What Is API Security? A Complete Guide

Introduction

Application Programming Interfaces (APIs) are an essential part of modern software. They allow websites, mobile apps, cloud services, and other systems to communicate and exchange data. For example, when a mobile banking app displays your account information, an API may connect the app with the bank’s backend services.

Because APIs can expose application functionality and sensitive information, protecting them is an important part of cybersecurity. OWASP describes API security as the strategies and solutions used to understand and reduce the unique security risks and vulnerabilities associated with APIs.

In this guide, we will explain what API security means, why it matters, common API security risks, and practical ways organizations can protect their APIs.

1. API Security Protects Application Interfaces

API security is the practice of protecting APIs from unauthorized access, data exposure, abuse, and other security threats.

An API acts as a communication layer between different software components. If that communication is not properly protected, attackers may attempt to access information or functionality they are not supposed to use.

Good API security can involve:

• Authentication
• Authorization
• Encryption
• Input validation
• Rate limiting
• Monitoring
• Security testing
• Proper API inventory and management

Since APIs can be customer-facing, partner-facing, or internal, security needs can vary depending on how an API is used.

2. Authentication Verifies Who Is Making a Request

Authentication determines whether the person, application, or service making an API request is legitimate.

For example, an application may require a valid access token before allowing a user to retrieve account information.

Weak authentication can create serious risks. OWASP’s API Security Top 10 identifies Broken Authentication as one of the major API security risks. Problems can include poorly protected credentials, improperly validated tokens, and insufficient protection against repeated login attempts.

Developers should use established authentication standards and carefully protect credentials and authentication tokens.

3. Authorization Controls What Users Can Access

Authentication answers “Who are you?”, while authorization answers “What are you allowed to access?”

For example, a customer may be authenticated successfully but should only be able to view their own orders—not another customer’s orders.

Broken authorization is a major API security concern. OWASP’s 2023 API Security Top 10 includes Broken Object Level Authorization and Broken Function Level Authorization among its listed risks.

APIs should therefore verify permissions whenever users attempt to access resources or perform sensitive functions.

4. Encryption Protects Data During Transmission

APIs frequently transmit information between applications and servers. Encryption helps protect that information while it travels across a network.

Using HTTPS with properly configured TLS is an important foundation for protecting API communications.

Encryption does not solve every API security problem, however. An encrypted connection cannot prevent an authorized user from accessing data they should not have permission to see. API security therefore requires multiple layers of protection.

5. API Security Helps Prevent Data Exposure

APIs may provide access to personal information, business information, account details, and other sensitive data.

An API should return only the information that a legitimate requester needs. Excessive information in API responses can increase the impact of a security problem.

OWASP’s 2023 API Security Top 10 includes Broken Object Property Level Authorization, which addresses improper authorization at the level of individual properties within an object.

Developers should carefully define which fields each API response is allowed to expose.

6. Rate Limiting Helps Control Excessive Requests

APIs can receive a large number of requests. Without appropriate controls, excessive requests can consume network bandwidth, CPU, memory, storage, or other resources.

Rate limiting places reasonable restrictions on how frequently requests can be made.

OWASP lists Unrestricted Resource Consumption as API4:2023 and notes that excessive consumption can contribute to denial-of-service conditions or increased operational costs.

Rate limits should be designed according to the API’s purpose and expected traffic rather than using the same limit for every endpoint.

7. Input Validation Helps Block Malicious Requests

APIs should not automatically trust data supplied by clients.

Input validation checks whether incoming data follows the expected format, type, size, and other rules before the application processes it.

For example, if an API expects a numeric product ID, it should validate that the received value meets the expected requirements.

Validation should be combined with other security controls rather than treated as a complete security solution.

8. API Monitoring Helps Detect Suspicious Activity

Security does not end after an API is deployed.

Organizations should monitor API activity to identify unusual patterns, errors, authentication problems, and potentially suspicious behavior.

Useful monitoring can include:

• Request and response metrics
• Authentication failures
• Unusual traffic patterns
• Error rates
• Unexpected access attempts
• Changes in API behavior

Logging and monitoring can help security teams investigate incidents and understand what happened when something goes wrong.

9. Regular API Testing Finds Security Weaknesses

API security testing can help developers discover weaknesses before attackers exploit them.

Testing can examine areas such as:

• Authentication
• Authorization
• Input validation
• Access controls
• Configuration
• Rate limits
• Error handling
• Sensitive data exposure

Organizations should also maintain an accurate inventory of their APIs. OWASP highlights Improper Inventory Management because outdated API versions, undocumented endpoints, and exposed development or debugging interfaces can increase security risk.

10. API Security Requires Continuous Protection

API security is not a one-time task. APIs change as applications grow, new features are introduced, and integrations are added.

OWASP’s 2023 API Security Top 10 identifies ten major risk categories, including Broken Object Level Authorization, Broken Authentication, Broken Object Property Level Authorization, Unrestricted Resource Consumption, Broken Function Level Authorization, SSRF, Security Misconfiguration, Improper Inventory Management, and Unsafe Consumption of APIs.

Organizations should therefore review API security throughout the development and operational lifecycle.

Conclusion

API security is an important part of modern application security. APIs connect applications, services, and users, but those connections can also create opportunities for unauthorized access and data exposure.

Strong API security combines authentication, authorization, encryption, input validation, rate limiting, monitoring, testing, and proper API management.

By building security into the API development lifecycle and regularly reviewing potential weaknesses, organizations can reduce security risks while continuing to provide reliable services to users.

For developers and security teams, the OWASP API Security Top 10 is a useful reference for understanding common API security risks and building stronger API protections.