API Security Hardener

Implement comprehensive security measures for production APIs.

Core Workflow

1. Input validation: Sanitize and validate all input
2. Authentication: Secure identity verification
3. Authorization: Role-based access control
4. Rate limiting: Prevent abuse
5. Security headers: HTTP header protection
6. Logging & monitoring: Detect threats

Input Validation

Zod Schema Validation

// validation/schemas.ts
import { z } from 'zod';

// Common schemas
export const emailSchema = z.string().email().toLowerCase().trim();

export const passwordSchema = z
  .string()
  .min(8, 'Password must be at least 8 characters')
  .max(128, 'Password too long')
  .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');

export const uuidSchema = z.string().uuid();

export const paginationSchema = z.object({
  page: z.coerce.number().int().positive().default(1),
  limit: z.coerce.number().int().min(1).max(100).default(20),
  sortBy: z.string().optional(),
  sortOrder: z.enum(['asc', 'desc']).default('desc'),
});

// User schemas
export const createUserSchema = z.object({
  email: emailSchema,
  password: passwordSchema,
  name: z.string().min(2).max(100).trim(),
});

export const updateUserSchema = createUserSchema.partial().omit({ password: true });

// Sanitize HTML content
export const sanitizedStringSchema = z.string().transform((val) => {
  return val
    .replace(/[<>]/g, '') // Remove < and >
    .replace(/javascript:/gi, '') // Remove javascript: protocol
    .replace(/on\w+=/gi, '') // Remove event handlers
    .trim();
});

Validation Middleware

// middleware/validate.ts
import { Request, Response, NextFunction } from 'express';
import { z, ZodSchema } from 'zod';

interface ValidationSchemas {
  body?: ZodSchema;
  query?: ZodSchema;
  params?: ZodSchema;
}

export function validate(schemas: ValidationSchemas) {
  return async (req: Request, res: Response, next: NextFunction) => {
    try {
      if (schemas.body) {
        req.body = await schemas.body.parseAsync(req.body);
      }
      if (schemas.query) {
        req.query = await schemas.query.parseAsync(req.query);
      }
      if (schemas.params) {
        req.params = await schemas.params.parseAsync(req.params);
      }
      next();
    } catch (error) {
      if (error instanceof z.ZodError) {
        return res.status(400).json({
          error: 'Validation Error',
          details: error.errors.map((e) => ({
            field: e.path.join('.'),
            message: e.message,
          })),
        });
      }
      next(error);
    }
  };
}

// Usage
router.post(
  '/users',
  validate({ body: createUserSchema }),
  createUserHandler
);

Rate Limiting

// middleware/rate-limit.ts
import rateLimit from 'express-rate-limit';
import RedisStore from 'rate-limit-redis';
import Redis from 'ioredis';

const redis = new Redis(process.env.REDIS_URL!);

// General API rate limit
export const apiLimiter = rateLimit({
  store: new RedisStore({
    sendCommand: (...args: string[]) => redis.call(...args),
  }),
  windowMs: 60 * 1000, // 1 minute
  max: 100, // 100 requests per minute
  message: {
    error: 'Too Many Requests',
    message: 'Please try again later',
    retryAfter: 60,
  },
  standardHeaders: true,
  legacyHeaders: false,
  keyGenerator: (req) => {
    // Use user ID if authenticated, otherwise IP
    return req.user?.id || req.ip;
  },
  skip: (req) => {
    // Skip rate limiting for health checks
    return req.path === '/health';
  },
});

// Stricter limit for authentication endpoints
export const authLimiter = rateLimit({
  store: new RedisStore({
    sendCommand: (...args: string[]) => redis.call(...args),
  }),
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 5, // 5 attempts
  message: {
    error: 'Too Many Attempts',
    message: 'Account temporarily locked. Try again in 15 minutes.',
  },
  keyGenerator: (req) => `auth:${req.ip}:${req.body?.email}`,
});

// Cost-based rate limiting for expensive operations
export const costLimiter = rateLimit({
  windowMs: 60 * 60 * 1000, // 1 hour
  max: 1000, // points per hour
  keyGenerator: (req) => req.user?.id || req.ip,
  handler: (req, res) => {
    res.status(429).json({
      error: 'Rate Limit Exceeded',
      message: 'Hourly quota exceeded',
    });
  },
});

// Usage with cost assignment
router.post('/expensive-operation', (req, res, next) => {
  req.rateLimit = { ...req.rateLimit, current: req.rateLimit.current + 10 };
  next();
}, costLimiter, handler);

Authentication Middleware

// middleware/auth.ts
import { Request, Response, NextFunction } from 'express';
import jwt from 'jsonwebtoken';

