API Security Basics: Protecting Your Business Integrations

APIs (Application Programming Interfaces) are the connective tissue of modern software, enabling applications to communicate and share data. They also represent a massive and growing attack surface. Gartner predicted that APIs would become the most frequent attack vector by 2025, and the trend is clear: API attacks have grown 400 percent in recent years. Poorly secured APIs expose customer data, enable unauthorized access, and create pathways for system compromise.

Why APIs Are Vulnerable

Lack of visibility. Many organizations do not have a complete inventory of their APIs. Shadow APIs (undocumented or forgotten endpoints) are particularly dangerous because they are not covered by security controls. The average enterprise has 15,000 to 25,000 APIs, and security teams are typically aware of only a fraction.

Authentication and authorization failures. APIs that accept requests without proper authentication, use weak API keys, or fail to enforce proper authorization allow attackers to access data and functionality they should not reach. Broken Object Level Authorization (BOLA) is the OWASP API Security Top 10’s number one risk, allowing attackers to access other users’ data by modifying object identifiers in API requests.

Excessive data exposure. APIs often return more data than the client needs, relying on the client application to filter sensitive fields. An attacker calling the API directly receives all the data, including fields the UI was designed to hide.

Rate limiting failures. Without rate limiting, attackers can brute-force credentials, enumerate resources, or cause denial of service by flooding API endpoints with requests.

OWASP API Security Top 10

The key risks include: Broken Object Level Authorization, Broken Authentication, Broken Object Property Level Authorization, Unrestricted Resource Consumption, Broken Function Level Authorization, Unrestricted Access to Sensitive Business Flows, Server-Side Request Forgery, Security Misconfiguration, Improper Inventory Management, and Unsafe Consumption of APIs.

Protection Strategies

Authentication. Use OAuth 2.0 with short-lived access tokens and refresh tokens rather than static API keys. Require mutual TLS (mTLS) for server-to-server communication. Never embed API keys in client-side code.

Authorization. Validate authorization at the object level on every request. Do not rely on client-side enforcement. Implement RBAC (Role-Based Access Control) and verify that users can only access resources they own or are explicitly authorized to view.

Input validation. Validate all input against strict schemas. Reject requests that do not match expected formats, types, and ranges.

Rate limiting and throttling. Implement rate limits per user, per IP, and per endpoint. Use graduated responses: warning, temporary block, permanent block.

API gateway. Deploy an API gateway (Kong, Apigee, AWS API Gateway) as the single entry point for all API traffic. The gateway enforces authentication, rate limiting, logging, and request/response transformation.

For the secure coding practices that prevent API vulnerabilities, see our secure coding guide. To monitor API traffic for attacks, explore our security operations center guide.

API Discovery and Inventory

You cannot secure APIs you do not know about. Implement API discovery tools that analyze network traffic, code repositories, and gateway logs to identify all APIs in your environment. Many organizations discover significantly more APIs than their security team was aware of, including deprecated endpoints that are still accessible and internal APIs exposed to the internet.

Maintain an API inventory that documents each API’s purpose, owner, authentication method, data sensitivity, and consumer applications. Review this inventory quarterly and decommission APIs that are no longer needed. An unused API that remains accessible is an attack surface that provides no business value.

API Security Testing

Include API security testing in your development lifecycle. Automated API security scanners like OWASP ZAP, Burp Suite, and Postman can test for common vulnerabilities during development. Manual penetration testing of APIs should be conducted annually or before major releases. API-specific penetration testing focuses on authorization bypass, injection, excessive data exposure, and business logic flaws that automated scanners may miss.