API Security

What is API security?

API security is the set of practices, controls, and testing methods used to protect application programming interfaces from unauthorized access, data exposure, and exploitation. It encompasses the authentication and authorization mechanisms that govern who can access an API and what they can do, the validation and sanitization of inputs, the protection of data in transit and at rest, and the testing required to confirm that controls work as intended under adversarial conditions.

APIs have become the primary interface layer of modern applications. A web or mobile application may expose hundreds or thousands of API endpoints. A microservices architecture routes internal communication through APIs. Third-party integrations, partner data sharing, and AI agent workflows all operate through APIs. This makes APIs the most consequential attack surface in most organizations and one of the most consistently under-tested.

The challenge is structural. APIs are designed to be accessible. The same properties that make them useful also make them an attractive target. The security burden falls on every layer of how the API is built, deployed, and validated.

Why API security matters

The scale and severity of API-related breaches reflect a consistent pattern: APIs expose application logic and data directly, authorization controls are frequently incomplete, and the testing tools most organizations rely on are not designed to find the vulnerabilities that matter most.

Several factors compound the risk:

• APIs expose data at scale. A single vulnerable API endpoint can return hundreds of records per request. Attackers exploiting a data access flaw do not need to work through a user interface. They enumerate data systematically at the API layer, and the exposure is proportional to the number of records the API can return.
• API attack surfaces are large and poorly mapped. Organizations commonly have more API endpoints in production than their security teams are aware of. Shadow or undocumented endpoints created by developers, inherited from third-party libraries, or left over from deprecated versions extend the testable surface beyond what any manually scoped assessment can cover.
• Authorization flaws are endemic. The most consistently exploited API vulnerabilities are not injection attacks or protocol weaknesses. They are failures to verify, at the object level, that an authenticated user is permitted to access the specific data they are requesting. These failures are structurally invisible to the tools most commonly used in application security programs.
• AI and agent integrations extend the exposure. APIs now serve as the interface layer for AI agent workflows, MCP servers, and LLM-backed services. An API compromise in this context not only exposes data but can also affect automated systems with broad access to business operations.

Common API security vulnerabilities

The most critical API security vulnerabilities are documented in the OWASP API Security Top 10. The categories that generate the most real-world exploitation activity are:

• API1:2023 Broken Object Level Authorization (BOLA/IDOR). An API confirms the user is authenticated but fails to verify they are permitted to access the specific data object they have requested. It is the single most exploited API vulnerability. See BOLA for a complete treatment.
• API2:2023 Broken Authentication. Weaknesses in how an API implements authentication, such as token handling, API key management, credential flows, and session management. These allow an attacker to impersonate another user or bypass authentication entirely. A leaked or poorly scoped API key is a common route to exactly this kind of compromise.
• API3:2023 Broken Object Property Level Authorization. The API returns more fields or properties than the requesting user should have access to, either by exposing sensitive properties on otherwise authorized objects or by failing to enforce restrictions on fields that should be read-only.
• API4:2023 Unrestricted Resource Consumption. The API does not rate-limit requests, response size, or computation, allowing an attacker to degrade availability or generate high costs through high-volume or resource-intensive requests.
• API5:2023 Broken Function Level Authorization (BFLA). The API exposes administrative or sensitive functionality without restricting which authenticated users can invoke it. Where BOLA is about unauthorized access to data objects, BFLA is about unauthorized access to actions and endpoints.
• API6:2023 Unrestricted Access to Sensitive Business Flows. The API exposes business workflows like account creation, payment submission, and content publication, without behavioral controls, allowing automated abuse at scale.
• API7:2023 Server-Side Request Forgery (SSRF). The API can be induced to fetch or interact with internal resources the attacker cannot reach directly, using the server as a proxy into the internal network.
• API8:2023 Security Misconfiguration. Insecure defaults, exposed debugging endpoints, an API key hardcoded in client-side code or committed to a repository, overly permissive CORS configuration, unnecessary HTTP methods, or missing security headers.
• API9:2023 Improper Inventory Management. Undocumented or deprecated API versions that remain accessible in production, frequently without the same security controls as current versions.
• API10:2023 Unsafe Consumption of Third-Party APIs. The application trusts data returned from third-party APIs without validation, allowing a compromised upstream service to affect the application’s behavior or security posture.

API security and the OWASP API Security Top 10

The OWASP API Security Top 10 is the industry’s primary reference for API vulnerabilities. Published in 2019 and updated in 2023, it documents the categories of API vulnerability that cause the most real-world harm, prioritized by prevalence and impact across real environments.

The 2023 edition reflects a shift in how API vulnerabilities are understood. The list moved away from a technology-centric view toward one grounded in how APIs are exploited by attackers. Authorization failures retained their positions at the top. New categories addressed property-level authorization, business flow abuse, and exploitation via unsafe consumption of third-party APIs.

For cyber security teams, penetration testers, and compliance programs, the OWASP API Security Top 10 provides a practical testing checklist: An API validated against every category in the Top 10 has been tested against the vulnerabilities most likely to be exploited by a real attacker. It is the standard reference for defining what comprehensive API security testing means.

