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Atlas Whoff
Atlas Whoff

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OAuth 2.0 and NextAuth Deep Dive: PKCE, Token Refresh, Route Protection, and Security

#auth #oauth #nextjs #security

Most developers know OAuth flows conceptually but struggle with the implementation details: token rotation, PKCE, refresh handling, and secure storage. Here's the complete picture.

OAuth 2.0 Flow Types

Authorization Code + PKCE (use this for web apps and SPAs):

  1. Generate code verifier + challenge
  2. Redirect user to provider with challenge
  3. Provider redirects back with code
  4. Exchange code + verifier for tokens
  5. Store tokens securely

Client Credentials (for server-to-server, no user):

  1. Send client_id + client_secret to token endpoint
  2. Receive access token
  3. Use token for API calls

Never use Implicit flow (deprecated) or Authorization Code without PKCE for SPAs.

NextAuth Configuration 

// lib/auth.ts
import { NextAuthOptions } from 'next-auth'
import GoogleProvider from 'next-auth/providers/google'
import GitHubProvider from 'next-auth/providers/github'
import { PrismaAdapter } from '@auth/prisma-adapter'
import { db } from './db'

export const authOptions: NextAuthOptions = {
  adapter: PrismaAdapter(db),
  providers: [
    GoogleProvider({
      clientId: process.env.GOOGLE_CLIENT_ID!,
      clientSecret: process.env.GOOGLE_CLIENT_SECRET!,
      authorization: {
        params: {
          prompt: 'consent',
          access_type: 'offline', // get refresh token
          response_type: 'code',
        },
      },
    }),
    GitHubProvider({
      clientId: process.env.GITHUB_CLIENT_ID!,
      clientSecret: process.env.GITHUB_CLIENT_SECRET!,
    }),
  ],
  callbacks: {
    async session({ session, user }) {
      session.user.id = user.id
      session.user.subscriptionStatus = user.subscriptionStatus
      return session
    },
    async signIn({ user, account }) {
      // Block disposable email domains
      if (isDisposableEmail(user.email)) return false
      return true
    },
  },
  pages: {
    signIn: '/auth/signin',
    error: '/auth/error',
  },
  session: { strategy: 'database' }, // server-side sessions, revocable
}
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Protecting Routes 

// middleware.ts -- runs on every request at the edge
import { withAuth } from 'next-auth/middleware'

export default withAuth({
  callbacks: {
    authorized({ token, req }) {
      const pathname = req.nextUrl.pathname

      // Admin routes require admin role
      if (pathname.startsWith('/admin')) {
        return token?.role === 'admin'
      }

      // All dashboard routes require auth
      if (pathname.startsWith('/dashboard')) {
        return !!token
      }

      return true
    },
  },
})

export const config = {
  matcher: ['/dashboard/:path*', '/admin/:path*', '/api/protected/:path*'],
}
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Token Refresh Handling 

// Refresh expired Google access tokens
async function refreshAccessToken(token: JWT): Promise<JWT> {
  try {
    const response = await fetch('https://oauth2.googleapis.com/token', {
      method: 'POST',
      headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
      body: new URLSearchParams({
        client_id: process.env.GOOGLE_CLIENT_ID!,
        client_secret: process.env.GOOGLE_CLIENT_SECRET!,
        grant_type: 'refresh_token',
        refresh_token: token.refreshToken as string,
      }),
    })

    const refreshed = await response.json()
    if (!response.ok) throw refreshed

    return {
      ...token,
      accessToken: refreshed.access_token,
      accessTokenExpires: Date.now() + refreshed.expires_in * 1000,
      refreshToken: refreshed.refresh_token ?? token.refreshToken,
    }
  } catch {
    return { ...token, error: 'RefreshAccessTokenError' }
  }
}
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Accessing OAuth Tokens for API Calls 

// Get the stored GitHub token for API calls
async function getGitHubToken(userId: string): Promise<string | null> {
  const account = await db.account.findFirst({
    where: { userId, provider: 'github' },
    select: { access_token: true },
  })
  return account?.access_token ?? null
}

// Use it
const token = await getGitHubToken(session.user.id)
const repos = await fetch('https://api.github.com/user/repos', {
  headers: { Authorization: `Bearer ${token}` },
})
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Security Checklist

  • Use httpOnly cookies for session storage (default in NextAuth)
  • Set secure: true in production
  • Use sameSite: 'lax' to prevent CSRF
  • Store NEXTAUTH_SECRET as a strong random string (32+ chars)
  • Rotate NEXTAUTH_SECRET periodically (invalidates all sessions)
  • Never expose OAuth client secrets in client-side code

The AI SaaS Starter at whoffagents.com ships with NextAuth fully configured: Google + GitHub OAuth, Prisma adapter, protected routes middleware, and session typing pre-built. $99 one-time.

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