read:data               │
      │─────────────────────────────────>│
      │                                  │
      │  { access_token, expires_in }    │
      │<─────────────────────────────────│

// Node.js implementation
async function getClientCredentialsToken(config) {
  const response = await fetch(config.tokenEndpoint, {
    method: 'POST',
    headers: {
      'Content-Type': 'application/x-www-form-urlencoded',
      Authorization: `Basic ${Buffer.from(
        `${config.clientId}:${config.clientSecret}`
      ).toString('base64')}`,
    },
    body: new URLSearchParams({
      grant_type: 'client_credentials',
      scope: config.scope,
    }),
  });

  const data = await response.json();

  // Cache the token until near expiry
  // tokenCache.set(cacheKey, data.access_token, data.expires_in - 60);

  return data.access_token;
}

# Python implementation
async def get_client_credentials_token(config: dict) -> str:
    async with httpx.AsyncClient() as client:
        response = await client.post(
            config["token_endpoint"],
            auth=(config["client_id"], config["client_secret"]),
            data={
                "grant_type": "client_credentials",
                "scope": config["scope"],
            },
        )
        response.raise_for_status()
        data = response.json()
        return data["access_token"]

Best practices:

  • Cache tokens until near expiry (subtract 60 seconds from expires_in)
  • Use mutual TLS (mTLS) for additional security in high-trust environments
  • Rotate client secrets periodically

Device Code Grant

For CLI tools, smart TVs, and devices without a browser or with limited input.

┌──────────┐          ┌───────────────┐          ┌──────────┐
│  Device   │          │ Authorization │          │  User's  │
│ (CLI/TV)  │          │    Server     │          │ Browser  │
└─────┬─────┘          └───────┬───────┘          └────┬─────┘
      │                        │                       │
      │ POST /device/code      │                       │
      │ client_id=xxx          │                       │
      │ scope=profile          │                       │
      │───────────────────────>│                       │
      │                        │                       │
      │ { device_code,         │                       │
      │   user_code: "ABCD-1234",                      │
      │   verification_uri,    │                       │
      │   interval: 5 }        │                       │
      │<───────────────────────│                       │
      │                        │                       │
      │ Display to user:       │                       │
      │ "Visit https://auth.example.com/device"        │
      │ "Enter code: ABCD-1234"│                       │
      │                        │                       │
      │                        │  User visits URL      │
      │                        │  and enters code      │
      │                        │<──────────────────────│
      │                        │                       │
      │                        │  User authenticates   │
      │                        │  and authorizes       │
      │                        │<──────────────────────│
      │                        │                       │
      │ Poll: POST /token      │                       │
      │ grant_type=urn:ietf:   │                       │
      │   params:oauth:        │                       │
      │   grant-type:device_code                       │
      │ device_code=xxx        │                       │
      │───────────────────────>│                       │
      │                        │                       │
      │ { access_token }       │                       │
      │<───────────────────────│                       │

// CLI implementation
async function deviceCodeFlow(config) {
  // 1. Request device code
  const codeResponse = await fetch(`${config.authServer}/device/code`, {
    method: 'POST',
    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
    body: new URLSearchParams({
      client_id: config.clientId,
      scope: 'openid profile',
    }),
  });

  const { device_code, user_code, verification_uri, interval } =
    await codeResponse.json();

  // 2. Display to user
  console.log(`Visit: ${verification_uri}`);
  console.log(`Enter code: ${user_code}`);

  // 3. Poll for completion
  while (true) {
    await new Promise((r) => setTimeout(r, interval * 1000));

    const tokenResponse = await fetch(`${config.authServer}/token`, {
      method: 'POST',
      headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
      body: new URLSearchParams({
        grant_type: 'urn:ietf:params:oauth:grant-type:device_code',
        device_code,
        client_id: config.clientId,
      }),
    });

    const data = await tokenResponse.json();

    if (data.error === 'authorization_pending') continue;
    if (data.error === 'slow_down') {
      interval += 5;
      continue;
    }
    if (data.error) throw new Error(data.error_description);

    return data; // { access_token, refresh_token, ... }
  }
}

Token Exchange (RFC 8693)

Allows a service to exchange one token for another, maintaining user context across microservices.

