API 5:2023 Broken Function Level Authorization: Unseen Authorizations, Critical Threats
APIs have become the most critical components that open the functions of software systems to the outside world as digital transformation accelerates. Modern approaches such as microservices architectures, multi-layer applications and multi-platform strategies place APIs at the center of systems. This growth brings not only operational advantages but also new security risks that need to be carefully managed. At this point, many organizations have the following question in mind: If Authentication and Access Control are already in place, is there a need for a security approach specific to function level authorization?
At first glance, this question may seem justified. Authentication systems control users’ access to the system, while Access Control Lists (ACLs) limit who can access certain resources. However, Broken Function Level Authorization (BFLA) is not just about “who”, but “what operations can they perform”. Incomplete or incorrect implementation of role, group and hierarchy-based authorization policies can lead to attackers accessing API functions that are not authorized. This is a serious security risk inherent to the API architecture and hidden in the business logic.
Today’s attacks are not limited to bypassing the login control; attackers aim to break the functional structure of the system and call functions that are not defined for them. In many cases, this is done through direct requests to API endpoints. For example, it may be possible to send a request with a normal user token to a function like /admin/deleteUser that only the admin role should access. If there is no function level authorization check on the backend side, the system can approve this request. Moreover, in some cases, the API may leave unprotected endpoints that are not visible on the client-side but are present in the documentation or test environments.
This is where API documentation becomes a valuable resource for attackers. By examining Swagger or OpenAPI outputs, the entire function map can be extracted. Thus, the attacker does not just try brute-force endpoints; he acts with a systematic, targeted attack plan. However, classical security solutions are limited at this point because most of them do not have the ability to understand the context of the requests, interpret the user role and analyze the function access logic.
Function level authorization is not just access permission; it is a security layer that works in the data and role context, defining which functions the user can execute and under what conditions. Modern API security solutions should be able to evaluate each API call not only at surface-level, but also in role and function relationship. The role of the user, the purpose of the function called, its place in the business logic and the system’s response to it are critical components for real protection.
Today, authentication, ACL, API Gateway policies and function level authorization controls and API Security solutions should be parts of an integrated security architecture. The complete and harmonized functioning of these structures ensures effective defense. However, missing any one of them can create serious vulnerabilities, especially against API-specific authorization flaws. Because attackers target not only weaknesses at the entry point, but also misconfigurations of internal functions, unclear role boundaries and inconsistencies in business logic.
In particular, Role-Based Access Control (RBAC), permission matrix and function-based test scenarios are integral parts of modern security strategies. Systems that work with these methods not only detect attacks, but also offer proactive protection against potential vulnerabilities that have not yet been exploited. This approach is not only a technical necessity but also critical for regulatory compliance, data security and corporate reputation.
Solutions that address API security not only at the request level but also in the context of function and role make organizations much more resilient against today’s and future advanced threats. It is no longer enough to just protect the entrance door to the system; it is necessary to know who calls which functions, which operations, and the effects of these operations on system integrity. Since modern attacks are targeted, layered and sophisticated, defense mechanisms must be equally comprehensive and context-aware.
Apifort, one of the local solutions developed in this direction, directly focuses on strengthening the function level authorization controls of corporate systems and closing this critical API-specific security gap.
