APIs are under attack, and Broken Function Level Authorization (BFLA) is a major culprit.
BFLA happens when APIs fail to enforce proper permission checks, letting users access restricted functions. It ranks #5 on the OWASP API Top 10 (2023) and has led to breaches at companies like Uber, Instagram, and GitHub.
Here’s what you need to know upfront:
- What is BFLA? It allows attackers to exploit API functions (not just individual objects) to bypass permissions.
- Why does it happen? Common causes include misconfigured roles, over-reliance on client-side controls, and flawed API gateway setups.
- How to fix it? Use tools like Postman and OWASP ZAP to test APIs, enforce server-side authorization, and adopt least privilege access.
Key takeaway: APIs need robust, server-side authorization checks at every function level to prevent BFLA.
Read on for detailed examples, testing techniques, and long-term strategies to secure your APIs.
What Is Broken Function Level Authorization?#
Broken Function Level Authorization (BFLA) is a security flaw that occurs when APIs fail to enforce proper permission checks, allowing users to perform actions they shouldn't have access to. Unlike object-level issues that target specific API objects, BFLA focuses on entire API functions. Imagine a scenario where someone with a visitor badge can stroll into the CEO's office and access confidential files - this is essentially what happens when BFLA vulnerabilities exist. The system doesn't properly verify whether the user has the right level of access.
BFLA is listed as API5 in the OWASP API Security Top 10, ranking fifth in severity as of 2023. This vulnerability is particularly dangerous because it allows attackers to exploit legitimate API calls to access restricted resources, bypassing standard user permissions.
"Complex access control policies with different hierarchies, groups, and roles, and an unclear separation between administrative and regular functions, tend to lead to authorization flaws. By exploiting these issues, attackers gain access to other users' resources and/or administrative functions." - OWASP API Security Top 10 2019 Report
BFLA can be thought of as a broader version of Broken Object Level Authorization (BOLA). While BOLA focuses on individual API objects, BFLA targets overarching API functions, making its potential impact even greater. APIs are particularly vulnerable because their structured nature often makes it easier for attackers to identify and exploit flaws.
How to Identify BFLA#
Spotting BFLA vulnerabilities requires a keen eye for unusual API behavior. One of the clearest signs is when users can access functions or endpoints that should be restricted based on their role or permission level.
Some common red flags include:
- Users accessing administrative endpoints or functions meant for higher privilege levels.
- Bypassing role restrictions by changing HTTP methods, like switching from
GETtoPOST. - Successful API calls to endpoints that should require elevated permissions.
- Unauthorized actions, such as creating, modifying, or deleting resources.
Attackers often reverse engineer client-side code or intercept application traffic to uncover these vulnerabilities. For example, during security testing, a simple change in an HTTP method or endpoint parameter might reveal unauthorized access to sensitive functions. This happens when APIs rely too heavily on client-side controls or lack robust server-side authorization checks. Recognizing these warning signs is critical because attackers frequently exploit these flaws in real-world scenarios.
Common Attack Examples#
Take the example of an Invitation Hijack Attack. In this scenario, an application that requires an invitation to join uses the following API call to retrieve invitation details:
GET /api/invites/{invite_guid}
An attacker, however, manipulates the request by changing the method to POST
and targeting a different endpoint:
POST /api/invites/new
This endpoint, meant only for administrators, allows the attacker to send a payload like this:
POST /api/invites/new
{ "email": "attacker@somehost.com", "role": "admin" }
If proper authorization checks are missing, the attacker can create an administrative invitation for themselves, effectively taking over the application.
Real-world examples highlight how damaging BFLA can be. For instance, breaches at a state insurance department and a major telecommunications provider involved attackers exploiting these vulnerabilities. In another case, New Relic Synthetics faced a privilege escalation issue where restricted users could modify alerts on monitors without proper permissions.
These types of attacks pose serious risks to businesses, including data exposure, data loss, corruption, and service interruptions. These examples emphasize just how critical it is to perform rigorous testing to identify and address BFLA vulnerabilities.
Why Function Level Authorization Breaks#
To secure APIs effectively, it’s crucial to understand why function level authorization often fails. These failures usually arise from poor design and flawed implementations. According to OWASP's 2021 findings, 94% of applications were tested for some form of broken access control, with a 3.81% incidence rate for such issues. This places broken access control as the #1 application security risk. Let’s dive into the specific misconfigurations that weaken function level authorization.
