A Deep Dive into Broken Functionality Level Authorization Vulnerability (BFLA)

What is Broken Functionality Level Authorization?

Broken Functionality Level Authorization (BFLA) is a security flaw that allows unauthorized users to access functions they shouldn’t. This issue arises when an application fails to enforce proper access controls based on user roles. For example, a regular user might gain the ability to delete another user’s account due to inadequate authorization measures. BFLA is a significant risk in APIs and can lead to serious security breaches if not addressed properly.

Example of BFLA Vulnerability 

Let’s say an application has an admin functionality that allows users to view all user accounts.

HTTP Request

GET /admin/view_users HTTP/1.1
Host: example.com
Authorization: Bearer regular_user_token

In this request, the attacker sends a GET request to the /admin/view_users endpoint. They include an authorization token typically granted to regular users, indicating a lack of adequate permission checks on the back-end server.

HTTP Response

The server responds with a list of all user accounts:

HTTP/1.1 200 OK
Content-Type: application/json

[
    {"user_id": 1, "name": "Alice"},
    {"user_id": 2, "name": "Bob"},
    {"user_id": 3, "name": "Charlie"}
]

In this response, the server returns a successful status code (200 OK) along with a JSON array containing user account details. This output highlights the vulnerability, as it exposes sensitive user information without verifying the requester’s authorization level.

Vulnerable Code

In this code, the application does not verify whether the requester has admin privileges before executing the function to view all users.

@app.route('/admin/view_users', methods=['GET'])
def view_all_users():
    # Fetches all user accounts
    users = fetch_all_users()  # Assume this retrieves all users from the database
    return jsonify(users)

This code defines a Flask route for the /admin/view_users endpoint, allowing only GET requests. When a user accesses this endpoint, the view_all_users function is triggered. Inside the function, it calls fetch_all_users() to retrieve all user accounts from the database. The retrieved user data is then returned in JSON format using jsonify(). This setup exposes user data without checking if the requester has admin privileges, creating a security vulnerability.

Exploitation Steps

1. The attacker discovers the API endpoint for viewing all users at GET /admin/view_users. This endpoint is intended for admin access only, but the attacker sees an opportunity.
2. Acting as a regular user, the attacker sends a GET request to the endpoint.
3. The server processes the request without checking the requester’s admin rights. This oversight allows the attacker to bypass security measures and access restricted information.

Difference Between BOLA and BFLA

Broken Function Level Authorization (BFLA) and Broken Object Level Authorization (BOLA) are both security vulnerabilities related to improper access control, but they focus on different aspects of authorization.

Demonstrating BFLA Vulnerability: A Practical Case

This case highlights a specific instance of BFLA vulnerability in action. It shows how a user can exploit flaw to perform unauthorized action.

User Registration 

There is a web application with multiple API endpoints, and /api/register allows users to register an account and receive an access token.

The provided cURL command sends a POST request to register a new user in an application. It specifies the content type as JSON and includes the user’s email and password in the request body. The command targets the URL http://127.0.0.1:5000/api/register, which serves as the registration endpoint on a local server. Upon successful registration, the server responds with a JSON object that contains an access token and a confirmation message indicating that the user has been registered successfully.

Fetching User Details 

The provided cURL command sends a GET request to retrieve a user’s profile from an application. It includes an authorization header with a bearer token, which verifies the user’s identity and permissions. The command targets the URL http://127.0.0.1:5000/api/user-profile, indicating that it is accessing the user profile endpoint. Upon receiving the request, the server checks the validity of the bearer token and retrieves the profile information associated with the user. The response contains details such as the user’s email, password, and role, confirming successful access to their profile data.

Unauthorized DELETE Operation 

There is a DELETE HTTP method that is intended only for admins to delete a user’s account, but any user can modify the request and delete a profile.

This cURL command sends a DELETE request to remove a user’s profile from an application. Upon receiving the request, the server validates the bearer token and proceeds to delete the specified user profile. The response confirms the action with a message stating, “User profile deleted,” indicating that the deletion was successful.

