Best Practices for Building Secure APIs

In today’s digital world, APIs (Application Programming Interfaces) play a crucial role in communication between systems. From mobile applications to complex cloud platforms, APIs enable fast and efficient integration and data exchange. However, every line of code carrying data between a server and a client can become a target for cyberattacks, which is why designing APIs with security in mind is so important.

Securing an API is not just a matter of company reputation or user trust. It is primarily about protecting personal data, system integrity, and business continuity. A seemingly minor code error can result in a serious breach, leading to financial losses, legal problems, and even business closure.

In this article, we will look at the most important practices related to API security, focusing on technical and organizational aspects that every development team should keep in mind.

Authorization and Authentication

API security begins with proper authentication and authorization. Although these terms are often mistakenly used interchangeably, they have different meanings:

• Authentication – The process of verifying the identity of a user or application. This is most often done using a login and password, but modern solutions also use JWT (JSON Web Tokens), OAuth 2.0, or biometrics.
• Authorization – The process of determining what resources a user has access to after successful authentication. An example could be a permission system where a user with an “admin” role has access to all data, while a “standard” user only has access to their own resources.

A well-secured API should use strong authentication mechanisms. API keys should be generated dynamically and have limited permissions. Access tokens with limited validity should also be used, minimizing the risk of interception.

A Few Best Practices:

• Use HTTP methods according to standards – Use GET requests for reading data, POST for creating, PUT for modifying, and DELETE for deleting. This way, we don’t unnecessarily expose ourselves to attacks. For example, using the GET method for deleting users exposes us to a CSRF attack (described later).
• Think of the API as a separate application – The frontend and backend are practically inseparable, which doesn’t change the fact that they are two separate applications. We cannot assume that an endpoint exposed to the world will never be used in a certain way just because there is no frontend access to it. Therefore, data validation and authorization are important on both the frontend and backend sides. Not doing so exposes us to attacks such as BOLA (described below).
• Implement secure solutions from the very beginning – Creating new features with security in mind and a solid, secure foundation is MUCH simpler than securing a ready and working system. Postponing this exposes us to implementation errors because it’s easy to overlook something, and we give ourselves more work because application security will have to be done someday anyway.

As Sun Tzu once said:

If you know the enemy and know yourself, you need not fear the result of a hundred battles.

Therefore:

Attacks on Website APIs

Website APIs often fall victim to various attacks that can lead to data loss or system compromise.

According to the current OWASP TOP 10, the most common include:

Descriptions of Selected Attacks:

• Broken Object Level Authorization (BOLA) – An attack involving unauthorized access to user data by manipulating resource identifiers in API requests. Example: a user changes their user ID in the URL to someone else’s (e.g., /api/users/123 → /api/users/124) and gains access to another person’s data.
• Man-in-the-Middle (MitM) Attacks – Intercepting API traffic between the client and server, allowing attackers to eavesdrop on or modify transmitted data. Example: an unsecured HTTP connection instead of HTTPS allows interception of an access token.
• Excessive Data Exposure – A situation where the API returns more information than necessary. Example: an endpoint returning full user data, including password hash, login IP address, and system role, even though the application only displays the first and last name.
• Rate Limit Bypass – An attacker bypasses mechanisms limiting the number of requests, allowing for brute-force or DDoS attacks. Example: changing headers or using different IP addresses to send a large number of requests.
• API Injection – Attempts to inject malicious code, e.g., SQL Injection or Command Injection, to gain control over the server or steal data. Example: a form field allows submitting the string ‘; DROP TABLE users; –, which, if improperly processed by the server, can lead to the deletion of the entire user table.
• Cross-Origin Resource Sharing (CORS) – Incorrect configuration of the CORS mechanism can allow attackers to make unauthorized requests from external domains, resulting in data leakage or bypassing authentication mechanisms. Proper configuration prevents the browser from displaying the result of a request sent from an untrusted domain.

Most of these attacks result from errors in API design and implementation. Regular security tests, log analysis, automated tests, and the use of frameworks supporting secure programming practices significantly reduce the risk of their occurrence. It is also worth using tools for automated security testing.

Additionally, test and development environments should be subject to a_s rigorous rules as the production environment – attacks often begin with the least protected system components, including test endpoints with real data.

CSRF (Cross-Site Request Forgery) Attacks

Although APIs in a REST architecture are generally not as susceptible to CSRF attacks as traditional web applications, there are still situations where they are possible – especially if the API handles requests authenticated with cookies. CSRF involves an attacker forcing a logged-in user to perform an unauthorized operation without their knowledge. Example: a user visits a malicious website that invisibly sends a POST request to the API of a service where the user is logged in, changing their password or deleting their account. It’s worth noting that making a GET request is much simpler than POST or DELETE, for example, because it can be sent with a simple click on a link.

To protect APIs from CSRF, avoid cookie-based authentication where possible – using tokens in Authorization headers is a better solution. For applications using cookies, it is worth implementing mechanisms such as CSRF tokens (anti-CSRF tokens).

Protection Against Injection Attacks

Although modern frameworks often validate input data used in database queries themselves, vulnerabilities to injection attacks still occur very frequently (they rank third in the OWASP ranking). Therefore, it is crucial to use prepared statements and properly filter and validate input data. It is also advisable to avoid returning detailed error messages that could help attackers understand the system’s structure.

WAF (Web Application Firewall) solutions, which monitor and filter incoming traffic, detecting injection attempts and other malicious operations, can also be helpful.

Data Encryption

All data transmitted via API should be secured with the HTTPS protocol using an up-to-date TLS certificate. Sensitive data, such as passwords or API keys, should be stored in encrypted or hashed form using secure algorithms, e.g., Argon2, bcrypt, or PBKDF2. This way, an attacker does not have access to passwords even with access to the database.

Rate Limiting and Traffic Monitoring

To prevent brute force or DDoS attacks, it is worth implementing mechanisms for limiting the number of requests (rate limiting). Tools such as API Gateway or WAF can be used for this purpose. Additionally, log monitoring and traffic pattern analysis allow for quick detection of anomalies and potential attacks.

AI and ML-based solutions, which automatically detect unusual user behavior and suspicious traffic patterns, are also increasingly being used. Such systems can identify an attack before it can cause real damage.

Principle of Least Privilege

The API should only provide the minimum access required to perform specific tasks. Each operation should be restricted according to the user’s permission policy. It is also worth applying separation of privileges between different environments (production, test, development).

Implementing an RBAC (Role-Based Access Control) or ABAC (Attribute-Based Access Control) policy allows for precisely defining who has access to specific resources and in what context. This avoids situations where users gain unintended access to critical data or functions.

Summary

A secure API is not just a matter of one-time implementation but a process requiring continuous analysis, testing, and monitoring. Regular security audits, penetration tests, and the implementation of best practices allow for minimizing risk and ensuring application stability.

It is worth treating security as an integral part of the software development process, not an add-on. Team education, automated security tests, and continuous evaluation of applied solutions should become standard in every organization that cares about stability, customer trust, and long-term business success.

Addendums

• More about CORS: https://outpost24.com/blog/exploiting-permissive-cors-configurations/
• OWASP TOP 10: https://owasp.org/www-project-top-ten/