What is API Security?

Why APIs Are Uniquely Vulnerable

APIs expose application functionality and data through defined interfaces designed to be accessible — to partners, mobile apps, and third-party integrations. Unlike web applications where functionality is accessed through a browser interface that provides some natural friction, APIs are designed for direct programmatic access, making them efficient channels for both legitimate use and abuse.

OWASP API Security Top 10

Broken Object Level Authorization — BOLA — is the most critical API vulnerability class. It occurs when an API endpoint returns data based on a user-supplied identifier without verifying that the requesting user is authorized to access that specific object. An API endpoint that returns user profile data for whatever user ID is provided in the request, without checking whether the requester is authorized to view that profile, has BOLA. By simply incrementing or modifying user IDs, an attacker can access any user's data. BOLA enabled the 2022 Optus breach, exposing 9.8 million Australian customer records.

Broken Authentication allows unauthenticated access to functionality that should require valid credentials. Excessive Data Exposure returns more data than the requesting application needs. Mass Assignment allows attackers to set fields they should not be able to modify.

API Security Controls

API gateways provide centralized authentication enforcement, rate limiting, input validation, and logging for API traffic. Deploying an API gateway as the single entry point for all API traffic ensures authentication is enforced consistently rather than relying on individual API implementations. Rate limiting prevents abuse through high-volume automated requests. Input validation blocks malformed requests before they reach backend services.

API inventory management — knowing what APIs exist and what they expose — is the foundational challenge. Shadow APIs, deprecated APIs, and undocumented internal APIs all create exposure that cannot be assessed or protected if they are not known. API discovery tools scan network traffic and application configurations to build comprehensive API inventories.

API Security for PE Portfolio Companies

PE-backed technology companies with significant API exposure should evaluate: Is there an API inventory documenting all internal and external APIs? Are all API endpoints authenticated, and is authentication enforced at the gateway level? Is authorization validated at the object level for every data access, not just at the endpoint level? Is API traffic logged and monitored for anomalous access patterns? Has API security testing been conducted, including testing for BOLA and other OWASP API Top 10 vulnerabilities?

Real-World Example: Optus Breach 2022 — BOLA at Scale

The 2022 Optus data breach exposed the personal data of 9.8 million Australians through a BOLA vulnerability in a publicly accessible API. The API required no authentication and returned customer records when queried with sequential customer ID numbers. The attacker simply iterated through customer IDs, collecting records until the breach was detected. The API had been exposed to the public internet without authentication as part of a migration, representing a catastrophic API governance failure.