Modern applications are rarely just websites.
A user may interact with a polished interface, but behind it there are APIs handling logins, payments, profiles, orders, integrations, mobile applications and communication between services. In many cases, the website is simply one client consuming those APIs.
So protecting only what users see is a bit like installing an impressive security door while leaving the service entrance permanently open.
That is where API Security becomes its own security discipline.
What Is an API?
An API, or Application Programming Interface, defines how software components communicate with each other. A frontend may request user information from an API, a mobile application may submit an order through one, and two backend services may use APIs to exchange data.
A simplified flow looks like this:
The API exposes specific operations through endpoints. Instead of clicking a button labeled “View Order,” for example, the application may send a request to an endpoint responsible for retrieving that order.
This separation is extremely useful for development. It also means that attackers do not necessarily need to use the interface developers intended them to use.
Why APIs Change the Attack Surface
A browser interface controls what users conveniently see and interact with. An API exposes the underlying operations more directly.
An attacker can inspect requests, discover endpoints and communicate with those endpoints without clicking through the website at all.
This matters because security cannot rely on the frontend hiding a button or preventing a certain action. The API itself must decide whether every request is legitimate.
The OWASP API Security Project highlights authorization, authentication, resource consumption, sensitive business flows and API inventory among the major security risks specific to APIs. (OWASP)
An endpoint may therefore accept a completely valid HTTP request and still perform something the requester should never have been allowed to do.
The Main API Security Risks
Authentication answers who is making the request. Authorization answers what that identity is allowed to do.
Both matter.
Imagine an API exposing:
GET /api/orders/1427
Being authenticated does not automatically mean a user should be able to access order 1427. The API must verify that the requested object belongs to that user or that their role allows access to it.
The same principle applies to administrative functions, sensitive fields and business operations.
There is also the problem of visibility. Organizations frequently accumulate old API versions, test endpoints and services that nobody actively manages anymore. OWASP specifically identifies improper inventory management as an API security risk because deprecated or forgotten endpoints can remain exposed long after teams stop thinking about them. (OWASP)
Why a WAF Is Not Enough
A Web Application Firewall remains valuable in front of APIs. It can analyze HTTP traffic, block known malicious patterns and enforce useful protections at the edge.
But it cannot understand every business decision the application must make.
Consider two requests:
GET /api/orders/100GET /api/orders/101
Both may be perfectly valid HTTP. Neither contains SQL Injection, XSS or an obviously malicious payload.
The security problem appears if user A owns order 100 but is also allowed to retrieve order 101, which belongs to user B.
That is an authorization failure. The application must understand ownership and permissions because the WAF usually does not possess enough business context to make that decision.
Protecting an API therefore requires controls deeper than traffic filtering.
What Good API Security Looks Like
API Security starts with knowing which APIs actually exist.
Organizations should maintain an inventory of endpoints, versions and owners, then remove APIs that are no longer necessary. Authentication must reliably establish identities, while authorization must be enforced for individual objects, functions and sensitive operations.
Requests should also be validated against expected schemas so that the API only accepts the structure and data it was designed to process. Rate limits and resource controls help prevent automation and excessive consumption, while useful logging makes suspicious behavior visible.
API gateways can centralize capabilities such as authentication enforcement, routing, rate limiting and request validation, but they are not a substitute for authorization inside the application.
This lifecycle approach is also reflected in the NIST SP 800-228 Guidelines for API Protection for Cloud-Native Systems, updated in March 2026, which organizes API risks and security controls across development and runtime rather than treating API protection as a single perimeter control. (NIST)
Conclusion
API Security is not simply about placing another security product in front of an endpoint.
It means understanding what APIs expose, controlling who can access them, validating what those users can actually do, monitoring their behavior and managing APIs throughout their entire lifecycle.
The website may be the interface humans see, but APIs often expose the operations attackers actually care about.
Securing the front door is useful. Just remember that modern applications have significantly more than one door.