Role Permission Setup Errors#
Errors in Role-Based Access Control (RBAC) are a frequent cause of function level authorization vulnerabilities. These issues arise when roles are misconfigured, hierarchies are poorly designed, or access to specific functions is not properly restricted. Missteps like overly broad permissions or inconsistent role structures can result in users gaining more access than intended, leaving critical functions exposed.
Real-world examples highlight the consequences of these errors. For instance, in 2022, GitHub faced a privilege escalation bug that allowed users to gain unauthorized access to higher-level repository functions. A common culprit is the failure to follow the principle of least privilege - starting users with minimal permissions and granting additional access only when necessary. Overlooking this principle creates exploitable gaps in backend functions or APIs.
Trusting Client-Side Controls#
Another major vulnerability lies in misplaced trust in client-side controls. Relying on mechanisms like JavaScript validation, hidden form fields, or disabled buttons for enforcing access controls is a risky practice. These client-side methods can be easily bypassed by users, as everything on the client side is under their control.
Access control decisions must always be enforced on the server, where they cannot be tampered with. CWE-602 specifically warns against delegating security enforcement to the client. While client-side validation can help catch simple errors and provide immediate feedback, it should only complement server-side controls, never replace them.
API Gateway Configuration Problems#
Misconfigured API gateways are another common source of function level authorization weaknesses. According to OWASP, security misconfiguration ranks as the fifth most common API vulnerability risk. Problems like default settings, insufficient CORS protection, missing authentication, and exposed admin APIs can all lead to unauthorized access.
The impact of these misconfigurations can be devastating. For example, in late 2022, T-Mobile suffered a data breach that exposed the personal information of 37 million customers due to a misconfigured API with inadequate authorization settings. Around the same time, Optus experienced a breach affecting 10 million customer accounts because an API endpoint didn’t require credentials.
"In the instance where a public API endpoint did not require authentication, anyone on the internet with knowledge of that endpoint URL could use it."
- Corey J Ball, Senior Manager of Cyber Security Consulting for Moss Adams
Common gateway misconfigurations include improper CORS settings, excessive access permissions, lack of rate limiting, and missing request validation. Exposed admin APIs and absent firewall rules further widen the attack surface. Modern API gateway setups are often complex, and when multiple teams manage different components, inconsistent policies and overlooked security settings can create persistent vulnerabilities for attackers to exploit.
How to Find and Fix Authorization Problems#
Once you've identified potential BFLA (Broken Function Level Authorization) vulnerabilities, the next step is tackling authorization weaknesses head-on. This requires a blend of technical tools and a hacker's mindset - thinking about how an attacker might exploit your system to access functions meant for higher-privilege users. The trick? Simulate real-world attack methods and test thoroughly.
Using Postman and OWASP ZAP for Testing#
To uncover authorization flaws, leverage powerful tools like Postman and OWASP ZAP.
Postman is a great starting point for manual testing. Begin by documenting your API endpoints and testing them with tokens from different user roles. Specifically, capture requests made by privileged users and replay them using lower-privilege tokens. This mirrors how attackers might attempt to access restricted functionality that isn’t visible in the user interface.
For automated testing, OWASP ZAP steps in with advanced features. Its active scanner can test combinations of headers and tokens, helping identify endpoints that bypass proper authorization checks. It’s particularly effective at uncovering endpoints vulnerable to unauthorized access.
Take, for example, a 2018 case where cyber researcher Jon Bottarini found a flaw in New Relic Synthetics. He discovered that a restricted user could modify alerts on monitors without proper permissions. Using Portswigger Burp Suite, he intercepted privileged session traffic and manipulated API requests to expose hidden vulnerabilities. This highlights how intercepting and replaying requests can shine a light on authorization issues.
Checking Tokens and Server Logs#
JWT (JSON Web Token) analysis is another crucial step in authorization
testing. Decode the token payload and verify that its claims accurately reflect
the user's role and permissions. Pay close attention to fields like aud
(audience), scopes, and any custom permission claims.
A common problem arises when tokens are formatted correctly but carry incorrect permissions for the requested function. Improper scope verification is often at the root of such issues.
Server logs can also uncover patterns that automated tools might miss. Look for unusual activity, such as non-admin users attempting to access admin endpoints or users performing actions outside their normal behavior. Standardizing log formats with key-value pairs makes analysis easier. Ensure logs capture essential details like user IDs, event categories, outcomes, and IP addresses.
