Auth Implementation Patterns

Authentication & Authorization Implementation Patterns

Build secure, scalable authentication and authorization systems using industry-standard patterns and modern best practices.

Use this skill when

• Implementing user authentication systems
• Securing REST or GraphQL APIs
• Adding OAuth2/social login or SSO
• Designing session management or RBAC
• Debugging authentication or authorization issues

Do not use this skill when

• You only need UI copy or login page styling
• The task is infrastructure-only without identity concerns
• You cannot change auth policies or credential storage

Instructions

• Define users, tenants, flows, and threat model constraints.
• Choose auth strategy (session, JWT, OIDC) and token lifecycle.
• Design authorization model and policy enforcement points.
• Plan secrets storage, rotation, logging, and audit requirements.
• If detailed examples are required, open resources/implementation-playbook.md.

Safety

• Never log secrets, tokens, or credentials.
• Enforce least privilege and secure storage for keys.

Resources

• resources/implementation-playbook.md for detailed patterns and examples.

Authentication and Authorization Implementation Patterns Implementation Playbook

This file contains detailed patterns, checklists, and code samples referenced by the skill.

Core Concepts

1. Authentication vs Authorization

Authentication (AuthN): Who are you?

• Verifying identity (username/password, OAuth, biometrics)
• Issuing credentials (sessions, tokens)
• Managing login/logout

Authorization (AuthZ): What can you do?

• Permission checking
• Role-based access control (RBAC)
• Resource ownership validation
• Policy enforcement

2. Authentication Strategies

Session-Based:

• Server stores session state
• Session ID in cookie
• Traditional, simple, stateful

Token-Based (JWT):

• Stateless, self-contained
• Scales horizontally
• Can store claims

OAuth2/OpenID Connect:

• Delegate authentication
• Social login (Google, GitHub)
• Enterprise SSO

JWT Authentication

Pattern 1: JWT Implementation

// JWT structure: header.payload.signature
import jwt from 'jsonwebtoken';
import { Request, Response, NextFunction } from 'express';

interface JWTPayload {
    userId: string;
    email: string;
    role: string;
    iat: number;
    exp: number;
}

// Generate JWT
function generateTokens(userId: string, email: string, role: string) {
    const accessToken = jwt.sign(
        { userId, email, role },
        process.env.JWT_SECRET!,
        { expiresIn: '15m' }  // Short-lived
    );

    const refreshToken = jwt.sign(
        { userId },
        process.env.JWT_REFRESH_SECRET!,
        { expiresIn: '7d' }  // Long-lived
    );

    return { accessToken, refreshToken };
}

// Verify JWT
function verifyToken(token: string): JWTPayload {
    try {
        return jwt.verify(token, process.env.JWT_SECRET!) as JWTPayload;
    } catch (error) {
        if (error instanceof jwt.TokenExpiredError) {
            throw new Error('Token expired');
        }
        if (error instanceof jwt.JsonWebTokenError) {
            throw new Error('Invalid token');
        }
        throw error;
    }
}

// Middleware
function authenticate(req: Request, res: Response, next: NextFunction) {
    const authHeader = req.headers.authorization;
    if (!authHeader?.startsWith('Bearer ')) {
        return res.status(401).json({ error: 'No token provided' });
    }

    const token = authHeader.substring(7);
    try {
        const payload = verifyToken(token);
        req.user = payload;  // Attach user to request
        next();
    } catch (error) {
        return res.status(401).json({ error: 'Invalid token' });
    }
}

// Usage
app.get('/api/profile', authenticate, (req, res) => {
    res.json({ user: req.user });
});

Pattern 2: Refresh Token Flow

interface StoredRefreshToken {
    token: string;
    userId: string;
    expiresAt: Date;
    createdAt: Date;
}

class RefreshTokenService {
    // Store refresh token in database
    async storeRefreshToken(userId: string, refreshToken: string) {
        const expiresAt = new Date(Date.now() + 7 * 24 * 60 * 60 * 1000);
        await db.refreshTokens.create({
            token: await hash(refreshToken),  // Hash before storing
            userId,
            expiresAt,
        });
    }

