Introduction
As web applications have evolved from static to dynamic, the complexity of managing access has grown. The need for stringent access control mechanisms has become paramount, especially as data sensitivity and security concerns rise. Enter Broken Access Control (BAC) — a prevalent vulnerability in web applications that poses significant security risks. BAC can lead to unauthorized access to resources, enabling attackers to escalate privileges, manipulate data, and breach user privacy.
In this article, we'll explore what BAC is, the various forms it can take, common attack strategies, and practical ways to test for and mitigate this vulnerability. By the end, you'll understand how BAC works, how to recognize it, and the tools and techniques you can use to defend against it.
What is Broken Access Control (BAC)?
Broken Access Control is, quite simply, a failure in enforcing restrictions on what authenticated users are allowed to do. It manifests primarily in two types of privilege escalation:
- Horizontal Privilege Escalation: Occurs when users can access data or perform actions meant for other users with the same privilege level. A classic example is an Insecure Direct Object Reference (IDOR), where users can view or manipulate other users' data by manipulating parameters (e.g., IDs) in requests.
- Vertical Privilege Escalation: Happens when a lower-privilege user, such as a regular user, gains access to actions or data reserved for higher-privilege users, like administrators.
For BAC to exist, specific conditions are typically in place:
- The application supports multiple user roles or privilege levels.
- Different functionalities or data are accessible based on privilege levels (e.g., regular users vs. admins).
- The application does not correctly enforce restrictions on these functionalities, allowing unauthorized access.
Examples of BAC
- Admin vs. Normal User: Testing admin-specific functionalities, such as user management, as a regular user.
- Paid User vs. Trial User: Paid users have access to premium features, while trial users do not. If trial users can bypass these limitations, a BAC vulnerability exists.
- Same Authorization Level: Two users with the same access level can access each other's data by altering object identifiers. This is often seen in IDOR vulnerabilities.
Attack Strategies
There are several methods for exploiting BAC vulnerabilities. These range from manual techniques to semi-automated approaches using specific tools.
Manual Testing
Manual testing can be highly effective for detecting BAC, especially when performed by individuals familiar with the application's access control structure. Here are some common manual testing techniques:
Developer Console and JavaScript Testing
The developer console in modern browsers offers access to JavaScript functions that may be obscured from the user interface (UI). Often, developers disable UI elements to restrict access to certain features, but the underlying JavaScript functions may still be executable.
Steps:
- Open the developer console (usually via F12 or Ctrl+Shift+I in most browsers).
- Use Intellisense (an auto-suggest feature) to identify hidden JavaScript functions by typing a letter, which will reveal related functions across the application.
- Attempt to execute these functions directly in the console, which may reveal restricted functionalities or sensitive data.
URL Manipulation
If high-privilege users (like admins) have access to specific URLs, you can log in as an admin, navigate to these URLs, and copy them. Then, log in as a lower-privilege user, paste the copied URL, and check if access is granted.
This technique requires understanding the application's privilege levels to identify accessible and restricted functionalities. It's simple yet powerful, revealing access issues in scenarios where privilege separation isn't enforced correctly.
Testing via Man-in-the-Middle (MiTM) Proxy
Using a MiTM proxy tool like Burp Suite, you can intercept and manipulate requests to simulate different privilege levels. Send restricted requests to Burp Suite Repeater and replace high-privilege authorization tokens (e.g., JWTs or session cookies) with those of a low-privilege user to see if BAC is enforced.
The goal is to test if the application differentiates between user levels in terms of access permissions. Successful access with a low-privilege token indicates BAC issues.
Semi-Automated Testing
Tools like Burp Suite offer extensions and plugins that simplify BAC testing. Two commonly used extensions are Authorize and Match and Replace:
- Authorize: This extension automatically tests whether access control is enforced by comparing requests from a high-privilege user with those from a low-privilege user.
- Match and Replace: Allows replacing certain parts of requests to simulate requests with varying privileges. This helps in identifying unauthorized access points efficiently.
Burp Suite's AutoRepeater is another plugin that can be configured to automatically send modified requests for BAC testing. However, compatibility issues with newer Burp Suite versions may limit its functionality.
Types of IDOR in BAC
IDOR is one of the most common forms of BAC. It involves accessing objects directly through identifiers (e.g., user IDs, product IDs). Here are the main distinctions in IDOR:
1. Object ID vs. User ID-Based
- Object ID-Based: Refers to direct access to objects via their ID, such as
invoiceID=1. - User ID-Based: Refers to access based on ownership, such as
ownerID=123, which indirectly allows access to objects owned by a user.
While the approach to finding these vulnerabilities remains the same, understanding the type of IDOR can help tailor your testing to specific application logic.
2. Primary vs. Secondary IDOR
- Primary IDOR: Involves direct checks within a single system to enforce access control.
- Secondary IDOR: Occurs when multiple systems handle access control, and authorization checks are not correctly passed along to secondary systems. This often happens when authorization tokens are not verified in subsequent systems, which can lead to unauthorized access.
Multi-Tenancy and BAC
In multi-tenant applications, multiple clients share a single server, and access control becomes more complex. Here, BAC vulnerabilities can lead to unauthorized cross-client data access, known as multi-tenancy IDOR. This is particularly dangerous in business environments, where accidental access to another client's data can lead to reputational damage, legal issues, and even corporate espionage.
Security Risks of Multi-Tenancy IDOR Multi-tenancy IDOR can result in data leaks, unauthorized actions, and potential breaches. Therefore, separating and enforcing access control rigorously between clients on shared servers is essential to avoid costly and potentially damaging breaches.
Preventing and Mitigating BAC Preventing BAC vulnerabilities requires a combination of secure coding practices, regular testing, and monitoring. Here are some best practices:
- Role-Based Access Control (RBAC): Ensure role-based permissions are enforced at both the UI and backend levels to prevent unauthorized access.
- Parameter Validation: Validate all parameters that reference sensitive objects, such as user IDs or order IDs, to ensure users can only access their own data.
- Frequent Security Audits: Regularly conduct manual and automated security audits to test access controls and detect BAC vulnerabilities.
- Separation of Multi-Tenant Data: Implement rigorous separation of client data in multi-tenant systems and conduct regular access reviews.
- Least Privilege Principle: Limit user access to only what is necessary for their role and avoid excessive permissions that increase risk.
- Security Training for Developers: Educate developers about BAC vulnerabilities and best practices in secure coding to reduce BAC incidents.