In 2020, Datadog emphasized the importance of monitoring authentication logs to identify security threats. They suggested tracking failed login attempts from a single user within short timeframes to detect brute force attacks, as well as monitoring logins from multiple user IDs originating from the same IP address to catch credential stuffing attempts.
"An API log is a comprehensive record generated by an API that documents all the requests sent to and responses received from the API." - Daniel Olaogun, @Merge
Tools like Datadog or the ELK Stack are invaluable for collecting and analyzing API logs. Establishing baselines for typical HTTP access patterns - both per endpoint and per user - allows you to spot deviations that might signal an authorization bypass. From there, test edge scenarios to further challenge your API's defenses.
Testing Edge Cases#
Edge case testing is where you’ll often find the hidden cracks in your authorization logic. These scenarios explore how your API behaves under rare or unexpected conditions, which is often where vulnerabilities lurk.
Start by testing token lifecycle scenarios, such as expired tokens, revoked permissions, or modified claims. For multi-tenant systems, try using a valid token from one tenant to access resources in another. Also, test boundary values at the edges of your role hierarchy or system ranges.
Other edge case tests include using corrupted tokens or omitting essential headers to ensure your system fails securely. For example, entering special characters in user roles can disrupt authorization logic if the system doesn’t handle these inputs properly. Similarly, feeding the system extreme input values - like unusually large user IDs - can reveal flaws or even crash the system if validation is inadequate.
How to Fix API Authorization Issues#
Once you've identified vulnerabilities using tools like Postman, OWASP ZAP, and log reviews, the next step is addressing these issues. Rather than treating security as an afterthought, it's essential to integrate authorization directly into the API design and deployment process. Here's how to do it effectively.
Setting Function-Level Rules with OpenAPI#
OpenAPI specifications are a powerful way to define and enforce authorization rules at the function level. By embedding security directly into your API documentation, you create a single source of truth that both developers and security tools can rely on.
To start, define your security schemes in the components/securitySchemes
section of your OpenAPI document. OpenAPI supports several types of
authentication and authorization schemes, including HTTP, apiKey, oauth2,
and openIdConnect, each with its specific properties.
After defining the security schemes, apply them using the security keyword.
You can do this at the root level to cover the entire API or at the operation
level for more granular control. This setup allows you to safeguard sensitive
functions while keeping public endpoints accessible.
For OAuth 2 and OpenID Connect, scope-based permissions offer a detailed way to manage access. This ensures that only users with the correct privileges can perform administrative tasks, aligning with best practices for integrating security into OpenAPI.
The real strength of OpenAPI lies in its ability to combine multiple authentication types using logical OR and AND operations in the security section. This flexibility supports different client types while maintaining strict authorization controls, helping to prevent Broken Function Level Authorization (BFLA) vulnerabilities.
Choosing Between RBAC and ABAC#
Once you've defined your authorization rules, the next step is selecting the right access control model. The choice between Role-Based Access Control (RBAC) and Attribute-Based Access Control (ABAC) significantly impacts both security and operational complexity.
- RBAC: Assigns permissions based on predefined roles. It's straightforward to set up and works well for small to medium-sized organizations with clear hierarchies.
- ABAC: Provides finer control by using attributes like user roles, location, or time of access. While more complex to configure initially, it scales better for larger organizations with more nuanced access requirements.
Smaller organizations often start with RBAC due to its simplicity. However, as companies grow and require more granular access controls, maintaining RBAC can become cumbersome. ABAC, on the other hand, offers the flexibility needed for diverse and evolving scenarios.
A hybrid approach can be particularly effective. Use RBAC for broad user categories and ABAC for more sensitive operations that require contextual decision-making. This combination creates robust authorization controls that help prevent BFLA vulnerabilities.
Implementing RBAC#
Here's a tutorial on how to implement RBAC on your API using Zuplo. There's also a written version.
Adding Authorization Tests to CI/CD#
Integrating authorization testing into your CI/CD pipeline ensures that vulnerabilities are caught before they reach production. This proactive approach addresses issues when they are cheaper and easier to fix.
Despite its importance, many organizations lag in this area. For example, a 2024 GitLab survey revealed that only 29% of companies fully integrate security into their DevOps processes. Meanwhile, IBM's 2023 report highlighted that the average cost of a breach has climbed to $4.88 million.
To avoid these risks, make security a continuous effort. Use tools like Newman (to run Postman collections) or OWASP ZAP for automated scanning to verify that your authorization rules work as intended across various user roles and scenarios.