    // Refresh access token
    async refreshAccessToken(refreshToken: string) {
        // Verify refresh token
        let payload;
        try {
            payload = jwt.verify(
                refreshToken,
                process.env.JWT_REFRESH_SECRET!
            ) as { userId: string };
        } catch {
            throw new Error('Invalid refresh token');
        }

        // Check if token exists in database
        const storedToken = await db.refreshTokens.findOne({
            where: {
                token: await hash(refreshToken),
                userId: payload.userId,
                expiresAt: { $gt: new Date() },
            },
        });

        if (!storedToken) {
            throw new Error('Refresh token not found or expired');
        }

        // Get user
        const user = await db.users.findById(payload.userId);
        if (!user) {
            throw new Error('User not found');
        }

        // Generate new access token
        const accessToken = jwt.sign(
            { userId: user.id, email: user.email, role: user.role },
            process.env.JWT_SECRET!,
            { expiresIn: '15m' }
        );

        return { accessToken };
    }

    // Revoke refresh token (logout)
    async revokeRefreshToken(refreshToken: string) {
        await db.refreshTokens.deleteOne({
            token: await hash(refreshToken),
        });
    }

    // Revoke all user tokens (logout all devices)
    async revokeAllUserTokens(userId: string) {
        await db.refreshTokens.deleteMany({ userId });
    }
}

// API endpoints
app.post('/api/auth/refresh', async (req, res) => {
    const { refreshToken } = req.body;
    try {
        const { accessToken } = await refreshTokenService
            .refreshAccessToken(refreshToken);
        res.json({ accessToken });
    } catch (error) {
        res.status(401).json({ error: 'Invalid refresh token' });
    }
});

app.post('/api/auth/logout', authenticate, async (req, res) => {
    const { refreshToken } = req.body;
    await refreshTokenService.revokeRefreshToken(refreshToken);
    res.json({ message: 'Logged out successfully' });
});

Session-Based Authentication

Pattern 1: Express Session

import session from 'express-session';
import RedisStore from 'connect-redis';
import { createClient } from 'redis';

// Setup Redis for session storage
const redisClient = createClient({
    url: process.env.REDIS_URL,
});
await redisClient.connect();

app.use(
    session({
        store: new RedisStore({ client: redisClient }),
        secret: process.env.SESSION_SECRET!,
        resave: false,
        saveUninitialized: false,
        cookie: {
            secure: process.env.NODE_ENV === 'production',  // HTTPS only
            httpOnly: true,  // No JavaScript access
            maxAge: 24 * 60 * 60 * 1000,  // 24 hours
            sameSite: 'strict',  // CSRF protection
        },
    })
);

// Login
app.post('/api/auth/login', async (req, res) => {
    const { email, password } = req.body;

    const user = await db.users.findOne({ email });
    if (!user || !(await verifyPassword(password, user.passwordHash))) {
        return res.status(401).json({ error: 'Invalid credentials' });
    }

    // Store user in session
    req.session.userId = user.id;
    req.session.role = user.role;

    res.json({ user: { id: user.id, email: user.email, role: user.role } });
});

// Session middleware
function requireAuth(req: Request, res: Response, next: NextFunction) {
    if (!req.session.userId) {
        return res.status(401).json({ error: 'Not authenticated' });
    }
    next();
}

// Protected route
app.get('/api/profile', requireAuth, async (req, res) => {
    const user = await db.users.findById(req.session.userId);
    res.json({ user });
});

// Logout
app.post('/api/auth/logout', (req, res) => {
    req.session.destroy((err) => {
        if (err) {
            return res.status(500).json({ error: 'Logout failed' });
        }
        res.clearCookie('connect.sid');
        res.json({ message: 'Logged out successfully' });
    });
});

OAuth2 / Social Login

Pattern 1: OAuth2 with Passport.js

import passport from 'passport';
import { Strategy as GoogleStrategy } from 'passport-google-oauth20';
import { Strategy as GitHubStrategy } from 'passport-github2';

