REST API Security Interview Questions and Answers

Top 10 REST API Security interview questions with detailed answers, production scenarios, and senior-level best practices.

REST APIs are the backbone of modern web and mobile applications. They expose business functionality over HTTP, making them one of the primary attack surfaces. Every backend developer should understand how to secure REST APIs using industry-standard practices.

Q1. What is REST API Security?

Answer

REST API Security is the process of protecting RESTful APIs from unauthorized access, attacks, and data breaches while ensuring confidentiality, integrity, and availability.

A secure REST API should provide:

Authentication
Authorization
HTTPS Encryption
Input Validation
Rate Limiting
Logging & Monitoring

Example

Insecure API

GET /api/accounts/1001

Anyone who knows the endpoint can access customer information.

Secure API

GET /api/accounts/1001
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5...

The server validates the JWT token before processing the request.

Q2. What are the common security threats in REST APIs?

Answer

Common threats include:

Broken Authentication
Broken Authorization
SQL Injection
Cross-Site Scripting (XSS)
Cross-Site Request Forgery (CSRF)
Sensitive Data Exposure
API Key Leakage
Brute Force Attacks
Credential Stuffing
Denial of Service (DoS)

Production Scenario

A banking API exposes:

GET /accounts/1001

Without authorization checks, changing the account ID to:

GET /accounts/1002

could expose another customer's data.

Q3. Why should REST APIs always use HTTPS?

Answer

HTTPS encrypts all communication between clients and servers.

Without HTTPS:

Passwords can be intercepted.
JWT tokens can be stolen.
Sensitive customer information can be exposed.
Attackers can perform Man-in-the-Middle (MITM) attacks.

Benefits

Encryption
Data Integrity
Server Authentication

Interview Tip

Never expose login or payment APIs over HTTP.

Q4. Why should REST APIs be stateless?

Answer

REST follows a stateless architecture.

Every request should contain all the information required to authenticate and authorize the user.

Example:

GET /orders
Authorization: Bearer <JWT_TOKEN>

The server does not maintain client session information.

Benefits

Better scalability
Easier load balancing
Simpler horizontal scaling
Better suited for microservices

Q5. Which authentication methods are commonly used in REST APIs?

Answer

Common authentication methods include:

JWT Authentication
OAuth2
API Keys
Basic Authentication (legacy/internal systems)
Mutual TLS (mTLS)
OpenID Connect (OIDC)

Enterprise Recommendation

API Type	Recommended Authentication
Public APIs	OAuth2 + JWT
Internal Microservices	mTLS + JWT
Partner APIs	OAuth2 Client Credentials
Legacy Systems	Basic Authentication over HTTPS

Q6. How do you secure REST APIs in Spring Boot?

Answer

Spring Boot applications are commonly secured using Spring Security.

Typical security measures include:

Spring Security
JWT Authentication
OAuth2 Resource Server
Method-Level Security
Role-Based Access Control (RBAC)
Input Validation
Exception Handling
HTTPS
Security Headers

Example:

@PreAuthorize("hasRole('ADMIN')")
@GetMapping("/users")
public List<User> getUsers() {
    return service.findAll();
}

Only users with the ADMIN role can access this endpoint.

Q7. How do you prevent unauthorized access to REST APIs?

Answer

Use multiple layers of protection:

JWT Validation
OAuth2
RBAC
ABAC
API Gateway
Token Expiration
Refresh Tokens
Method-Level Authorization

Example

A customer should not be allowed to execute:

DELETE /users/100

Only administrators should have permission.

Q8. What HTTP security headers should REST APIs return?

Answer

Important security headers include:

Strict-Transport-Security (HSTS)
Content-Security-Policy (CSP)
X-Content-Type-Options
X-Frame-Options
Referrer-Policy
Cache-Control

Example:

Strict-Transport-Security:
max-age=31536000

These headers help prevent browser-based attacks and improve overall security.

Q9. How do you protect REST APIs from brute-force and DDoS attacks?

Answer

Use multiple protection mechanisms:

Rate Limiting
Request Throttling
CAPTCHA (for login endpoints)
Web Application Firewall (WAF)
API Gateway
IP Blocking
Redis-based Rate Limiting

Example:

100 Requests / Minute

If the limit is exceeded:

HTTP/1.1 429 Too Many Requests

Q10. What are the REST API security best practices?

Answer

Follow these production best practices:

Always use HTTPS
Never expose sensitive data in URLs
Use OAuth2 or JWT authentication
Validate every client input
Implement least privilege authorization
Use an API Gateway
Enable request logging and auditing
Rotate secrets regularly
Use short-lived access tokens
Scan for vulnerabilities regularly
Follow OWASP API Security recommendations

Production Security Architecture

                Internet
                    │
              HTTPS / TLS
                    │
              Load Balancer
                    │
              API Gateway
                    │
        Authentication (JWT/OAuth2)
                    │
          Rate Limiting / WAF
                    │
           Spring Security Filter
                    │
           Authorization (RBAC)
                    │
            Business Services
                    │
               Database

Senior Interview Tip

Enterprise-grade API security is never achieved with a single mechanism. Modern applications use layered security (Defense in Depth) by combining:

HTTPS
API Gateway
Authentication
Authorization
Input Validation
Rate Limiting
Logging
Monitoring
Secret Management

This approach significantly reduces the attack surface and improves overall system resilience.

Quick Revision

REST APIs should always use HTTPS.
REST APIs should be stateless.
Use JWT or OAuth2 for authentication.
Apply RBAC or ABAC for authorization.
Validate all client inputs.
Protect against SQL Injection and XSS.
Use API Gateway for centralized security.
Implement rate limiting to prevent abuse.
Enable logging and monitoring.
Follow OWASP API Security best practices.