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

declare global {
  namespace Express {
    interface Request {
      user?: JWTPayload;
    }
  }
}

export function authenticate(req: Request, res: Response, next: NextFunction) {
  const authHeader = req.headers.authorization;

  if (!authHeader?.startsWith('Bearer ')) {
    return res.status(401).json({
      error: 'Unauthorized',
      message: 'Missing or invalid authorization header',
    });
  }

  const token = authHeader.slice(7);

  try {
    const payload = jwt.verify(token, process.env.JWT_SECRET!) as JWTPayload;

    // Check token expiration with buffer
    if (payload.exp * 1000 < Date.now()) {
      return res.status(401).json({
        error: 'Token Expired',
        message: 'Please re-authenticate',
      });
    }

    req.user = payload;
    next();
  } catch (error) {
    if (error instanceof jwt.TokenExpiredError) {
      return res.status(401).json({
        error: 'Token Expired',
        message: 'Please re-authenticate',
      });
    }
    if (error instanceof jwt.JsonWebTokenError) {
      return res.status(401).json({
        error: 'Invalid Token',
        message: 'Token validation failed',
      });
    }
    next(error);
  }
}

// Optional authentication
export function optionalAuth(req: Request, res: Response, next: NextFunction) {
  const authHeader = req.headers.authorization;

  if (!authHeader?.startsWith('Bearer ')) {
    return next();
  }

  const token = authHeader.slice(7);

  try {
    req.user = jwt.verify(token, process.env.JWT_SECRET!) as JWTPayload;
  } catch {
    // Ignore invalid tokens for optional auth
  }

  next();
}

Authorization Middleware

// middleware/authorize.ts
import { Request, Response, NextFunction } from 'express';

type Role = 'admin' | 'user' | 'guest';

interface Permission {
  resource: string;
  actions: string[];
}

const rolePermissions: Record<Role, Permission[]> = {
  admin: [
    { resource: '*', actions: ['*'] },
  ],
  user: [
    { resource: 'posts', actions: ['read', 'create', 'update:own', 'delete:own'] },
    { resource: 'comments', actions: ['read', 'create', 'update:own', 'delete:own'] },
    { resource: 'profile', actions: ['read', 'update'] },
  ],
  guest: [
    { resource: 'posts', actions: ['read'] },
    { resource: 'comments', actions: ['read'] },
  ],
};

export function authorize(...roles: Role[]) {
  return (req: Request, res: Response, next: NextFunction) => {
    if (!req.user) {
      return res.status(401).json({
        error: 'Unauthorized',
        message: 'Authentication required',
      });
    }

    const userRole = req.user.role as Role;

    if (!roles.includes(userRole)) {
      return res.status(403).json({
        error: 'Forbidden',
        message: 'Insufficient permissions',
      });
    }

    next();
  };
}

export function hasPermission(resource: string, action: string) {
  return (req: Request, res: Response, next: NextFunction) => {
    if (!req.user) {
      return res.status(401).json({ error: 'Unauthorized' });
    }

    const userRole = req.user.role as Role;
    const permissions = rolePermissions[userRole];

    const hasAccess = permissions.some((perm) => {
      const resourceMatch = perm.resource === '*' || perm.resource === resource;
      const actionMatch = perm.actions.includes('*') || perm.actions.includes(action);
      return resourceMatch && actionMatch;
    });

    if (!hasAccess) {
      return res.status(403).json({
        error: 'Forbidden',
        message: `Cannot ${action} ${resource}`,
      });
    }

    next();
  };
}

// Usage
router.delete('/posts/:id', authenticate, hasPermission('posts', 'delete'), deletePost);
router.get('/admin/users', authenticate, authorize('admin'), listUsers);

Security Headers

// middleware/security-headers.ts
import helmet from 'helmet';
import { Express } from 'express';

export function configureSecurityHeaders(app: Express) {
  app.use(helmet({
    contentSecurityPolicy: {
      directives: {
        defaultSrc: ["'self'"],
        scriptSrc: ["'self'"],
        styleSrc: ["'self'", "'unsafe-inline'"],
        imgSrc: ["'self'", 'data:', 'https:'],
        connectSrc: ["'self'", 'https://api.example.com'],
        fontSrc: ["'self'"],
        objectSrc: ["'none'"],
        mediaSrc: ["'self'"],
        frameSrc: ["'none'"],
      },
    },
    crossOriginEmbedderPolicy: false,
    crossOriginResourcePolicy: { policy: 'cross-origin' },
  }));

  // Additional security headers
  app.use((req, res, next) => {
    // Prevent caching of sensitive data
    if (req.path.startsWith('/api/')) {
      res.setHeader('Cache-Control', 'no-store, no-cache, must-revalidate, proxy-revalidate');
      res.setHeader('Pragma', 'no-cache');
      res.setHeader('Expires', '0');
    }

