API (Application Programming Interface)

What is an API?

An Application Programming Interface (API) defines how applications interact with each other. APIs expose functions or endpoints that enable data access or actions without revealing internal implementation details. APIs are foundational to modern software, enabling web services, mobile apps, cloud platforms, and integrations across systems.

Why APIs matter

APIs are critical because they:

• Enable interoperability between services and platforms
• Power cloud-native and microservices architectures
• Allow automation and integration at scale
• Decouple systems for faster development and deployment

In enterprise environments, APIs are often the primary access layer to business data and services.

Common API types

APIs can be categorized by style and scope:

1) REST APIs

• Stateless, resource-based, HTTP methods (GET, POST, PUT, DELETE)
• Most widely used for web services

2) GraphQL APIs

• Clients request exactly the data they need
• Reduces over/under-fetching

3) SOAP APIs

• XML-based, strict contracts
• Still used in legacy or regulated systems

4) Internal vs external APIs

• Internal: used within an organization
• External/Public: exposed to partners or developers

API authentication and authorization

APIs typically require access controls, including:

• API keys (simple, limited security)
• OAuth 2.0 tokens (scoped, time-limited)
• Mutual TLS (mTLS) (strong service-to-service identity)
• JWTs (signed tokens with claims)

Improper API security is a common cause of breaches.

APIs as a security attack surface

APIs are frequently targeted because they:

• Expose direct access to data and business logic
• Are often less monitored than user-facing apps
• Can be abused via broken authentication or authorization

Common API attack patterns include:

• Broken Object Level Authorization (BOLA)
• Excessive data exposure
• Token leakage or reuse
• Rate limit abuse and scraping
• Injection and deserialization flaws

API security best practices

Effective API protection typically includes:

• Strong authentication (OAuth, mTLS)
• Least-privilege scopes and roles
• Input validation and schema enforcement
• Rate limiting and abuse detection
• Centralized logging and monitoring
• Regular security testing and inventory management

API security is a core pillar of Zero Trust architectures.

API vs SDK

• API: the interface and contract exposed by a service
• SDK: a client-side library that simplifies API usage

SDKs consume APIs but do not replace them.

APIs in cloud and enterprise environments

APIs underpin:

• SaaS integrations (CRM, HR, finance)
• CI/CD and DevOps automation
• Identity platforms and IAM
• Microservices and service meshes

In many organizations, APIs outnumber users by orders of magnitude.

Common misconceptions

• "APIs are only for developers"
• "APIs are secure by default"
• "Hiding endpoints is sufficient security"
• "APIs don't need monitoring"