Implementing OAuth 2.0 and OpenID Connect from Scratch in Flask

OAuth 2.0 is an authorization framework that enables third-party applications to obtain limited access to a user's account without exposing credentials. OpenID Connect (OIDC) adds an identity layer on top, providing a standard way to authenticate users via external providers.

OAuth 2.0 Flows Overview

• Authorization Code + PKCE: For web and mobile apps. Most secure, recommended default.
• Client Credentials: Machine-to-machine authentication (no user involved).
• Implicit: Deprecated. Never use for new applications.
• Device Code: For devices without browsers (smart TVs, CLI tools).

Authorization Code Flow with PKCE

import secrets, hashlib, base64
from authlib.integrations.flask_client import OAuth

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

@app.route("/auth/google/login")
def google_login():
    redirect_uri = url_for("google_callback", _external=True)
    return google.authorize_redirect(redirect_uri)

@app.route("/auth/google/callback")
def google_callback():
    token = google.authorize_access_token()
    userinfo = token.get("userinfo")
    # userinfo contains: sub (unique ID), email, name, picture
    return create_or_login_user(userinfo)

Token Validation

Always validate the ID token's signature, expiry, audience (aud), and issuer (iss) before trusting its contents. Use the provider's JWKS endpoint to verify the signature cryptographically. Never decode a JWT without validating the signature first.

Storing Tokens Securely

Store access tokens in httpOnly, Secure, SameSite=Lax cookies—never in localStorage (vulnerable to XSS). Refresh tokens should be rotated on every use and stored with the same cookie attributes. Implement token binding to prevent token theft from proxy servers.

Production JWT Verification Middleware

Here is an enterprise-grade Flask decorator verifying JWT auth tokens, verifying claims, and tracking client IP rate-limiting using Redis:

import jwt
from functools import wraps
from flask import request, jsonify
import redis

r_client = redis.Redis(host='localhost', port=6379, db=0)
SECRET_KEY = "super_secret_claims"

def requires_auth_and_rate_limit(f):
    @wraps(f)
    def decorated(*args, **kwargs):
        token = request.headers.get('Authorization', '').split(' ')[-1]
        if not token:
            return jsonify({"error": "Unauthorized. Access token is missing"}), 401
        try:
            payload = jwt.decode(token, SECRET_KEY, algorithms=['HS256'])
            client_id = payload['sub']
            
            # Rate-limiting: Max 100 requests per minute
            current = r_client.incr(f"rate_limit:{client_id}")
            if current == 1: r_client.expire(f"rate_limit:{client_id}", 60)
            if current > 100:
                return jsonify({"error": "Too many requests. Throttled"}), 429
                
        except jwt.ExpiredSignatureError:
            return jsonify({"error": "Token expired"}), 401
        except jwt.InvalidTokenError:
            return jsonify({"error": "Invalid token"}), 401
        return f(*args, **kwargs)
    return decorated

Data Flow & Security Verification Profile

Below is the benchmark analysis showing transactional latency, decryption overheads, and write throughput during high-frequency transaction testing:

US & UK Cybersecurity Standards and Risk Frameworks

Organizations operating across Transatlantic corridors face overlapping cybersecurity compliance environments. In the US, enterprise contracts demand compliance with the NIST Cybersecurity Framework (CSF), and government-facing SaaS requires FedRAMP authorization. In the UK, companies align with the National Cyber Security Centre (NCSC) Cyber Essentials Plus certification and global information security policies under ISO/IEC 27001. Implementing active intrusion monitoring, vulnerability scanning (SAST/DAST), and strict access control lists are essential components to secure enterprise client workloads.