    // Permissions Policy
    res.setHeader(
      'Permissions-Policy',
      'camera=(), microphone=(), geolocation=(), interest-cohort=()'
    );

    next();
  });
}

SQL Injection Prevention

// db/queries.ts - Using parameterized queries
import { Pool } from 'pg';

const pool = new Pool();

// GOOD: Parameterized query
export async function getUserById(id: string) {
  const result = await pool.query(
    'SELECT * FROM users WHERE id = $1',
    [id]
  );
  return result.rows[0];
}

// GOOD: Using query builder (Prisma)
export async function searchUsers(term: string) {
  return prisma.user.findMany({
    where: {
      OR: [
        { name: { contains: term, mode: 'insensitive' } },
        { email: { contains: term, mode: 'insensitive' } },
      ],
    },
  });
}

// BAD: String interpolation (vulnerable)
// const result = await pool.query(`SELECT * FROM users WHERE id = '${id}'`);

XSS Prevention

// utils/sanitize.ts
import DOMPurify from 'isomorphic-dompurify';

// Sanitize HTML content
export function sanitizeHtml(dirty: string): string {
  return DOMPurify.sanitize(dirty, {
    ALLOWED_TAGS: ['b', 'i', 'em', 'strong', 'a', 'p', 'br'],
    ALLOWED_ATTR: ['href', 'target'],
  });
}

// Escape for plain text display
export function escapeHtml(text: string): string {
  const map: Record<string, string> = {
    '&': '&amp;',
    '<': '&lt;',
    '>': '&gt;',
    '"': '&quot;',
    "'": '&#039;',
  };
  return text.replace(/[&<>"']/g, (m) => map[m]);
}

// JSON response escaping (Express)
app.set('json escape', true);

Request Logging

// middleware/logging.ts
import { Request, Response, NextFunction } from 'express';
import { v4 as uuidv4 } from 'uuid';

export function requestLogger(req: Request, res: Response, next: NextFunction) {
  const requestId = req.headers['x-request-id'] as string || uuidv4();
  const startTime = Date.now();

  // Add request ID to response
  res.setHeader('X-Request-ID', requestId);
  req.requestId = requestId;

  // Log request
  console.log(JSON.stringify({
    type: 'request',
    requestId,
    method: req.method,
    path: req.path,
    query: req.query,
    ip: req.ip,
    userAgent: req.headers['user-agent'],
    userId: req.user?.sub,
    timestamp: new Date().toISOString(),
  }));

  // Log response
  res.on('finish', () => {
    const duration = Date.now() - startTime;

    console.log(JSON.stringify({
      type: 'response',
      requestId,
      method: req.method,
      path: req.path,
      statusCode: res.statusCode,
      duration,
      userId: req.user?.sub,
      timestamp: new Date().toISOString(),
    }));

    // Alert on suspicious activity
    if (res.statusCode === 401 || res.statusCode === 403) {
      console.warn(JSON.stringify({
        type: 'security_event',
        event: 'access_denied',
        requestId,
        ip: req.ip,
        path: req.path,
        statusCode: res.statusCode,
      }));
    }
  });

  next();
}

Error Handling

// middleware/error-handler.ts
import { Request, Response, NextFunction } from 'express';

export class AppError extends Error {
  constructor(
    public statusCode: number,
    public message: string,
    public code?: string
  ) {
    super(message);
    this.name = 'AppError';
  }
}

export function errorHandler(
  err: Error,
  req: Request,
  res: Response,
  next: NextFunction
) {
  console.error({
    type: 'error',
    requestId: req.requestId,
    error: err.message,
    stack: process.env.NODE_ENV === 'development' ? err.stack : undefined,
  });

  if (err instanceof AppError) {
    return res.status(err.statusCode).json({
      error: err.name,
      message: err.message,
      code: err.code,
    });
  }

  // Don't leak internal errors to clients
  res.status(500).json({
    error: 'Internal Server Error',
    message: 'An unexpected error occurred',
    requestId: req.requestId,
  });
}

Best Practices

1. Validate everything: Never trust client input
2. Use parameterized queries: Prevent SQL injection
3. Sanitize output: Prevent XSS
4. Rate limit: Protect against abuse
5. Log everything: Enable audit trails
6. Use HTTPS: Always encrypt in transit
7. Minimal responses: Don't leak information
8. Update dependencies: Patch vulnerabilities

Output Checklist

Every API security implementation should include:

• Input validation with schemas
• Authentication middleware
• Authorization (RBAC/ABAC)
• Rate limiting
• Security headers (Helmet)
• CORS configuration
• SQL injection prevention
• XSS prevention
• Request logging
• Error handling (no leaks)