// Service A has user's token, needs to call Service B
async function exchangeToken(userToken, targetAudience, config) {
  const response = await fetch(config.tokenEndpoint, {
    method: 'POST',
    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
    body: new URLSearchParams({
      grant_type: 'urn:ietf:params:oauth:grant-type:token-exchange',
      subject_token: userToken,
      subject_token_type: 'urn:ietf:params:oauth:token-type:access_token',
      audience: targetAudience, // Service B's identifier
      scope: 'read:data',
    }),
  });

  return response.json();
  // Returns token with `act` claim showing delegation chain:
  // { "sub": "user_123", "act": { "sub": "service_a" } }
}

OpenID Connect (OIDC)

OIDC is an identity layer on top of OAuth2. While OAuth2 handles authorization (access to resources), OIDC handles authentication (who the user is).

What OIDC Adds to OAuth2

OAuth2 Only OIDC Adds
Access token (opaque) ID token (JWT with user info)
Resource access User identity
Scopes for permissions Standard identity scopes
No user info standard UserInfo endpoint
No discovery .well-known/openid-configuration

ID Token

The ID token is a JWT containing user identity information.

{
  "iss": "https://auth.example.com",
  "sub": "user_abc123",
  "aud": "client_id_xyz",
  "exp": 1700001500,
  "iat": 1700000600,
  "nonce": "random_nonce_value",
  "auth_time": 1700000500,
  "name": "Alice Smith",
  "email": "alice@example.com",
  "email_verified": true,
  "picture": "https://example.com/alice.jpg"
}

Standard OIDC Scopes

Scope Claims Returned
openid sub (required scope for OIDC)
profile name, family_name, given_name, picture, locale
email email, email_verified
address address (structured object)
phone phone_number, phone_number_verified

Discovery Document

GET https://auth.example.com/.well-known/openid-configuration

{
  "issuer": "https://auth.example.com",
  "authorization_endpoint": "https://auth.example.com/authorize",
  "token_endpoint": "https://auth.example.com/token",
  "userinfo_endpoint": "https://auth.example.com/userinfo",
  "jwks_uri": "https://auth.example.com/.well-known/jwks.json",
  "scopes_supported": ["openid", "profile", "email"],
  "response_types_supported": ["code"],
  "grant_types_supported": ["authorization_code", "client_credentials"],
  "subject_types_supported": ["public"],
  "id_token_signing_alg_values_supported": ["RS256", "ES256"],
  "code_challenge_methods_supported": ["S256"]
}

UserInfo Endpoint

// Fetch additional user info
const userInfo = await fetch('https://auth.example.com/userinfo', {
  headers: { Authorization: `Bearer ${accessToken}` },
}).then((r) => r.json());

// Response:
// {
//   "sub": "user_abc123",
//   "name": "Alice Smith",
//   "email": "alice@example.com",
//   "email_verified": true,
//   "picture": "https://example.com/alice.jpg"
// }

Provider Integration

Auth0

// Next.js with Auth0 SDK
// npm install @auth0/nextjs-auth0

// app/api/auth/[auth0]/route.ts
import { handleAuth } from '@auth0/nextjs-auth0';
export const GET = handleAuth();

// app/layout.tsx
import { UserProvider } from '@auth0/nextjs-auth0/client';
export default function RootLayout({ children }) {
  return <UserProvider>{children}</UserProvider>;
}

// Protected page
import { withPageAuthRequired, getSession } from '@auth0/nextjs-auth0';
export default withPageAuthRequired(async function Dashboard() {
  const session = await getSession();
  return <div>Welcome {session.user.name}</div>;
});

// API route protection
import { withApiAuthRequired, getSession } from '@auth0/nextjs-auth0';
export const GET = withApiAuthRequired(async (req) => {
  const session = await getSession();
  return Response.json({ user: session.user });
});

Clerk

// Next.js with Clerk
// npm install @clerk/nextjs

// middleware.ts
import { clerkMiddleware, createRouteMatcher } from '@clerk/nextjs/server';

const isProtectedRoute = createRouteMatcher(['/dashboard(.*)']);

export default clerkMiddleware(async (auth, request) => {
  if (isProtectedRoute(request)) {
    await auth.protect();
  }
});

