API Security Testing: The Overlooked Frontline in Application Penetration Testing 

Introduction

Within the past 15 years, Application Programming Interfaces (APIs) have become the backbone of modern applications. From mobile apps and web platforms to microservices and cloud-native architectures, APIs facilitate seamless communication between systems, services, and users. With the rise of digital work, the number of exposed APIs has surged—driven by the need for faster integration, automation, and scalability. 

However, this rapid proliferation has not gone unnoticed by malicious actors. APIs are increasingly becoming prime targets for cyberattacks, and unlike traditional web applications, APIs often expose backend logic, sensitive data, and business functions directly to clients. Attackers exploit common vulnerabilities such as broken authentication, excessive data exposure, and improper access controls to compromise applications and exfiltrate data. Recently, API-related breaches have been on the rise, with attackers leveraging misconfigurations and logic flaws that go undetected in conventional security assessments. 

This is where API security testing must play a central role in any application penetration testing strategy. But, what is API security? Traditional pentesting approaches often overlook APIs or treat them as secondary concerns, yet APIs require targeted testing methods that account for unique threats—like abuse of API-specific functionality, inadequate rate limiting, or lack of schema validation. Effective API security testing helps uncover these hidden risks, assess real-world impact, and guide secure development practices. Integrating API security testing into your regular pentest cycle is not just a best practice—it’s now a necessity. As your applications grow more interconnected, securing the APIs that bind them becomes critical to protecting your entire attack surface. 

The Growing Threat Landscape for APIs 

As web and mobile apps continue to develop and evolve, APIs have become central pillars—powering everything from microservices and fintech platforms to integrated SaaS, IoT, and AI-driven tools. Organizations now expose hundreds or even thousands of API endpoints, driving agility and innovation, but also multiplying potential attack paths. One misconfigured API call can open the floodgates to massive data leaks or business-level abuse. 

Why APIs Are a Major Target 

APIs often surface backend logic and sensitive data directly to clients. Attackers then exploit weak controls—broken authentication, object-level flaws, improper rate limiting, wild injection points, and business logic weaknesses—to launch automated, high-impact attacks. A single unsecured API can yield jackpot access to PII, financial systems, or critical workflows. 

For example, in early July of 2025, Qantas experienced a large-scale breach, exposing phone numbers, birth dates, and addresses of 5.7 million customers via an API misconfiguration incident. “Since the incident, we have put in place a number of additional cyber security measures to further protect our customers’ data, and are continuing to review what happened,” Qantas Group CEO Vanessa Hudson said. Though, proactivity would have been an more effective line of defense. 

The Role of OWASP API Security Top 10 

The OWASP API Security Top 10 (2023) is the benchmark for identifying and testing real-world API threats. Key categories include: 

1. Broken object-level & function-level authorization: attack paths to exfiltrate unauthorized data. 
2. Broken authentication: enables impersonation. 
3. Broken object‑property authorization: unauthorized read/write to sensitive fields. 
4. Unrestricted resource consumption & lack of rate limiting: enabling DoS and brute attacks. 
5. Business logic abuse & SSRF: abuses permitted flows to business detriment. 
6. Security misconfiguration & inventory issues: neglected endpoints often go untested. 
7. Unsafe third-party API consumption: trusting external APIs blindly can introduce vulnerabilities. 

By mapping penetration testing to the OWASP API Top 10, security teams simulate sophisticated attacks—ranging from object enumeration and fuzzing to logic manipulation, SSRF, and bot-driven threats. 

Why APIs Are a Prime Target for Attackers 

APIs have become a top attack vector because they often expose the very core of an application’s data and logic. Designed for machine-to-machine communication, APIs frequently bypass traditional user interfaces—making them less visible but highly valuable to attackers. These interfaces directly connect to backend systems, exposing endpoints that handle sensitive data, such as personally identifiable information (PII), financial records, and business-critical operations. 

A major weakness lies in poorly implemented authentication and authorization mechanisms. APIs commonly lack strong identity checks or fail to enforce user-specific access controls. This makes it easier for attackers to impersonate users or escalate privileges. Broken user authentication can allow unauthorized access to protected endpoints, enabling attackers to log in as other users or even administrators. 

