Introduction

APIs (Application Programming Interfaces) play a crucial role in modern application development, allowing different software systems to communicate and exchange data. However, APIs can introduce security vulnerabilities if not implemented and secured properly. One such vulnerability is Broken Function Level Authorization, which can lead to unauthorized access to sensitive functionality and data within an application. In this article, we will explore the threat agents, attack vectors, security weakness, and impacts associated with Broken Function Level Authorization. We will also provide three examples with code to demonstrate how this vulnerability can be exploited and discuss preventive measures.

Understanding Broken Function Level Authorization

Threat Agents and Attack Vectors

Threat agents in Broken Function Level Authorization attacks are typically attackers who aim to gain unauthorized access to privileged functionality or sensitive data within an application. The attack vectors involve exploiting the lack of proper authorization checks for API endpoints. Attackers can send legitimate API calls to endpoints they should not have access to, either as anonymous users or regular non-privileged users.

Security Weakness

The security weakness in Broken Function Level Authorization lies in the improper implementation of authorization checks for functions or resources. Authorization checks are usually managed through configuration or code level, where access rights are granted based on user roles, groups, or hierarchies. However, implementing proper checks can be a complex task, especially in modern applications with numerous roles, groups, and intricate user hierarchies. This complexity makes it easier for attackers to discover flaws in APIs, as APIs are more structured and accessing different functions is often more predictable.

Impacts

Exploiting Broken Function Level Authorization vulnerabilities can have severe technical and business-specific impacts. Attackers gaining unauthorized access to sensitive functionality can lead to various consequences, such as:

1. Data Disclosure: Attackers may access confidential information or sensitive data that should only be accessible to authorized users. This can result in the exposure of personal data, trade secrets, or other critical information.
2. Data Loss: Unauthorized access to administrative functions can potentially lead to data loss or data corruption. Attackers might manipulate or delete data, causing significant damage to the application and its users.
3. Service Disruption: By exploiting administrative functions, attackers can disrupt the normal operation of the application. This can result in service downtime, loss of availability, and potential financial losses for the business.

Given the severity of these impacts, it is crucial to identify and address Broken Function Level Authorization vulnerabilities in APIs.

Is the API Vulnerable?

To determine if an API is vulnerable to Broken Function Level Authorization, a comprehensive analysis of the authorization mechanism is required. It is essential to consider the user hierarchy, different roles or groups in the application, and ask relevant questions. Here are some key questions to consider:

1. Can a regular user access administrative endpoints?
2. Can a user perform sensitive actions that they should not have access to simply by changing the HTTP method?
3. Can a user from one group access a function that should only be exposed to users from another group by guessing the endpoint URL and parameters?

It is important not to assume that an API endpoint is regular or administrative solely based on the URL path. While developers may choose to expose most administrative endpoints under a specific relative path, such as /api/admins, it is common to find administrative endpoints mixed with regular endpoints under other relative paths, like /api/users.

Performing a thorough analysis of the API's authorization mechanisms and asking these questions can help identify potential vulnerabilities.

Example Attack Scenarios

Let's examine three attack scenarios that demonstrate how Broken Function Level Authorization vulnerabilities can be exploited.

Scenario #1: Unauthorized Admin Privileges

During the registration process of an application that allows only invited users to join, a mobile application triggers an API call to

retrieve invite details:

GET /api/invites/{invite_guid}

The response contains a JSON with details about the invite, including the user's role and email. An attacker duplicates the request, manipulates the HTTP method and endpoint, and sends a malicious request to create a new invite with admin privileges:

POST /api/invites/new

{
"email": "attacker@somehost.com",
"role":"admin"
}

Since the endpoint responsible for creating invites does not implement proper authorization checks, the attacker successfully creates an admin account and gains full access to the system.

Scenario #2: Unauthorized Access to User Data

An API contains an endpoint that should be exposed only to administrators:

GET /api/admin/v1/users/all

This endpoint returns details of all users in the application. However, it does not implement proper function level authorization checks. An attacker, familiar with the API's structure, makes an educated guess and accesses this endpoint, exposing sensitive user details.

Scenario #3: Guessing Endpoint URLs

In this scenario, the API has different endpoints for exporting user data based on user groups. An attacker attempts to access an endpoint that should be exposed only to users from group Y but mistakenly guesses the URL and parameters:

/api/v1/users/export_all

By exploiting this flaw in function-level authorization, the attacker gains unauthorized access to the export functionality meant for users from group Y.

These scenarios illustrate the real-world risks associated with Broken Function Level Authorization vulnerabilities and emphasize the importance of implementing proper authorization checks.

How To Prevent Broken Function Level Authorization

Preventing Broken Function Level Authorization requires implementing robust authorization mechanisms and conducting thorough security assessments. Here are some preventive measures:

1. Consistent Authorization Module: Your application should have a consistent and easy-to-analyze authorization module that is invoked from all business functions. This module should enforce proper access control by denying all access by default and requiring explicit grants for specific roles to access each function.
2. Review API Endpoints: Regularly review your API endpoints for potential function level authorization flaws. Consider the business logic of the application and the hierarchy of user groups. Ensure that sensitive functions or administrative controllers implement appropriate authorization checks based on the user's group and role.
3. Administrative Abstract Controller: Implement an administrative abstract controller that handles authorization checks for all administrative functions. All administrative controllers should inherit from this abstract controller to ensure consistent and proper authorization across administrative endpoints.

By following these preventive measures, you can significantly reduce the risk of Broken Function Level Authorization vulnerabilities in your APIs.

Conclusion

Broken Function Level Authorization in APIs can lead to unauthorized access to privileged functionality and sensitive data, potentially resulting in severe consequences for businesses and users. Understanding the threat agents, attack vectors, security weaknesses, and impacts associated with this vulnerability is crucial for building secure APIs. By analyzing the authorization mechanisms, asking relevant questions, and implementing preventive measures, developers can mitigate the risks and ensure the proper security of their APIs. Remember, securing your APIs is an ongoing process, and regular security assessments are essential to stay protected against evolving threats.