// app/layout.tsx
import { ClerkProvider } from '@clerk/nextjs';
export default function RootLayout({ children }) {
  return <ClerkProvider>{children}</ClerkProvider>;
}

// Components
import { SignIn, SignUp, UserButton } from '@clerk/nextjs';
// <SignIn /> - full sign-in component
// <UserButton /> - user avatar with dropdown

Supabase Auth

// Supabase Auth with Row Level Security
import { createClient } from '@supabase/supabase-js';

const supabase = createClient(SUPABASE_URL, SUPABASE_ANON_KEY);

// Sign up
const { data, error } = await supabase.auth.signUp({
  email: 'user@example.com',
  password: 'secure-password',
});

// Sign in
const { data, error } = await supabase.auth.signInWithPassword({
  email: 'user@example.com',
  password: 'secure-password',
});

// OAuth (Google)
const { data, error } = await supabase.auth.signInWithOAuth({
  provider: 'google',
  options: { redirectTo: 'https://app.example.com/callback' },
});

// Get current session
const { data: { session } } = await supabase.auth.getSession();

// RLS policy (in PostgreSQL)
// CREATE POLICY "Users can read own data"
// ON profiles FOR SELECT
// USING (auth.uid() = user_id);

AWS Cognito

// AWS Cognito with Amplify
import { Amplify } from 'aws-amplify';
import { signIn, signUp, getCurrentUser } from 'aws-amplify/auth';

Amplify.configure({
  Auth: {
    Cognito: {
      userPoolId: 'us-east-1_xxxxx',
      userPoolClientId: 'xxxxx',
      loginWith: {
        oauth: {
          domain: 'auth.example.com',
          scopes: ['openid', 'profile', 'email'],
          redirectSignIn: ['https://app.example.com/callback'],
          redirectSignOut: ['https://app.example.com/'],
          responseType: 'code',
        },
      },
    },
  },
});

const { isSignedIn } = await signIn({
  username: 'user@example.com',
  password: 'secure-password',
});

Keycloak

// Keycloak with keycloak-js
import Keycloak from 'keycloak-js';

const keycloak = new Keycloak({
  url: 'https://keycloak.example.com',
  realm: 'my-realm',
  clientId: 'my-app',
});

await keycloak.init({
  onLoad: 'check-sso',
  pkceMethod: 'S256',
});

if (keycloak.authenticated) {
  const token = keycloak.token;
  const userInfo = await keycloak.loadUserInfo();
}

// Token refresh
keycloak.onTokenExpired = () => {
  keycloak.updateToken(30).catch(() => keycloak.login());
};

Social Login

Google

// Google OAuth2 specifics
const googleConfig = {
  authorizationEndpoint: 'https://accounts.google.com/o/oauth2/v2/auth',
  tokenEndpoint: 'https://oauth2.googleapis.com/token',
  scopes: 'openid email profile',
  // Quirks:
  // - Use `prompt=consent` to force consent screen (get refresh token)
  // - Use `access_type=offline` for refresh tokens
  // - Google ID tokens include `hd` (hosted domain) for Google Workspace
};

GitHub

// GitHub OAuth2 specifics
const githubConfig = {
  authorizationEndpoint: 'https://github.com/login/oauth/authorize',
  tokenEndpoint: 'https://github.com/login/oauth/access_token',
  userEndpoint: 'https://api.github.com/user',
  emailEndpoint: 'https://api.github.com/user/emails',
  // Quirks:
  // - No OIDC support (no ID token)
  // - Must fetch user info separately
  // - Email may be private; use /user/emails endpoint
  // - Token endpoint returns form-encoded by default
  //   (set Accept: application/json header)
  // - No refresh tokens (tokens don't expire unless revoked)
};

Apple

// Apple Sign In specifics
const appleConfig = {
  authorizationEndpoint: 'https://appleid.apple.com/auth/authorize',
  tokenEndpoint: 'https://appleid.apple.com/auth/token',
  // Quirks:
  // - Client secret is a JWT signed with your Apple private key
  // - User info (name, email) only returned on FIRST sign-in
  //   (must store it immediately)
  // - Users can hide email (relay address)
  // - Must validate ID token, Apple doesn't have UserInfo endpoint
  // - response_mode=form_post for web
};

