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APIs form the backbone of today's connected applications, powering financial systems, healthcare platforms, and SaaS ecosystems. But as APIs grow more complex, they also create new opportunities for attackers. One of the most dangerous yet often overlooked vulnerabilities in modern APIs is Broken Function Level Authorization (BFLA), ranked #5 in the OWASP API Security Top 10.
This issue arises when APIs fail to enforce proper access controls on functions like create, update, or delete, allowing unauthorized users to perform sensitive operations. Left unchecked, BFLA can lead to data manipulation, privilege escalation, and severe business disruption.
What is Broken Function Level Authorization (BFLA)?
BFLA occurs when an API endpoint allows users to perform actions that should be restricted to administrators or privileged roles.
Unlike Broken Object Level Authorization (BOLA), which deals with unauthorized access to data objects, BFLA focuses on unauthorized actions. It happens when APIs expose administrative or sensitive functions to all users without validating whether they are permitted to perform those actions.
For instance, an endpoint like /modify-user-type should only be accessible to administrators. If regular users can call this API to change their account status from free to premium, that is a clear case of BFLA.
How Broken Function Level Authorization Works
Attackers often discover these flaws by analyzing how an application interacts with its backend. Many API operations use predictable HTTP methods, such as GET for reading data and PUT, POST, or DELETE for modifying or removing data. Understanding HTTP verb tampering is essential for recognizing how attackers exploit these patterns.
When developers do not properly secure these endpoints, a malicious user can manipulate requests to perform unauthorized actions.
For example:
- A user intercepts a GET request to view account details
- They modify the method to PUT or DELETE and resend it
- Without proper function-level authorization, the server processes the request, updating or deleting data it should not
This happens because the API fails to check whether the user has the right role or privilege to execute that function.
Common Examples of BFLA Vulnerabilities
Broken Function Level Authorization can manifest in many ways. Some of the most frequent cases include:
- Unauthorized Role Changes: A regular user gains access to an endpoint that lets them upgrade their role, turning themselves into an admin.
- Balance Manipulation: Endpoints like /update-balance are exposed without restriction, allowing users to alter financial data. These types of business logic vulnerabilities can devastate financial systems.
- Unauthorized Deletions or Modifications: APIs that do not enforce strict access control might let users delete other users' data by calling an unrestricted endpoint.
- Admin Function Exposure: Developers unintentionally leave endpoints like /delete-account or /add-user accessible to all roles rather than limiting them to admins.
These vulnerabilities are not just theoretical. They have led to major real-world breaches across industries, including the Citi hack that exposed millions of customer records.
The Real-World Risks of BFLA
The impact of Broken Function Level Authorization can be catastrophic. A single exposed endpoint can give attackers the power to:
- Change system data, like modifying balances or user roles
- Delete critical business information, resulting in data loss
- Perform privilege escalation, gaining admin-level access
- Undermine trust and compliance, especially in regulated industries
For instance, imagine an API endpoint allowing users to reset their account balance. If unrestricted, attackers could zero out debts, trigger fraudulent transactions, or completely disrupt financial systems.
Bumble: A Real-World Example of BFLA in Action
The Bumble case is a clear example of what happens when an API exposes a function that should have been kept private.
Bumble had an API that allowed users to check their account type. A normal request might look like:
GET /api/accountType
That endpoint simply returned whether the account was free or premium. It was safe and intended for users to see.
The problem started when researchers found another endpoint that allowed users to update their account type:
POST /api/accountType
body: { "type": "premium" }
That meant anyone could send a request telling the system they were a premium user, and the API accepted it. No payment was required, and no backend verification took place. A regular user could promote themselves to a premium plan just by changing the request.
This is exactly what Broken Function Level Authorization means. The API did not check whether the person calling that function had permission to do it. Functions that should only be accessible to internal systems (like upgrading an account after payment) were left open to everyone.
The correct setup would have been simple. The user-facing API should only allow checking the account type, not changing it. The ability to upgrade should have been restricted to Bumble's backend systems, triggered only after payment was confirmed.
The lesson is straightforward. Never expose sensitive or administrative API functions to end users. Even a small oversight, like allowing a public update call, can let attackers change roles, gain privileges, or access features they should not have.
How to Prevent Broken Function Level Authorization
Securing APIs against BFLA requires a combination of strong access control policies, consistent testing, and proper architectural design.
- Implement Role-Based Access Control (RBAC): Clearly define which user roles can access which functions. Ensure each endpoint enforces these role checks consistently.
- Apply the Principle of Least Privilege: Restrict users to only the functions necessary for their role. Customer users should not be able to access administrative update endpoints.
- Separate Administrative and User Functions: Never expose internal or administrative endpoints in the same API namespace as public ones.
- Validate HTTP Methods: Enforce strict control over HTTP verbs. Ensure users can only use methods appropriate for their permissions.
- Conduct Regular Authorization Testing: Continuously test your APIs for exposed or misconfigured endpoints. This should be an ongoing process as APIs evolve and new features are deployed.
How to Test for BFLA Vulnerabilities
Identifying BFLA flaws requires systematic testing across all API endpoints and user roles. Follow this step-by-step methodology to uncover authorization gaps before attackers do.