Set clear thresholds for blocking builds when critical authorization vulnerabilities are detected. Lower-severity issues can pass with alerts, but critical flaws should halt deployment. Treat authorization policies as first-class code by implementing unit and integration tests, and maintain detailed logs of every authorization decision. This approach not only catches vulnerabilities early but also lays the groundwork for long-term security, reducing the risk of BFLA attacks.
Over 10,000 developers trust Zuplo to secure, document, and monetize their APIs
Learn MoreBuilding Long-Term Authorization Security#
Strengthening API security for the long haul requires more than quick fixes. It demands a strategic approach that adapts to your organization’s needs and the ever-evolving threat landscape. The idea is to weave security practices into your development process, making them a natural part of your workflow rather than an afterthought.
Using Least Privilege Access#
The principle of least privilege is a cornerstone of effective authorization systems. By limiting access rights to only what’s necessary, you significantly reduce the potential attack surface. This isn’t just theory - removing local admin rights and controlling execution can mitigate 75% of Microsoft’s critical vulnerabilities. That’s a statistic you can’t afford to ignore.
Start by conducting a privilege audit to identify unused accounts, shadow admin credentials, and outdated permissions. Transition all users to standard privileges by default, granting elevated access only when absolutely necessary. For high-risk API functions, implement time-bound privileges - temporary permissions granted for specific tasks. This approach ensures that administrative access isn’t left open indefinitely.
Hardcoded credentials should be replaced with API-based authentication systems that can be monitored and revoked instantly. This not only improves security but also provides better control over who has access to what, and when.
"Authorization issues are typically difficult to detect in an automated fashion. The structure of the codebase should be set up in a way that it is difficult to make authorization errors on specific endpoints. To achieve this, authorization measures should be implemented as far up the stack as possible. Potentially at a class level, or using middleware." – Hakluke and Farah Hawa
As organizations scale, managing access becomes exponentially complex, especially with machine identities growing at twice the rate of human identities. This makes implementing and maintaining least privilege access a critical step in long-term security planning.
Managing Policies with GitOps#
GitOps offers a systematic way to manage authorization policies by treating them as code stored in Git repositories. This approach provides a single source of truth, enabling version control, automated deployments, and quick rollbacks when needed.
The benefits of GitOps shine during crises. For instance, when Weaveworks faced a system outage caused by a risky change, they restored their entire system - including clusters, applications, and monitoring tools - in just 40 minutes, thanks to their Git-based configuration files.
"GitOps is a set of best practices encompassing using Git repositories as the single source of truth to deliver infrastructure as code." – Hossein Ashtari
To ensure security, use pull requests for all changes to API access controls. This creates an audit trail and allows for automated reviews. Role-based access control can also be integrated into your GitOps workflow, defining who has permission to make changes to specific parts of your API configuration.
Even small adjustments to permissions should go through feature branches, ensuring proper review and testing before deployment. Automated testing for API configurations can catch errors early, preventing them from becoming vulnerabilities.
By separating API code from configuration releases, GitOps allows for faster updates and bug fixes while maintaining strict security oversight. If something goes wrong, you can quickly roll back changes without disrupting the application itself. This method also integrates seamlessly with CI/CD pipelines, reinforcing security at every stage.
"With GitOps, you can implement continuous deployment from any environment without having to switch tools. It's self-documenting, as changes are all recorded in the repo." – Cerbos
Regular policy reviews complement this automated approach, ensuring that your security measures remain effective over time.
Regular Permission Reviews#
Even the most well-designed authorization systems can drift without ongoing maintenance. Human error and stolen credentials remain leading causes of security breaches. Regular permission reviews are essential to prevent privilege creep and ensure that access rights align with current needs.
For most organizations, quarterly reviews are sufficient, but environments with higher security demands may require monthly audits. These reviews should examine not just user permissions but also API calls and the privileges granted to automated systems.
Key areas to focus on include removing access for former employees, revoking temporary privileges that have outlived their purpose, and ensuring that current employees don’t retain permissions from previous roles. In 2024, 61% of organizations reported cloud security issues, underscoring the importance of thorough permission reviews.
Involve multiple stakeholders in the process - not just the security team. Employees and managers who understand the business context behind access requirements can provide valuable insights. Document every step of the review process to create a record that supports continuous improvement.
