API-05 Broken Function Level Authorization

In this article, we’ll dive into the concept of Broken Function Level Authorization, which is a type of access control vulnerability in APIs. We will explore its implications, potential risks, and best practices to mitigate this security issue, which is the 5th item in the OWASP API Security Top 10.

What is Broken Function Level Authorization?

Broken Function Level Authorization is a type of access control vulnerability that allows an attacker to perform administrative actions or access privileged functionalities without the necessary permissions. It occurs when an API fails to properly enforce authorization checks, enabling unauthorized users to exploit the system.

Understanding the Access Control Hierarchy

Access control hierarchies define the permissions and privileges granted to different user roles within an application. In the case of broken function level authorization, the vulnerability lies in the improper enforcement of these permissions. This can manifest in two ways:

1. Vertical Privilege Escalation: Vertical privilege escalation occurs when a regular user gains access to administrative functionalities or performs actions reserved for higher-level roles. For example, a regular user might be able to delete or modify sensitive data that should only be accessible to administrators.
2. Horizontal Privilege Escalation: Horizontal privilege escalation involves a user gaining unauthorized access to functionalities or data belonging to other organizations or tenants within a multi-tenancy application. This breach of isolation can lead to unauthorized actions and potential data breaches.

Implications and Risks

The consequences of Broken Function Level Authorization can be severe. It can lead to unauthorized access, data manipulation, exposure of sensitive information, and compromise of the entire system’s security. Additionally, it can undermine the trust of users and damage the reputation of the application or organization.

Resolving Broken Function Level Authorization

To mitigate the risks associated with broken function level authorization, consider implementing the following best practices:

1. Perform Authorization Checks: Make sure that every API request includes proper authorization checks to verify the user’s permissions before allowing any action. This involves validating the user’s role and ensuring they have the necessary privileges to perform the requested operation.
2. Early Access Control Consideration: Integrate access management early in the development life cycle. Define and design the access control hierarchy, considering discretionary elements and permission systems. Simplify the permission structure if possible to reduce complexity and potential vulnerabilities.
3. Avoid Middleware Authorization: Avoid implementing authorization solely in the middleware layer. Instead, enforce authorization checks at the API level to provide consistent and reliable access control.
4. Comprehensive Application Security Program: Establish a comprehensive application security program that supports developers in producing secure code. This program should include regular security training, code reviews, and automated testing to identify and address vulnerabilities, including Broken Function Level Authorization.