// Google OAuth
passport.use(
    new GoogleStrategy(
        {
            clientID: process.env.GOOGLE_CLIENT_ID!,
            clientSecret: process.env.GOOGLE_CLIENT_SECRET!,
            callbackURL: '/api/auth/google/callback',
        },
        async (accessToken, refreshToken, profile, done) => {
            try {
                // Find or create user
                let user = await db.users.findOne({
                    googleId: profile.id,
                });

                if (!user) {
                    user = await db.users.create({
                        googleId: profile.id,
                        email: profile.emails?.[0]?.value,
                        name: profile.displayName,
                        avatar: profile.photos?.[0]?.value,
                    });
                }

                return done(null, user);
            } catch (error) {
                return done(error, undefined);
            }
        }
    )
);

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

app.get(
    '/api/auth/google/callback',
    passport.authenticate('google', { session: false }),
    (req, res) => {
        // Generate JWT
        const tokens = generateTokens(req.user.id, req.user.email, req.user.role);
        // Redirect to frontend with token
        res.redirect(`${process.env.FRONTEND_URL}/auth/callback?token=${tokens.accessToken}`);
    }
);

Authorization Patterns

Pattern 1: Role-Based Access Control (RBAC)

enum Role {
    USER = 'user',
    MODERATOR = 'moderator',
    ADMIN = 'admin',
}

const roleHierarchy: Record<Role, Role[]> = {
    [Role.ADMIN]: [Role.ADMIN, Role.MODERATOR, Role.USER],
    [Role.MODERATOR]: [Role.MODERATOR, Role.USER],
    [Role.USER]: [Role.USER],
};

function hasRole(userRole: Role, requiredRole: Role): boolean {
    return roleHierarchy[userRole].includes(requiredRole);
}

// Middleware
function requireRole(...roles: Role[]) {
    return (req: Request, res: Response, next: NextFunction) => {
        if (!req.user) {
            return res.status(401).json({ error: 'Not authenticated' });
        }

        if (!roles.some(role => hasRole(req.user.role, role))) {
            return res.status(403).json({ error: 'Insufficient permissions' });
        }

        next();
    };
}

// Usage
app.delete('/api/users/:id',
    authenticate,
    requireRole(Role.ADMIN),
    async (req, res) => {
        // Only admins can delete users
        await db.users.delete(req.params.id);
        res.json({ message: 'User deleted' });
    }
);

Pattern 2: Permission-Based Access Control

enum Permission {
    READ_USERS = 'read:users',
    WRITE_USERS = 'write:users',
    DELETE_USERS = 'delete:users',
    READ_POSTS = 'read:posts',
    WRITE_POSTS = 'write:posts',
}

const rolePermissions: Record<Role, Permission[]> = {
    [Role.USER]: [Permission.READ_POSTS, Permission.WRITE_POSTS],
    [Role.MODERATOR]: [
        Permission.READ_POSTS,
        Permission.WRITE_POSTS,
        Permission.READ_USERS,
    ],
    [Role.ADMIN]: Object.values(Permission),
};

function hasPermission(userRole: Role, permission: Permission): boolean {
    return rolePermissions[userRole]?.includes(permission) ?? false;
}

function requirePermission(...permissions: Permission[]) {
    return (req: Request, res: Response, next: NextFunction) => {
        if (!req.user) {
            return res.status(401).json({ error: 'Not authenticated' });
        }

        const hasAllPermissions = permissions.every(permission =>
            hasPermission(req.user.role, permission)
        );

        if (!hasAllPermissions) {
            return res.status(403).json({ error: 'Insufficient permissions' });
        }

        next();
    };
}

// Usage
app.get('/api/users',
    authenticate,
    requirePermission(Permission.READ_USERS),
    async (req, res) => {
        const users = await db.users.findAll();
        res.json({ users });
    }
);

Pattern 3: Resource Ownership

// Check if user owns resource
async function requireOwnership(
    resourceType: 'post' | 'comment',
    resourceIdParam: string = 'id'
) {
    return async (req: Request, res: Response, next: NextFunction) => {
        if (!req.user) {
            return res.status(401).json({ error: 'Not authenticated' });
        }

        const resourceId = req.params[resourceIdParam];

        // Admins can access anything
        if (req.user.role === Role.ADMIN) {
            return next();
        }