Step 1: Enumerate All Endpoints and HTTP Methods
Start by mapping every API endpoint in your application. Review API documentation, OpenAPI specifications, and network traffic to build a complete inventory. Document which HTTP methods each endpoint accepts, including GET, POST, PUT, PATCH, and DELETE. Many BFLA vulnerabilities hide in endpoints that accept methods developers never intended to expose.
Step 2: Create Tokens for Different User Roles
Generate valid authentication tokens or sessions for each user role in your system. At a minimum, create credentials for a regular user, a privileged user, and an administrator. Label each token clearly so you can track which role you are testing. Store these securely during your testing session.
Step 3: Test Each Endpoint with Lower-Privilege Credentials
Using your regular user token, attempt to access every endpoint in your inventory. Pay special attention to endpoints containing keywords like admin, manage, delete, update, or config. Record the response codes and messages for each request. A 200 OK response on an admin endpoint using a regular user token indicates a BFLA vulnerability.
Step 4: Attempt HTTP Method Manipulation
For endpoints that return data via GET requests, try sending PUT, POST, PATCH, or DELETE requests using lower-privilege credentials. APIs that fail to validate authorization per method often accept destructive operations from unauthorized users. Test whether changing the HTTP method bypasses access controls.
Step 5: Document and Validate Findings
Record every successful unauthorized access attempt with full request and response details. Verify findings are reproducible and not false positives caused by caching or session issues. Prioritize vulnerabilities by business impact, focusing first on endpoints that modify sensitive data or user privileges.
Tools like Burp Suite and OWASP ZAP simplify this process by intercepting requests, modifying headers, and automating role-based testing across large API surfaces. For a detailed comparison, see our guide on Burp Suite vs ZAP.
Practical Testing Examples
The following examples demonstrate common BFLA tests using cURL commands. Replace the tokens and endpoints with values from your own environment.
1. Testing DELETE method as a regular user:
curl -X DELETE https://api.example.com/admin/users/12345 \
-H "Authorization: Bearer regular_user_token_here."
If this request returns a 200 status or successfully deletes the user, the endpoint lacks proper function-level authorization.
2. Attempting admin endpoint access with user token:
curl -X GET https://api.example.com/admin/settings \
-H "Authorization: Bearer regular_user_token_here."
Admin configuration endpoints should return 401 Unauthorized or 403 Forbidden for non-admin users. Any other response warrants investigation.
3. Manipulating role parameters in requests:
curl -X POST https://api.example.com/users/profile \
-H "Authorization: Bearer regular_user_token_here" \
-H "Content-Type: application/json" \
-d '{"userId": "12345", "role": "admin"}'
Attempt to escalate privileges by including role or permission parameters in request bodies. APIs should reject unauthorized role modifications regardless of what the client sends.
Automating Function-Level Security Testing with APIsec
Manually testing every API endpoint for proper authorization logic is time-consuming and error-prone, especially as applications scale. Selecting the right API security testing tools is critical for effective automation. This is where APIsec provides a major advantage.
APIsec automates function-level security testing by scanning APIs to identify endpoints that allow unauthorized actions. Its AI-driven platform simulates real-world attacks to verify whether authorization checks are correctly implemented across different roles and environments.
Here is how APIsec strengthens your API defenses:
- Detects misconfigured endpoints that expose privileged operations
- Validates function-level access controls in real time
- Integrates directly into your CI/CD pipeline to ensure vulnerabilities are caught before deployment, supporting shift-left security practices
- Provides detailed proof-of-concept reports to help teams fix issues faster
By integrating APIsec into your API security workflow, you can ensure that Broken Function Level Authorization, along with other OWASP vulnerabilities, is proactively identified and remediated.
Conclusion
Broken Function Level Authorization (BFLA) is one of the most dangerous yet preventable API vulnerabilities. It allows attackers to exploit weak access control logic, manipulate data, and perform actions reserved for admins or system processes.
Securing your APIs against BFLA requires verifying and enforcing authorization at every functional level. Following API security best practices and using systematic testing methodologies with automation tools like APIsec helps organizations protect their APIs from misuse while maintaining performance, compliance, and user trust.
Do not wait for attackers to find your API's weak spots. Start your free APIsec trial and see how automated security testing can protect your APIs from OWASP's most critical risks.
Frequently Asked Questions
1. What is Broken Function Level Authorization (BFLA)?
BFLA happens when an API lets unauthorized users access or execute functions like modifying, deleting, or creating objects that should be restricted.
2. How is BFLA different from BOLA?
BOLA allows unauthorized access to data, while BFLA allows unauthorized execution of functions such as updating or deleting that data.
3. What are some examples of BFLA vulnerabilities?
Common cases include users changing account types, modifying balances, or accessing admin-only functions.
4. How can developers prevent BFLA?
Use RBAC, enforce least privilege, validate HTTP methods, and test regularly for exposed endpoints.
5. How do I test for BFLA manually?
Enumerate all endpoints, create tokens for different roles, test each endpoint with lower-privilege credentials, attempt HTTP method manipulation, and document findings.
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