Vulnerable Code

The provided code for user profile retrieval and deletion contains a Broken Function Level Authorization (BFLA) vulnerability. Both the GET and DELETE methods rely on the get_jwt_identity() function to identify the current user based on their JWT token. However, the code does not implement any checks to ensure that only admins can delete the profiles. An attacker could potentially manipulate their requests to access or delete another user’s profile if they can guess or obtain a valid JWT token associated with that user. This lack of strict authorization checks allows unauthorized actions, compromising the application’s security.

Secure Code

The updated code addresses the Broken Function Level Authorization (BFLA) vulnerability by implementing stricter access controls for user profile operations.

In the GET method, the application retrieves the current user’s identity using get_jwt_identity() and checks if a user with that email exists in the users list. The code adds an additional authorization check by verifying if the current user has an ‘admin’ role before allowing profile deletion.

This change prevents regular users from deleting profiles and restricts this action to users with the appropriate privileges. If a non-admin user attempts to delete a profile, the application responds with a “403 Access denied” message, effectively enforcing role-based access control and enhancing overall security. By implementing these checks, the code mitigates the risk of unauthorized access and ensures that users can only perform actions they are permitted to execute.

Key Causes of Broken Function Level Authorization Vulnerabilities

Several factors contribute to this vulnerability, which can lead to significant security risks within an application.

Insufficient or Weak Access Controls

BFLA vulnerabilities often arise from insufficient or poorly implemented access controls within APIs. When role-based access controls (RBAC) are not adequately defined, unauthorized users can access functions they should not be able to execute. This issue is particularly prevalent in applications with complex user hierarchies, where multiple roles and permissions complicate the enforcement of strict access policies.

Inadequate Separation Between Admin and General Functions

A lack of clear separation between administrative and general user functions can lead to BFLA vulnerabilities. When developers do not establish distinct boundaries for different user roles, unauthorized users may inadvertently gain access to sensitive functionalities. This situation often occurs in applications where the complexity of user roles makes it challenging to enforce strict controls.

Lack of Proper Input Validation

APIs that fail to validate user input effectively can become susceptible to BFLA attacks. Attackers may manipulate authorization tokens or parameters in their requests, bypassing security measures designed to restrict access. This lack of validation can lead to unauthorized users executing functions that should be off-limits.

Mitigating BFLA Vulnerabilities: Best Practices

Implementing effective prevention strategies is essential to mitigate the risks associated with Broken Function Level Authorization (BFLA). By adopting best practices in access control and security measures, developers can significantly reduce the likelihood of unauthorized access to sensitive functions.

Implement Role-Based Access Control (RBAC)

Establishing a robust Role-Based Access Control (RBAC) system is crucial for preventing BFLA vulnerabilities. Clearly define user roles and their associated permissions within the application. This approach ensures that users can only access functions relevant to their roles, minimizing the risk of unauthorized actions.

Enforce Strong Input Validation

Implementing strict input validation measures is vital for preventing unauthorized access through manipulated requests. Ensure that all incoming data, including authorization tokens and parameters, are validated against expected formats and values. This practice helps prevent attackers from bypassing authorization checks by exploiting input fields.

Function-Level Validation

Proper function-level validation is crucial for mitigating BFLA risks. This process involves verifying user permissions before executing any action or function request. Conducting thorough permission checks throughout the application ensures that users can only access functions relevant to their roles.

Centralized Access Management

A centralized access management system guarantees consistent enforcement of authorization policies across the application. This strategy simplifies the administration of roles and permissions, making it easier to update and apply policies uniformly.

Minimize Client-Side Authorization

Relying on client-side authorization poses risks due to its susceptibility to manipulation. To reduce BFLA risks, essential authorization checks should be conducted on the server side. Additionally, limiting the transmission of sensitive data to the client lowers the chances of exposure or tampering.

Final Thoughts

Addressing Broken Function Level Authorization (BFLA) is crucial for maintaining the security and integrity of web applications. By understanding the causes and implementing effective prevention strategies, organizations can significantly reduce the risk of unauthorized access to sensitive functionalities. It is essential to prioritize robust access controls, regular security audits, and strong input validation practices. Additionally, fostering a culture of security awareness among developers and stakeholders can further enhance the resilience of applications against BFLA vulnerabilities.