        // Check ownership
        let resource;
        if (resourceType === 'post') {
            resource = await db.posts.findById(resourceId);
        } else if (resourceType === 'comment') {
            resource = await db.comments.findById(resourceId);
        }

        if (!resource) {
            return res.status(404).json({ error: 'Resource not found' });
        }

        if (resource.userId !== req.user.userId) {
            return res.status(403).json({ error: 'Not authorized' });
        }

        next();
    };
}

// Usage
app.put('/api/posts/:id',
    authenticate,
    requireOwnership('post'),
    async (req, res) => {
        // User can only update their own posts
        const post = await db.posts.update(req.params.id, req.body);
        res.json({ post });
    }
);

Security Best Practices

Pattern 1: Password Security

import bcrypt from 'bcrypt';
import { z } from 'zod';

// Password validation schema
const passwordSchema = z.string()
    .min(12, 'Password must be at least 12 characters')
    .regex(/[A-Z]/, 'Password must contain uppercase letter')
    .regex(/[a-z]/, 'Password must contain lowercase letter')
    .regex(/[0-9]/, 'Password must contain number')
    .regex(/[^A-Za-z0-9]/, 'Password must contain special character');

// Hash password
async function hashPassword(password: string): Promise<string> {
    const saltRounds = 12;  // 2^12 iterations
    return bcrypt.hash(password, saltRounds);
}

// Verify password
async function verifyPassword(
    password: string,
    hash: string
): Promise<boolean> {
    return bcrypt.compare(password, hash);
}

// Registration with password validation
app.post('/api/auth/register', async (req, res) => {
    try {
        const { email, password } = req.body;

        // Validate password
        passwordSchema.parse(password);

        // Check if user exists
        const existingUser = await db.users.findOne({ email });
        if (existingUser) {
            return res.status(400).json({ error: 'Email already registered' });
        }

        // Hash password
        const passwordHash = await hashPassword(password);

        // Create user
        const user = await db.users.create({
            email,
            passwordHash,
        });

        // Generate tokens
        const tokens = generateTokens(user.id, user.email, user.role);

        res.status(201).json({
            user: { id: user.id, email: user.email },
            ...tokens,
        });
    } catch (error) {
        if (error instanceof z.ZodError) {
            return res.status(400).json({ error: error.errors[0].message });
        }
        res.status(500).json({ error: 'Registration failed' });
    }
});

Pattern 2: Rate Limiting

import rateLimit from 'express-rate-limit';
import RedisStore from 'rate-limit-redis';

// Login rate limiter
const loginLimiter = rateLimit({
    store: new RedisStore({ client: redisClient }),
    windowMs: 15 * 60 * 1000,  // 15 minutes
    max: 5,  // 5 attempts
    message: 'Too many login attempts, please try again later',
    standardHeaders: true,
    legacyHeaders: false,
});

// API rate limiter
const apiLimiter = rateLimit({
    windowMs: 60 * 1000,  // 1 minute
    max: 100,  // 100 requests per minute
    standardHeaders: true,
});

// Apply to routes
app.post('/api/auth/login', loginLimiter, async (req, res) => {
    // Login logic
});

app.use('/api/', apiLimiter);

Best Practices

1. Never Store Plain Passwords: Always hash with bcrypt/argon2 2. Use HTTPS: Encrypt data in transit 3. Short-Lived Access Tokens: 15-30 minutes max 4. Secure Cookies: httpOnly, secure, sameSite flags 5. Validate All Input: Email format, password strength 6. Rate Limit Auth Endpoints: Prevent brute force attacks 7. Implement CSRF Protection: For session-based auth 8. Rotate Secrets Regularly: JWT secrets, session secrets 9. Log Security Events: Login attempts, failed auth 10. Use MFA When Possible: Extra security layer

Common Pitfalls

• Weak Passwords: Enforce strong password policies
• JWT in localStorage: Vulnerable to XSS, use httpOnly cookies
• No Token Expiration: Tokens should expire
• Client-Side Auth Checks Only: Always validate server-side
• Insecure Password Reset: Use secure tokens with expiration
• No Rate Limiting: Vulnerable to brute force
• Trusting Client Data: Always validate on server

Resources

• references/jwt-best-practices.md: JWT implementation guide
• references/oauth2-flows.md: OAuth2 flow diagrams and examples
• references/session-security.md: Secure session management
• assets/auth-security-checklist.md: Security review checklist
• assets/password-policy-template.md: Password requirements template
• scripts/token-validator.ts: JWT validation utility