Equally dangerous is Broken Object Level Authorization, or BOLA, which is the number one threat in the previously mentioned OWASP API Security Top 10. BOLA vulnerabilities occur when APIs allow access to objects (e.g., user profiles, documents, account data) without properly validating the user’s ownership of that object. A common example is when changing a user ID in an API call retrieves another user’s data, leading to severe privacy breaches or account takeovers. 

Another critical issue is improper rate limiting, which leaves APIs open to brute force attacks, credential stuffing, and denial-of-service scenarios. When APIs fail to throttle traffic or restrict repeated requests, attackers can automate large-scale attacks with ease. Additionally, excessive data exposure—where APIs return more data than necessary—often provides attackers with internal schema, hidden fields, or sensitive details that can be exploited further. 

The Role of API Security Testing in Application Pentesting

API security testing is a critical component of modern application penetration testing. Unlike traditional app pentests that focus on web interfaces, API testing targets the underlying endpoints that power frontend and mobile interactions. Penetration testers assess APIs by inspecting traffic, analyzing documentation, and using tools or custom scripts to simulate real-world attack scenarios. 

API security testing differs from traditional app testing in its focus on machine-level interactions, lack of a GUI, and greater exposure of backend logic. APIs often allow direct access to objects and actions that would typically be restricted or hidden in web apps, making authorization flaws more impactful. 

Key security protocols in APIs include: 

• Authentication and Authorization Testing: Ensuring users cannot impersonate others, bypass access controls, or access unauthorized resources (e.g., testing for BOLA). 
• Rate Limiting and Abuse Protection: Simulating brute force or bot attacks to identify missing or ineffective rate limiting, throttling, or IP restrictions. 
• Input Validation and Injection Attacks: Fuzzing parameters for SQL/NoSQL injection, command injection, or XSS vulnerabilities. 
• Business Logic Testing: Probing for flaws in how APIs enforce rules—such as order manipulation, pricing errors, or unauthorized workflow steps. 

Best Practices for API Security Testing 

Securing APIs requires a proactive, layered approach that combines secure design, robust controls, and continuous testing. We recommend the following effective API security best practices that help prevent common vulnerabilities and ensure resilient APIs. 

1. Implement Secure Authentication Mechanisms
   At NetSPI, we recommend using industry-standard authentication methods like OAuth 2.0, JWT (JSON Web Tokens), or API keys to verify user and application identity. OAuth enables delegated access and token expiration, while JWT provides stateless, signed tokens that can carry user claims securely. Avoid using static credentials or sending tokens in URL parameters.
2. Enforce Least Privilege Access Control
   APIs should enforce strict authorization policies to ensure users can access only what they’re permitted to. This includes applying object-level authorization checks and role-based access controls for every endpoint. Misconfigured access is a leading cause of data leaks.
3. Validate and Sanitize User Input
   All input passed to an API must be validated against strict schemas and sanitized to prevent injection attacks. Input validation helps block SQL/NoSQL injection, command injection, and cross-site scripting, which are commonly exploited when APIs blindly trust client input.
4. Enable API Rate Limiting and Monitoring
   Use rate limiting, throttling, and IP blacklisting to protect APIs from brute force attacks, scraping, and abuse. Coupled with real-time monitoring and logging, this allows teams to detect anomalies early and respond to threats quickly. 
5. Use Automated API Security Testing Tools
   Integrate automated tools with test scripts, and specialized platforms to identify vulnerabilities continuously. These tools help simulate attacks, validate security controls, and ensure consistent testing across development cycles. 

By incorporating these best practices into your API security testing strategy, you build stronger defenses and reduce your attack surface.

How NetSPI Can Help

APIs are a crucial part of modern applications, but without proper security testing, they can become a major entry point for cyberattacks. Prioritizing API security in your application pentesting strategy helps protect sensitive data, prevent breaches, and ensure compliance with security standards. Don’t leave your organization exposed—take proactive steps to secure your APIs today.