Between formal reviews, continuous monitoring can help catch issues like failed login attempts or unusual access patterns. This proactive approach complements scheduled reviews and helps identify problems before they escalate.
The goal isn’t to achieve perfect security - it’s to build a system that evolves and improves over time. Regular permission reviews create the feedback loop necessary for continuous improvement, helping you address issues before they turn into costly mistakes.
Conclusion: Better API Security Through Proper Authorization#
Broken Function Level Authorization (BFLA) continues to pose a serious risk. Recent data reveals that 41% of organizations have faced an API security incident, with 63% of these incidents leading to data breaches - even though 90% already had authentication policies in place. This highlights a critical gap: while authentication may be in place, effective authorization controls are often lacking, leaving room for BFLA vulnerabilities to flourish.
Real-world cases show that no organization is completely safe from BFLA. Its combination of being hard to detect and easy to exploit makes it particularly dangerous.
Addressing this requires embedding robust authorization checks into every layer of your API architecture. Every API endpoint must enforce authorization by verifying user identity, roles, and permissions before granting access to sensitive functions or data. This goes beyond simply implementing Role-Based Access Control (RBAC) or Attribute-Based Access Control (ABAC) - it’s about adopting a mindset where security is treated as a core part of development, just like writing clean, efficient code.
Incorporating tools such as Postman and OWASP ZAP into your CI/CD pipeline can help ensure that authorization checks are consistently validated.
For long-term protection, strategies like enforcing the principle of least privilege, managing policies through GitOps, and conducting regular reviews of your authorization configurations are essential. These practices, combined with the testing and role configuration methods discussed earlier, create a stronger defense against threats.
BFLA is ranked #5 on the OWASP API Top 10. By implementing thorough function-level validation, maintaining server-side authorization controls, and adopting a zero-trust approach to API access, you’re not just patching vulnerabilities - you’re building systems designed to handle both current and emerging threats.
Use the techniques outlined in this guide as a starting point, and remember: API security is an ongoing process that demands constant vigilance and improvement.
FAQs#
What steps can organizations take to prevent Broken Function Level Authorization (BFLA) vulnerabilities in APIs?#
Preventing Broken Function Level Authorization (BFLA) in APIs#
Protecting APIs from Broken Function Level Authorization (BFLA) vulnerabilities requires a focus on well-implemented access controls and diligent security testing. Start by ensuring that every API function has strict, role-based authorization checks in place. These checks should prevent unauthorized users from accessing sensitive operations and must be applied consistently across all API endpoints.
Regular security assessments are equally important. Use tools like security scanners or conduct manual testing to uncover vulnerabilities before attackers can exploit them. Additionally, stay informed about the latest security practices, such as those outlined in the OWASP guidelines. By continuously reviewing and improving your authorization processes, you can strengthen your API's defenses and reduce the risk of breaches.
How can I identify if my API has Broken Function Level Authorization (BFLA) vulnerabilities, and what are the best ways to detect them?#
Broken Function Level Authorization (BFLA)#
BFLA vulnerabilities occur when users gain access to restricted functions or data by manipulating API requests. For instance, if altering a request parameter allows someone to access sensitive operations or resources they shouldn't, that's a clear sign of a BFLA issue. Another red flag is when access controls are inconsistent or missing across various API endpoints.
To identify these vulnerabilities, start by thoroughly reviewing your API's authorization logic. Use tools to simulate unauthorized access attempts and analyze the results. Combining manual code reviews with automated security testing can be particularly effective. Detailed logging of access patterns also helps in spotting potential weaknesses. Tools like Postman and OWASP ZAP are great for crafting test requests and examining responses to pinpoint gaps in your authorization setup. API gateways like Zuplo help you implement fixes at scale. Taking these steps can go a long way in strengthening your API's security.
Why should authorization testing be part of your CI/CD pipeline, and what tools can help?#
Incorporating authorization testing into your CI/CD pipeline is a smart way to identify security vulnerabilities early, ensuring that sensitive functions are only accessible to authorized users. By addressing these issues proactively, you can reduce risks, block unauthorized access, and maintain compliance with security standards. In the fast-paced world of CI/CD, relying solely on manual testing can leave gaps, but automated testing provides consistent and dependable results.
Tools like OWASP ZAP are excellent for dynamic application security testing, while SonarQube and Checkmarx specialize in static security testing. These tools integrate seamlessly into your pipeline, automating checks and enabling you to catch and resolve issues quickly - before they ever make it to production.
