Web & REST API Security Explained

Web and REST APIs are the engines of modern software, powering everything from your favorite mobile app to complex enterprise systems, as highlighted in this McKinsey report on APIs driving business growth. They allow different applications to communicate and share data seamlessly. But this connectivity comes with a risk: if an API isn't secure, it can become a wide-open door for attackers to steal data, disrupt services, and cause serious damage. Understanding web and REST API security is no longer just for security experts; it's essential knowledge for every developer, as emphasized by CISA’s API security advisory highlighting the rising risks and developer responsibilities.

TL;DR

Web and REST API security focuses on protecting API endpoints from unauthorized access and attacks. Key practices include implementing strong authentication (like OAuth 2.0), enforcing strict authorization to prevent data exposure, and validating all incoming data to block injection attacks. Following these REST API security best practices is crucial for building applications that are both functional and resilient.

What is Web & REST API Security?

At its core, web API security is the practice of protecting the integrity of APIs that are exposed over the internet. Since the vast majority of modern web APIs are built using the REST (Representational State Transfer) architectural style, the conversation quickly turns to REST API security.

REST APIs use standard HTTP methods (like GET, POST, PUT, DELETE) to perform operations on resources (data). For example, a GET /users/123 request fetches information about a specific user. Securing these APIs means ensuring that:

• Only legitimate users or services can make requests.
• Users can only access the data and perform the actions they are explicitly allowed to.
• The data exchanged is protected from eavesdropping or tampering.
• The API itself is resilient to attacks designed to crash it or abuse its logic.

Think of it like securing a building. You don't just lock the front door; you have security guards (authentication), key cards that only open certain doors (authorization), security cameras (monitoring), and reinforced windows (input validation). A layered defense is key.

Top 5 REST API Security Best Practices

Building a secure REST API isn't about finding a single magic bullet. It's about consistently applying a set of principles throughout the development lifecycle. Here are the five most important best practices to follow.

1. Robust Authentication: Verifying Who's Calling

Before your API does anything, it needs to answer the question: "Who are you?" Authentication is the process of verifying the identity of the client making the request. Sending requests to sensitive endpoints without authentication is like leaving your front door unlocked.

Best Practices:

• Avoid Basic Authentication: Don't send usernames and passwords with every request. This method is easy to intercept and compromise.
• Use Token-Based Authentication: The industry standard is to use tokens. The flow usually works like this:
• 1. The client sends its credentials (e.g., username/password) to an authentication server.
  2. The server validates the credentials and issues a signed token (like a JSON Web Token or JWT).
  3. The client includes this token in the Authorization header of every subsequent API request.
  4. The API server validates the token's signature and expiration on every call before processing the request.
• Implement OAuth 2.0: For applications where users grant access to their data, OAuth 2.0 is the definitive standard. It allows a user to grant a third-party application limited access to their resources without sharing their credentials.

2. Strict Authorization: Enforcing What They Can Do

Once you know who is making the request, you need to determine what they are allowed to do. Authorization is where many of the most critical API vulnerabilities occur, most notably Broken Object Level Authorization (BOLA).

BOLA happens when a user can access data belonging to another user simply by changing an ID in the request. For example, if an attacker can change GET /my-orders/123 to GET /my-orders/456 and see someone else's order, you have a BOLA vulnerability.

Best Practices:

• Never Trust the Client: Always perform authorization checks on the server side for every single request. Don't assume the client-side app will prevent a user from trying to access something they shouldn't.
• Check Ownership on Every Request: For any request that accesses a specific resource (e.g., /users/{userId}/profile), your code must verify that the authenticated user making the request is the owner of userId or has explicit permission (like an admin) to access it.
• Implement Role-Based Access Control (RBAC): Define clear roles (e.g., user, editor, admin) and associate specific permissions with each role. For example, only users with the admin role should be able to access an endpoint like DELETE /users/{userId}.

3. Rigorous Input Validation: Trusting No Data

You must treat all data sent from a client as potentially malicious. Without proper validation, attackers can send malformed data to crash your server or craft malicious payloads to execute injection attacks (like SQL or NoSQL injection).

Best Practices:

• Use a Schema: Define a strict schema for your API requests using a standard like the OpenAPI Specification. This schema should specify the data types, formats (e.g., regex for strings), and required fields for every endpoint.
• Validate on the Server: Your API gateway or application logic should validate every incoming request against this schema and immediately reject any request that doesn't conform with a 400 Bad Request error.
• Sanitize for Injection: Use parameterized queries or prepared statements when interacting with your database. Never build SQL queries by concatenating strings with user input. This is the most effective way to prevent SQL injection.

Automating these checks is essential. A platform like Aikido can identify endpoints that lack proper validation and other common vulnerabilities, integrating directly into your development workflow. You can start scanning your APIs for free with Aikido.

4. Encryption and Transport Layer Security (TLS)

Any data sent over a network without encryption can be read by anyone snooping on the traffic. This is especially dangerous for APIs that handle sensitive information like authentication tokens, personal data, or financial details.

Best Practices:

• Enforce HTTPS Everywhere: Configure your server to only accept connections over HTTPS (HTTP over TLS). This encrypts all data in transit, protecting it from man-in-the-middle attacks.
• Use Modern TLS Configurations: Disable outdated and insecure protocols like SSLv3 and older versions of TLS. Stick to TLS 1.2 or, preferably, TLS 1.3.
• Consider Encryption at Rest: For highly sensitive data, encrypt it in your database as well. This provides an additional layer of protection if an attacker gains access to your database files.

5. Rate Limiting and Monitoring: Spotting Abuse

Attackers often rely on automation to find vulnerabilities or disrupt your service. Rate limiting is your first line of defense against these automated attacks, helping prevent abuse and protect critical infrastructure, as outlined in Cloudflare's guide to API security.

Best Practices:

• Implement Rate Limiting: Set a reasonable limit on how many requests a single user or IP address can make in a given time frame (e.g., 100 requests per minute). This helps mitigate brute-force attacks on login endpoints and prevents denial-of-service (DoS) attacks from overwhelming your server.
• Log and Monitor API Activity: Log all important events, including successful requests, failed authentication attempts, and authorization failures. Feed these logs into a monitoring system that can alert you to suspicious patterns, such as a sudden spike in errors from a single IP address.

Web & REST API Security Checklist

Use this table as a quick reference to ensure you're covering the most important aspects of web and REST API security.

Conclusion

Securing web and REST APIs is not an afterthought; it is a fundamental part of the development process. By embedding these best practices into your workflow—strong authentication, strict authorization, rigorous validation, universal encryption, and vigilant monitoring— you can build APIs that are resilient by design. This not only protects your application and your users but also enables you to innovate with confidence, knowing your digital assets are secure. Try Aikido Now.