OAUTH

When to use

Use this skill when you need to:

• Implement or debug an OAuth 2.0/2.1 flow in a Fastify application
• Validate tokens, configure PKCE, or set up refresh token rotation
• Secure Fastify routes and plugins with access-control middleware
• Resolve RFC compliance questions or identify security anti-patterns

Step-by-step: Authorization Code + PKCE in Fastify

1. Install dependencies

npm install @fastify/oauth2 @fastify/cookie @fastify/session fastify-plugin

2. Register the OAuth plugin

// plugins/oauth.ts
import fp from 'fastify-plugin'
import oauth2, { OAuth2Namespace } from '@fastify/oauth2'
import { FastifyInstance } from 'fastify'

export default fp(async function (fastify: FastifyInstance) {
  fastify.register(oauth2, {
    name: 'oauth2',
    scope: ['openid', 'profile', 'email'],
    credentials: {
      client: {
        id: process.env.CLIENT_ID!,
        secret: process.env.CLIENT_SECRET!,
      },
      auth: {
        authorizeHost: process.env.AUTH_SERVER!,
        authorizePath: '/authorize',
        tokenHost: process.env.AUTH_SERVER!,
        tokenPath: '/token',
      },
    },
    startRedirectPath: '/login',
    callbackUri: process.env.CALLBACK_URI!,
    pkce: 'S256',               // RFC 7636 — always use for public clients
    generateStateFunction: (req) => req.session.state = crypto.randomUUID(),
    checkStateFunction: (req, callback) =>
      req.query.state === req.session.state ? callback() : callback(new Error('State mismatch')),
  })
})

Validation checkpoint: Confirm callbackUri exactly matches a registered redirect URI at the authorization server before proceeding (RFC 6749 §3.1.2).

3. Handle the callback and exchange the code

// routes/auth.ts
import { FastifyInstance } from 'fastify'

export default async function authRoutes(fastify: FastifyInstance) {
  fastify.get('/login/callback', async (request, reply) => {
    // @fastify/oauth2 verifies state and exchanges code automatically
    const tokenResponse = await fastify.oauth2.getAccessTokenFromAuthorizationCodeFlow(request)

    // Store only what you need; never log the raw token
    request.session.set('accessToken', tokenResponse.token.access_token)
    request.session.set('refreshToken', tokenResponse.token.refresh_token)

    return reply.redirect('/')
  })

  fastify.get('/logout', async (request, reply) => {
    await request.session.destroy()
    return reply.redirect('/')
  })
}

4. JWT validation middleware (token introspection hook)

// hooks/verifyToken.ts
import { FastifyRequest, FastifyReply } from 'fastify'
import jwt from '@fastify/jwt'

export async function verifyToken(request: FastifyRequest, reply: FastifyReply) {
  try {
    await request.jwtVerify()
    // Validate required claims (RFC 7519)
    const payload = request.user as Record<string, unknown>
    const now = Math.floor(Date.now() / 1000)

    if (typeof payload.exp === 'number' && payload.exp < now)
      return reply.code(401).send({ error: 'token_expired' })

    if (payload.iss !== process.env.EXPECTED_ISSUER)
      return reply.code(401).send({ error: 'invalid_issuer' })

    if (payload.aud !== process.env.EXPECTED_AUDIENCE)
      return reply.code(401).send({ error: 'invalid_audience' })

  } catch (err) {
    return reply.code(401).send({ error: 'invalid_token', error_description: (err as Error).message })
  }
}

Validation checkpoints:

• Verify exp, iss, aud, and sub on every request — never skip (RFC 7519 §4)
• Use fastify.jwt.verify (asymmetric RS256/ES256) rather than HS256 for tokens issued by a third-party server

5. Protecting routes

// routes/api.ts
import { FastifyInstance } from 'fastify'
import { verifyToken } from '../hooks/verifyToken'

export default async function apiRoutes(fastify: FastifyInstance) {
  fastify.addHook('onRequest', verifyToken)   // applies to all routes in this scope

  fastify.get('/me', {
    schema: {
      response: { 200: { type: 'object', properties: { sub: { type: 'string' } } } },
    },
  }, async (request) => {
    const user = request.user as { sub: string }
    return { sub: user.sub }
  })
}

6. Refresh token rotation

async function refreshAccessToken(fastify: FastifyInstance, refreshToken: string) {
  const newToken = await fastify.oauth2.getNewAccessTokenUsingRefreshTokenFlow({ refresh_token: refreshToken })

  // Always replace the stored refresh token if rotation is in use (RFC 6749 §10.4)
  return {
    accessToken: newToken.token.access_token,
    refreshToken: newToken.token.refresh_token ?? refreshToken,
  }
}

Security checklist

Common anti-patterns

• Storing tokens in localStorage — use HttpOnly, Secure, SameSite=Strict cookies instead
• Skipping audience validation — allows token reuse across services
• Using implicit flow — deprecated in OAuth 2.1; use authorization code + PKCE
• Accepting response_type=token in browser apps — tokens in URL fragments leak in logs/referrers
• Symmetric signing (HS256) for third-party tokens — use RS256/ES256 with JWKS endpoint

Further implementation references

• See DEVICE_FLOW.md for device authorization flow (RFC 8628) implementation
• See TOKEN_VALIDATION.md for JWKS rotation, caching strategies, and opaque token introspection
• See CLIENT_CREDENTIALS.md for machine-to-machine service authentication patterns
• See MOBILE_OAUTH.md for native/mobile app flows (RFC 8252) and custom URI schemes