API Security Testing: Why Your APIs Are Your Weakest Link
APIs are the fastest-growing attack surface in modern applications. Most organisations test their web interfaces but leave their APIs inadequately assessed. This guide explains what API security testing covers, why APIs are disproportionately vulnerable, and what findings to expect.
TL;DR
APIs power modern applications. They also represent the fastest-growing and most inadequately tested attack surface in most organisations. While web application security has matured, API security has lagged: weaker authentication enforcement, inconsistent access controls, excessive data exposure, and entire endpoints that security teams do not know exist. This guide explains what API security testing covers, what the most common API vulnerabilities look like in practice, and why a web application test is not a substitute for dedicated API testing.
Why APIs are disproportionately vulnerable
Several structural factors make APIs consistently more vulnerable than the web interfaces they support.
APIs expose raw data and functionality directly. A web interface filters and formats what it shows. An API returns data objects directly. If the access control on an API endpoint is missing or incomplete, an attacker gets the raw data, not a filtered view.
APIs are built faster than their security is reviewed. Development teams add endpoints quickly. Security review does not always keep pace. Authentication may be correctly implemented on the primary user-facing endpoints but missing on a batch export endpoint added later. Rate limiting may be enforced on the main flow but not on a secondary data retrieval endpoint.
Shadow APIs are common. Undocumented endpoints from legacy versions, development shortcuts, deprecated integrations, and third-party connections exist in most large applications. Security teams do not know they exist. They are not monitored. They often have weaker controls than documented endpoints.
API authentication tokens are long-lived and high value. API keys, OAuth tokens, and JWT tokens that are compromised grant persistent access. Unlike session cookies that expire after a browser session, API tokens are often long-lived and shared across multiple consumers.
The OWASP API Security Top 10
The most common and most damaging API vulnerabilities are well documented. API security testing uses the OWASP API Security Top 10 as a baseline testing framework.
Broken object level authorization (BOLA)
BOLA is the most exploited API vulnerability. It occurs when an API endpoint accepts an object reference from the client, such as a user ID, order ID, or document reference, without verifying that the requesting user is authorised to access that specific object. An attacker who changes the ID in a request to access another user's data is exploiting BOLA. The fix requires server-side verification of object ownership for every request, not just authentication.
Broken authentication
API authentication weaknesses include missing authentication on specific endpoints, weak token validation, JWT algorithm confusion attacks, API keys in URLs that appear in server logs, and missing authentication on secondary endpoints that developers assumed were not publicly accessible.
Broken function level authorization
Function level authorization controls whether a user can invoke a specific API endpoint. Vulnerabilities occur when regular users can access administrative endpoints, when different user roles can access each other's functions by directly calling the API, and when the user interface restricts an action but the underlying API endpoint does not enforce the same restriction.
Excessive data exposure
Many APIs return full data objects in responses and rely on the client to filter what is displayed to the user. This means that sensitive fields, internal identifiers, and data the user should not see are present in API responses but simply not shown in the interface. An attacker who can read API responses directly can access this data.
Unrestricted resource consumption
Missing or ineffective rate limiting allows attackers to make unlimited API calls, enabling credential stuffing, enumeration of user data, resource exhaustion, and the extraction of large datasets through repeated calls. Rate limiting must be enforced at the API level, not just in the user interface.
What API security testing covers in practice
API discovery and inventory. Before testing begins, the tester maps the full API surface, including endpoints discovered through the provided specification, through active exploration, and through passive observation of application traffic. Shadow endpoints discovered during this phase are often among the most significant findings.
Authentication and token testing. Testing covers how authentication tokens are generated, transmitted, validated, and revoked. This includes JWT validation, API key entropy and exposure, OAuth flow security, and whether authentication is consistently enforced across all discovered endpoints.
Authorization testing across roles. Testing uses multiple test accounts at different privilege levels to verify that object-level and function-level authorization is correctly enforced for every endpoint. Each endpoint is tested for horizontal and vertical privilege escalation.
Input validation and injection. API endpoints that process user-controlled input are tested for injection vulnerabilities including SQL injection, NoSQL injection, command injection, and mass assignment vulnerabilities where the API accepts object fields that should not be user-controlled.
Rate limiting and business logic. Testing verifies that rate limiting is enforced on all sensitive endpoints, that business logic rules are enforced at the API layer and not only in the client, and that sensitive operations cannot be repeated without appropriate controls.
FAQ
What is API security testing?
API security testing is a structured assessment of the security of application programming interfaces, covering authentication, authorisation, input validation, rate limiting, data exposure, and business logic enforcement. It uses both automated tools and manual testing to identify vulnerabilities that allow attackers to access data they should not see, perform actions they should not be able to perform, or abuse the API in ways the developer did not intend.
What is broken object level authorization in APIs?
Broken object level authorization, listed as the top vulnerability in the OWASP API Security Top 10, occurs when an API endpoint accepts object references from the client without verifying that the requesting user is authorised to access that specific object. An attacker who can change an order ID, user ID, or document reference in an API request to access another user's data is exploiting broken object level authorization. This is the most commonly exploited API vulnerability in real-world attacks.
How does API security testing differ from web application testing?
Web application testing typically focuses on the user-facing interface and what a browser-based attacker can do. API testing focuses on the underlying data and service layer, often accessible without any user interface at all. APIs frequently expose more functionality than the web interface uses, have weaker authentication enforcement on specific endpoints, and expose sensitive data directly in responses without the presentation layer filtering that a web interface applies. API testing requires understanding the API contract, not just what a browser shows.
What is the OWASP API Security Top 10?
The OWASP API Security Top 10 is a reference list of the most critical API security risks. It covers broken object level authorization, broken authentication, broken object property level authorization, unrestricted resource consumption, broken function level authorization, unrestricted access to sensitive business flows, server-side request forgery, security misconfiguration, improper inventory management, and unsafe consumption of APIs. API penetration testing uses this list as a baseline but extends well beyond it.
Why are undocumented or shadow APIs a security risk?
Shadow APIs are endpoints that exist in production but are not listed in official API documentation or inventories. They arise from legacy integrations, development shortcuts, deprecated versions left running, and third-party integrations. Because they are undocumented, they are not covered by security testing, are not monitored, and often have weaker authentication and access controls than documented endpoints. They represent a significant risk because attackers can discover them through reconnaissance.
What should be included in an API security assessment scope?
An API security assessment scope should include all API endpoints including undocumented and legacy versions, all authentication mechanisms including API keys, OAuth, and JWT tokens, all user roles and their expected access levels, any batch or export endpoints, any administrative endpoints, rate limiting and throttling controls, and the data models exposed by the API. The scope should be defined from an API specification document if available, supplemented by discovery during the test.
Related services and resources
Sectricity provides dedicated API security testing covering REST, GraphQL, and SOAP APIs across all user roles and privilege levels. We also provide web application penetration testing and on-demand testing through our RedSOC PTaaS platform. For a deeper look at web application security, see our guide on web application penetration testing. Start with a free security scan.