API security testing approaches

Effective API security requires testing approaches matched to the vulnerabilities being targeted. The main approaches differ significantly in what they find and what they miss:

• Vulnerability scanning detects known CVEs, missing headers, and common misconfigurations by comparing requests and responses against a signature database. Fast and scalable, but unable to detect authorization flaws, logic vulnerabilities, or any weakness without an established signature.
• Dynamic Application Security Testing (DAST) tests a running API for detectable vulnerability patterns like injection, authentication issues, and basic misconfigurations by sending crafted inputs and inspecting responses. Effective for signature-matchable vulnerabilities but limited against context-dependent weaknesses such as BOLA and business logic abuse.
• Manual penetration testing applies adversarial human judgment to find complex authorization failures, logic abuse, and chained exploits. Deep and accurate, but slow, expensive, and scoped to a point in time. Cannot scale with frequent API changes.
• AI penetration testing applies adversarial reasoning at machine speed, discovering shadow endpoints, probing authorization boundaries, abusing business logic, and validating exploitability across the full API surface. Closes the coverage and frequency gap that manual testing cannot resolve.
• API gateway and WAF security controls enforce runtime protections such as rate limiting, input validation, and authentication enforcement at the edge. An API gateway is a protective control, not a testing methodology; it reduces the impact of some attacks but does not validate whether the API itself is free of vulnerabilities.

A comprehensive API security program typically combines gateway-level controls for runtime protection with regular AI-driven penetration testing to validate those controls and surface vulnerabilities that the controls were never designed to catch.

API security vs. application security

API security is a component of application security, but the two disciplines require meaningfully different approaches in practice.

Application security, broadly, covers all security properties of a software application: its code, dependencies, infrastructure, authentication mechanisms, and interfaces. Static Application Security Testing (SAST) examines source code for vulnerability patterns. Software Composition Analysis (SCA) identifies vulnerable third-party dependencies. DAST tests the running application through its user interface and HTTP interface.

API security focuses on the interface layer, specifically the endpoints that expose application functionality and data to clients, other services, and third parties. The distinctive challenges of API security are:

• APIs expose application data more directly than user interfaces, making authorization failures immediately consequential and scalable for an attacker.
• APIs are consumed programmatically, so systematic enumeration and abuse require only basic tooling from an attacker’s perspective.
• The most prevalent API vulnerabilities are not reliably detected by the SAST, DAST, and scanning approaches that form the backbone of most application security programs.

These distinctions mean that organizations with mature application security programs still frequently carry significant API security risks, because the tools they have invested in are not designed to find the vulnerabilities that matter most at the API layer.

API security and compliance

Authorization and access control failures sit at the heart of API security risk, and several major regulatory frameworks create direct obligations around their testing and remediation:

• PCI DSS v4.0 requires penetration testing of all systems that store, process, or transmit cardholder data. APIs handling payment flows are directly in scope, and PCI DSS v4.0 explicitly expects testing that validates whether controls prevent exploitation, not merely that controls exist on paper.
• DORA requires financial entities to test their ICT systems under realistic adversarial conditions. APIs are the primary interface layer for most financial applications, and BOLA and authorization testing is a substantive component of any realistic adversarial assessment at the API layer.
• NIS2 requires essential and important entities to manage ICT security risk proportionately, including access control failures. APIs handling personal data, financial transactions, or operational systems represent material NIS2 risk when their authorization controls are unvalidated.
• GDPR does not prescribe specific testing methodologies but requires appropriate technical measures to protect personal data. An API that exposes personal data without adequate authorization controls represents a direct GDPR exposure, and testing that validates those controls is a proportionate technical measure.
• ISO 27001 requires access control effectiveness to be tested as part of an information security management system. Authorization failures at the API layer represent access control breakdowns that standard reviews do not surface.

Challenges and considerations

• Mapping the full attack surface. Organizations commonly have more API endpoints in production than their inventory records. Shadow endpoints, versioned APIs, and third-party integrations extend the testable surface beyond what a manually defined scope can cover reliably.
• Testing behind authentication. The most critical API vulnerabilities sit behind authentication and within multi-step, role-dependent workflows. Testing conducted without valid credentials and role context will not reach the vulnerabilities that matter most.
• Business logic is application-specific. Logic vulnerabilities cannot be detected by signature. They require understanding how the application is designed to behave and testing whether that behavior can be abused. This is the hardest class of API vulnerability to test at scale.
• Rate and abuse controls. Many APIs implement access controls that limit individual requests without protecting against systematic abuse conducted across a session, a time window, or a distributed set of clients. Effective testing must simulate realistic adversarial behavior, not just individual crafted requests.
• Third-party and supply chain risk. APIs that consume data from third-party sources without validation inherit the risk of those sources. Testing programs that cover only first-party APIs miss this class of exposure.

API security with Equixly

Equixly tests API security through its autonomous Agentic AI Hacker, which discovers the full API attack surface, including shadow endpoints the cyber security team was not aware of, and probes authorization boundaries, business logic, and all OWASP API Security Top 10 categories with adversarial reasoning that adapts to the target’s behavior.

Every finding is validated before it is surfaced. Equixly confirms that an authorization flaw is genuinely exploitable, demonstrating the specific request, identifier, and response that prove the vulnerability exists rather than flagging patterns that require manual verification. The result is a false positive rate below 1% across all findings.

Since Equixly operates continuously, API security validation stays current as endpoints change, new integrations are added, and data models evolve. The same adversarial testing that finds BOLA on day one runs automatically every time the API changes, without requiring a new scoping cycle.

Further detail is available on the continuous penetration testing platform page.