// Generate Apple client secret (JWT)
import { SignJWT, importPKCS8 } from 'jose';

async function generateAppleClientSecret(config) {
  const privateKey = await importPKCS8(config.privateKey, 'ES256');

  return new SignJWT({})
    .setProtectedHeader({ alg: 'ES256', kid: config.keyId })
    .setIssuer(config.teamId)
    .setSubject(config.clientId)
    .setAudience('https://appleid.apple.com')
    .setIssuedAt()
    .setExpirationTime('180d')
    .sign(privateKey);
}

Scope Design

Naming Conventions

# Resource-based (recommended)
read:users
write:users
delete:users
admin:users

# Action-based
users.read
users.write
users.delete

# Hierarchical (coarse to fine)
users           # Full access to users
users:read      # Read-only access
users:profile   # Access to profile only

Scope Design Principles

Principle Description
Least privilege Request only needed scopes
Granularity balance Too fine = user confusion, too coarse = over-permission
Hierarchical Broader scope implies narrower ones
Descriptive Scope name should be self-explanatory
Documented Each scope has a user-facing description

Consent Management

// Scope validation middleware
function requireScopes(...requiredScopes) {
  return (req, res, next) => {
    const tokenScopes = req.auth.scope?.split(' ') || [];
    const hasAll = requiredScopes.every((s) => tokenScopes.includes(s));
    if (!hasAll) {
      return res.status(403).json({
        error: 'insufficient_scope',
        required: requiredScopes,
        granted: tokenScopes,
      });
    }
    next();
  };
}

// Usage
app.get('/api/users', requireScopes('read:users'), getUsers);
app.post('/api/users', requireScopes('write:users'), createUser);
app.delete('/api/users/:id', requireScopes('delete:users'), deleteUser);

Token Lifecycle

Token Endpoint Responses

// Successful token response
{
  "access_token": "eyJhbGciOi...",
  "token_type": "Bearer",
  "expires_in": 900,
  "refresh_token": "dGhpcyBpcyBh...",
  "id_token": "eyJhbGciOi...",
  "scope": "openid profile email"
}

// Error response
{
  "error": "invalid_grant",
  "error_description": "The authorization code has expired"
}

Token Introspection (RFC 7662)

Allows a resource server to check if a token is still valid (useful for opaque tokens).

// Resource server checks token validity
async function introspectToken(token, config) {
  const response = await fetch(config.introspectionEndpoint, {
    method: 'POST',
    headers: {
      'Content-Type': 'application/x-www-form-urlencoded',
      Authorization: `Basic ${Buffer.from(
        `${config.clientId}:${config.clientSecret}`
      ).toString('base64')}`,
    },
    body: new URLSearchParams({
      token,
      token_type_hint: 'access_token',
    }),
  });

  const data = await response.json();
  // { active: true, sub: "user_123", scope: "read:users", exp: 1700001500 }
  // { active: false } -- token is invalid/expired/revoked
  return data;
}

Token Revocation (RFC 7009)

// Revoke a token (on logout)
async function revokeToken(token, tokenType, config) {
  await fetch(config.revocationEndpoint, {
    method: 'POST',
    headers: {
      'Content-Type': 'application/x-www-form-urlencoded',
      Authorization: `Basic ${Buffer.from(
        `${config.clientId}:${config.clientSecret}`
      ).toString('base64')}`,
    },
    body: new URLSearchParams({
      token,
      token_type_hint: tokenType, // 'access_token' or 'refresh_token'
    }),
  });
  // Always returns 200 (even if token was already invalid)
}

Implementation Libraries

Auth.js (NextAuth.js)

// app/api/auth/[...nextauth]/route.ts
import NextAuth from 'next-auth';
import Google from 'next-auth/providers/google';
import GitHub from 'next-auth/providers/github';
import Credentials from 'next-auth/providers/credentials';

export const { handlers, auth, signIn, signOut } = NextAuth({
  providers: [
    Google({
      clientId: process.env.GOOGLE_CLIENT_ID,
      clientSecret: process.env.GOOGLE_CLIENT_SECRET,
    }),
    GitHub({
      clientId: process.env.GITHUB_CLIENT_ID,
      clientSecret: process.env.GITHUB_CLIENT_SECRET,
    }),
    Credentials({
      credentials: {
        email: { label: 'Email' },
        password: { label: 'Password', type: 'password' },
      },
      authorize: async (credentials) => {
        const user = await verifyCredentials(
          credentials.email,
          credentials.password
        );
        return user || null;
      },
    }),
  ],
  callbacks: {
    async jwt({ token, user, account }) {
      if (user) {
        token.role = user.role;
      }
      return token;
    },
    async session({ session, token }) {
      session.user.role = token.role;
      return session;
    },
  },
  pages: {
    signIn: '/login',
    error: '/auth/error',
  },
});

