Quick answer: The OWASP API Security Top 10 is a reference list that prioritizes the most critical security risks specific to APIs. There is a separate list from the web application list because APIs carry a different attack surface: instead of a user interface they expose data and operation endpoints directly. At the top of the list is broken authorization at the object and function level; that is, a user being able to reach another user's data or an unauthorized operation. Other prominent risks are authentication weaknesses, excessive data exposure, resource consumption and abuse of business logic flows. The root fix is to enforce authorization on the server at the object and function level on every request, harden authentication and protect the API not like an interface but like a directly accessed data layer.

Modern applications talk to each other with APIs. Mobile apps, single page web applications and integrations all call APIs. This makes the API the most exposed and most valuable attack surface of the organization. The OWASP API Security Top 10 provides a common priority list to protect this surface. This article explains the logic of the list and the prominent risks.

Why APIs require a separate list

In a classic web application the attacker interacts with an interface; in an API they talk directly to data and operation endpoints. Details that the interface hides are exposed in the API. Also, APIs often expose many endpoints and each must be authorized separately. So web application risks do not carry one to one to the API; the API has its own priority list.

Prominent API risks

Risk What it does Result
Broken object level authorization Access to another user's object Data disclosure, IDOR
Broken function level authorization Access to an unauthorized function Privilege escalation
Authentication weakness Weak or bypassable identity Account takeover
Excessive data exposure Returning more fields than needed Sensitive data leak
Resource consumption Unlimited requests and queries Denial of service

Broken object level authorization is the top risk on the list and the most common API flaw in practice. A user reaches another user's data by giving a different identity to an endpoint designed for their own data.

Authentication and tokens

Authentication in APIs is often done with tokens. If a token is weakly generated, accepted without validation or leaks, all authority passes to the attacker. This is directly related to JWT security: the token signature must be validated, its expiry and scope checked. A leaked token is as dangerous as leaked API keys.

Excessive data exposure and resource consumption

APIs leak and get consumed through two common mistakes. Excessive data exposure is the endpoint returning more fields than needed and the data being present in the response even if the interface does not show it. Resource consumption is tiring the API with unlimited requests, pagination and queries; this relies on the same logic as the DoS risk in GraphQL security. Both are solved with limits and field control on the server side.

The correct defense

1. Object and function level authorization

Authorization must be enforced on the server on every request for both the accessed object and the called function. A user must be able to reach only their own data and the function they are authorized for.

2. Hardening authentication

The token must be strongly generated, its signature and scope validated on every request, and its lifetime limited. Weak authentication opens the whole API.

3. Field and data boundary

Endpoints must return only the needed fields and sensitive data must be removed from the response. The interface not showing it does not provide security.

4. Rate and resource limit

Limits must be set for requests, pagination and queries, and unlimited resource consumption must be prevented.

API security testing with KAOS

DSET's local AI security engine KAOS tests your APIs for OWASP API Security Top 10 risks with an evidence first approach. It tries authorization gaps at the object and function level, measures authentication weaknesses and verifies in a controlled way whether an authorization bypass actually returns another user's data. It reports only genuinely exploitable findings without false positive noise. So you see the real state of your API security in priority order.

Frequently asked questions

Why is there a separate OWASP list for APIs? Because APIs carry a different attack surface. They expose data and operation endpoints directly instead of an interface and each endpoint requires separate authorization. Web application risks do not fit the API one to one, so the API has its own priority list.

What is the most common API flaw? Broken object level authorization is the most common in practice and the top risk on the list. A user reaches another user's data by giving a different identity to an endpoint designed for their own data.

Is it a problem if data I do not show in the interface is returned in the response? Yes. Excessive data exposure is the data being present in the response even if the interface does not show it. Because the attacker reads the API response directly, sensitive data can leak even if it is hidden in the interface. The fix is to remove the data from the response on the server side.

Sources

To test your APIs for OWASP API Security Top 10 risks with a working proof, contact DSET. We provide API penetration testing and secure code review from our Ankara Hacettepe Teknokent laboratory.