Horizontal Broken Function Level Authorization (BFLA)
Description
Horizontal Broken Function Level Authorization (BFLA) occurs when an application fails to verify that a user is authorized to access resources or perform actions belonging to another user at the same privilege level. Unlike vertical privilege escalation where attackers gain higher-level permissions, horizontal BFLA allows users to access or manipulate data and functions of their peers by tampering with identifiers or parameters in requests. This vulnerability is particularly common in APIs and web applications that rely solely on client-side access controls or fail to validate resource ownership server-side.
Remediation
To prevent horizontal BFLA vulnerabilities, implement comprehensive server-side authorization controls:
1. Enforce resource ownership validation: Always verify that the authenticated user owns or has explicit permission to access the requested resource before processing any operation.
// Example: Validate resource ownership before access
app.get('/api/orders/:orderId', authenticate, async (req, res) => {
const order = await Order.findById(req.params.orderId);
if (!order) {
return res.status(404).json({ error: 'Order not found' });
}
// Verify the order belongs to the authenticated user
if (order.userId !== req.user.id) {
return res.status(403).json({ error: 'Access denied' });
}
return res.json(order);
});2. Implement centralized authorization logic: Create reusable authorization middleware or functions to ensure consistent access control checks across your application, reducing the risk of oversight.
3. Use indirect object references: Instead of exposing direct database IDs, use session-specific or user-scoped identifiers that cannot be easily manipulated to access other users' resources.
4. Apply the principle of least privilege: Grant users only the minimum permissions necessary for their role, and explicitly define which resources each user can access.
5. Implement role-based or attribute-based access control (RBAC/ABAC): Use structured access control systems that define clear policies for resource access based on user roles, attributes, and context.
6. Conduct regular security testing: Perform automated and manual testing specifically targeting authorization flaws, including testing with multiple user accounts at the same privilege level to identify horizontal access control issues.
7. Log and monitor authorization failures: Track failed authorization attempts to detect potential exploitation attempts and identify weaknesses in your access control implementation.