Passport.js (Express)

import passport from 'passport';
import { Strategy as GoogleStrategy } from 'passport-google-oauth20';

passport.use(
  new GoogleStrategy(
    {
      clientID: process.env.GOOGLE_CLIENT_ID,
      clientSecret: process.env.GOOGLE_CLIENT_SECRET,
      callbackURL: '/auth/google/callback',
    },
    async (accessToken, refreshToken, profile, done) => {
      const user = await db.users.upsert({
        where: { googleId: profile.id },
        create: {
          googleId: profile.id,
          email: profile.emails[0].value,
          name: profile.displayName,
        },
        update: { name: profile.displayName },
      });
      done(null, user);
    }
  )
);

// Routes
app.get('/auth/google', passport.authenticate('google', {
  scope: ['profile', 'email'],
}));

app.get('/auth/google/callback',
  passport.authenticate('google', { failureRedirect: '/login' }),
  (req, res) => res.redirect('/dashboard')
);

Python: Authlib / python-social-auth

# FastAPI with Authlib
from authlib.integrations.starlette_client import OAuth
from starlette.config import Config

oauth = OAuth()
oauth.register(
    name="google",
    server_metadata_url="https://accounts.google.com/.well-known/openid-configuration",
    client_id=config("GOOGLE_CLIENT_ID"),
    client_secret=config("GOOGLE_CLIENT_SECRET"),
    client_kwargs={"scope": "openid email profile"},
)

@app.get("/auth/google")
async def google_login(request: Request):
    redirect_uri = request.url_for("google_callback")
    return await oauth.google.authorize_redirect(request, redirect_uri)

@app.get("/auth/google/callback")
async def google_callback(request: Request):
    token = await oauth.google.authorize_access_token(request)
    userinfo = token.get("userinfo")
    # Create or update user, establish session
    return RedirectResponse(url="/dashboard")

Go: golang.org/x/oauth2

import (
    "golang.org/x/oauth2"
    "golang.org/x/oauth2/google"
)

var googleOAuthConfig = &oauth2.Config{
    ClientID:     os.Getenv("GOOGLE_CLIENT_ID"),
    ClientSecret: os.Getenv("GOOGLE_CLIENT_SECRET"),
    RedirectURL:  "https://app.example.com/auth/google/callback",
    Scopes:       []string{"openid", "profile", "email"},
    Endpoint:     google.Endpoint,
}

func handleGoogleLogin(w http.ResponseWriter, r *http.Request) {
    state := generateRandomState() // Store in session
    url := googleOAuthConfig.AuthCodeURL(state, oauth2.AccessTypeOffline)
    http.Redirect(w, r, url, http.StatusTemporaryRedirect)
}

func handleGoogleCallback(w http.ResponseWriter, r *http.Request) {
    // Validate state parameter
    code := r.URL.Query().Get("code")
    token, err := googleOAuthConfig.Exchange(r.Context(), code)
    if err != nil {
        http.Error(w, "Token exchange failed", http.StatusInternalServerError)
        return
    }

    // Use token to get user info
    client := googleOAuthConfig.Client(r.Context(), token)
    resp, _ := client.Get("https://www.googleapis.com/oauth2/v2/userinfo")
    // Parse response, create/update user, establish session
}

Deprecated: Implicit Grant

The OAuth2 Implicit grant (response_type=token) is deprecated and should not be used for new applications.

Why it was deprecated:

  • Access token exposed in URL fragment (browser history, referer headers)
  • No refresh tokens (user must re-authenticate)
  • No mechanism to verify the token was intended for your client
  • Vulnerable to token injection attacks

Migration: Use Authorization Code + PKCE instead, with a BFF for SPAs.