What is API security? Key risks and protection strategies

It applies to both internal and external APIs and covers more than verifying who sends a request. Effective API security also determines what an authenticated user or system is allowed to access or change, validates requests, limits abuse, and monitors API activity throughout the lifecycle.

API security protects the interfaces applications and services use to exchange data and trigger functionality. Because APIs can expose customer records, internal systems, business operations, and other sensitive resources, weaknesses in an API can provide direct access to valuable data or actions.

Security controls therefore need to protect both access to the API and what a caller can do after gaining access. Authentication verifies the identity making a request, while authorization determines whether that identity can access a specific object, function, or operation.

API security is part of broader application security, but it addresses risks that arise specifically from exposed endpoints, machine-to-machine communication, API permissions, and application logic.

API security risks often involve identity, permissions, exposed functionality, and the way applications process requests. The OWASP API Security Top 10 provides a widely used reference for API-specific risks.

Weak authentication can allow attackers to impersonate legitimate users or systems. Problems can include insecure credentials, poorly protected tokens, or incorrectly implemented authentication flows.

Authentication does not guarantee that a user should be allowed to perform every requested action. Broken Object Level Authorization (BOLA) occurs when an API fails to verify whether an authenticated caller may access a particular object or record.

Authorization problems can also expose functions that should only be available to certain roles or users.

APIs that accept unexpected or malicious data without sufficient validation can expose applications to injection attacks and other unintended behavior.

Attackers or automated clients can make excessive requests or trigger expensive operations. This can affect availability, infrastructure costs, or downstream services.

Old versions, debug endpoints, undocumented APIs, and incorrectly configured services can remain accessible without teams realizing they are part of the attack surface.

Applications also consume external APIs. Treating third-party responses as trusted input can introduce another security boundary that needs validation.

Teams secure APIs by combining identity, authorization, validation, resource controls, encryption, and monitoring rather than relying on a single gateway or security mechanism.

Authentication establishes who or what is making an API request. Depending on the system, mechanisms can include OAuth-based flows, access tokens, API keys, or mutual TLS for machine-to-machine communication.

Credentials need appropriate storage, rotation, expiration, and transport protection.

Authorization checks what an authenticated identity is permitted to do. Controls may need to evaluate access to a particular object, field, function, or action rather than granting broad access to an entire API.

For example, two authenticated customers may both be allowed to request invoice data, but each should only be able to access invoices associated with their own account.

APIs should validate inputs, expected schemas, data types, formats, and data received from external services. Rejecting malformed or unexpected requests reduces the chance that unintended input reaches application logic.

Rate limits, quotas, and resource restrictions can reduce brute-force attempts, automated abuse, excessive API consumption, and resource exhaustion.

Secure transport such as TLS protects API data while it moves between clients, services, and infrastructure. Sensitive information may require additional protection depending on the system and threat model.

Teams can monitor authentication failures, unusual request patterns, sensitive operations, permission failures, and unexpected changes in API behavior. Runtime visibility helps detect abuse that preventive controls do not stop.

API inventory matters because teams cannot consistently secure an endpoint they do not know exists.

An inventory can help track active endpoints, owners, versions, deprecated APIs, and undocumented or shadow APIs. This is especially important as systems evolve and old versions remain deployed alongside newer ones.

OWASP identifies Improper Inventory Management as an API security risk because outdated documentation, forgotten endpoints, and exposed legacy versions can expand the attack surface.

Inventory is therefore not just documentation. It provides the visibility needed to decide which APIs require authentication, testing, monitoring, deprecation, or removal.

API security is the broader set of controls and practices used to protect APIs throughout their lifecycle. API testing is the process of checking whether an API behaves as expected.

API testing can cover functional behavior, reliability, performance, error handling, and security. Security testing may check authentication, authorization, input handling, exposed endpoints, or known vulnerability patterns.

Testing is therefore one part of API security, not a synonym for it. Secure APIs also depend on design decisions, runtime controls, permissions, configuration, monitoring, and ongoing maintenance.

Learn more about API testing:

API security should be considered from design through production rather than added only before release.

During design, teams can define authentication, authorization, data exposure, and trust boundaries. During development and CI/CD, automated and manual testing can identify implementation problems before deployment.

Security continues at runtime because APIs change. New endpoints appear, permissions evolve, older versions may remain active, and new attack techniques emerge. Monitoring, inventory updates, access reviews, and recurring testing help teams identify risks introduced after the first release.

This lifecycle approach is particularly important for APIs shared across multiple products, teams, partners, or external customers.

No individual API security control provides complete protection because different controls address different parts of the attack surface.

Strong authentication, for example, confirms identity but does not prevent broken authorization. A valid user may still access another customer's records if the API does not enforce object-level permissions correctly.

Rate limiting and WAF rules can reduce common abuse, but they may not recognize business-logic flaws or legitimate-looking requests used maliciously. Automated scanners can identify many technical issues, yet authorization and multi-step workflow problems often require additional context.

API inventories can also be incomplete, especially when undocumented or shadow endpoints exist. Third-party APIs introduce additional trust boundaries that an organization does not fully control.

Controls therefore need maintenance as APIs, permissions, infrastructure, and integrations change. Effective API security combines identity, authorization, application logic, configuration, testing, inventory, and runtime monitoring.