APIs allow communication between services, platforms, and users. As organizations grow and use microservices, serverless architectures, and third-party connections, the number and complexity of APIs increase. Each public API endpoint adds another point of entry that attackers may try to exploit. APIs differ from traditional apps that expose backend services and private actions directly to users, making them attractive targets for attackers.

To secure user data and ensure organizational integrity, security engineers should perform continuous API security testing. This includes checking authentication systems, implementing access restrictions, finding input validation gaps, and detecting logic errors. Without a strong API security testing strategy, vulnerabilities can go undetected, resulting in data breaches, privilege escalation, or unauthorized access to systems.

This blog will explore API security testing, its importance, and types. Learn more about how to do API security testing, API security testing methodology, and examples.

What is API Security Testing?

API security testing is the process of analyzing APIs to identify vulnerabilities, misconfigurations, and flaws that could lead to unauthorized access, data leaks, or service disruption. It includes ensuring the accuracy and durability of authentication, authorization, input validation, data exposure controls, and rate-limiting systems.

Security engineers should use both automated and manual techniques to analyze APIs in various environments and use cases. This ensures that APIs follow security rules, can handle unexpected scenarios, and keep data secure. Its goal is to protect APIs from attacks like injection, elevated privileges, and data theft.

Why is API Security Testing Important?

APIs connect directly with important systems and data, making them attractive targets for attackers. Effective API security testing helps to prevent data breaches and operational interruptions. Here's why API security testing is important:

Prevents Unauthorized Access

APIs help to check that authentication and authorization are applied in the right place. If access controls are incorrect or missing, attackers can imitate real users or get privileges. API security testing helps to identify vulnerabilities in token validation, session management, and role enforcement. This ensures that only authorized users can access and use data and systems. This prevents identity-based attacks and lateral movement.

Protects Sensitive Data

APIs exchange personal, financial, or confidential information of users and can unintentionally expose sensitive information if they are not properly checked or protected. Security testing helps to find excessive data exposure by response analysis and schema validation. It ensures that APIs only return what is needed for each operation, helps to reduce data leakage, and ensures regulatory compliance.

Identifies Business Logic Vulnerabilities

Attackers often exploit the intended workflows of APIs, which lead to logical defects, like skipping payment processes or changing transaction flows. In addition to technical validation, testing helps to analyze how people interact with the system. It detects weaknesses in patterns, workflows, and dependencies and helps to address these issues to prevent the exploitation of core business processes.

Ensures Strength Against Abuse

Rate limiting, throttling, and quota enforcement are essential for preventing abuse. Without these, APIs can be overloaded with requests, resulting in a denial-of-service or resource exhaustion. Security testing validates and protects against brute force attacks, attack replays, and misuse behaviors. It also examines whether APIs log and alert on abuse attempts and help to ensure service availability and integrity.

Supports Shift-Left Security

Using API testing early in the development lifecycle reduces remediation costs and effort. Security engineers can combine tests into CI/CD pipelines to detect weaknesses before they are executed. This links development with security requirements from the beginning. It promotes secure design and quick iteration cycles. Continuous testing reduces the possibility of releasing exploited APIs.

Types of API Security Testing

API security testing includes various methods that focus on different layers of an API's functionality and architecture. Each type focuses on specific risk areas to ensure full coverage.

Static Application Security Testing (SAST)

Source: backslash

SAST checks source code, configuration files, and API specifications without running the program. It helps security engineers identify vulnerabilities like encoded secrets, insecure configurations, and poor input sanitization. This method of testing is effective in the early phases of development. Security engineers use SAST to enforce secure code practices. It helps to prevent common vulnerabilities before they are implemented.

Dynamic Application Security Testing (DAST)

DAST checks operating APIs by sending fake queries and evaluating the answers. It helps to find runtime vulnerabilities like injection issues, authentication bypasses, and incorrect headers. DAST is language-agnostic and does not require code access. It imitates how hackers interact with APIs in normal conditions. This helps to expose risks that are missed during static analysis.

Fuzz Testing

Fuzz testing includes sending incorrect, unexpected, or random data to API endpoints to verify inputs. It detects crashes, exceptions, and logic failures that happen during abnormal activities. Fuzzing helps to identify edge cases that structured testing can miss. Security engineers should use fuzz testing to check how APIs handle input that is not verified, and help to improve API security and reduce attack surfaces.

Authentication and Authorization Testing

Authentication and authorization testing checks how APIs enforce identity verification and access restriction. It helps with token handling, session management, privilege enforcement, and role segregation checks. API security testing ensures that unauthorized users cannot access data or services that are not assigned to their roles. It ensures that security policies are applied in all endpoints, so if there are any errors in authentication or authorization, it can result in data breaches.

Rate Limiting

Rate restriction allows the implementation of usage policies like rate limitations, quotas, and throttling. APIs that lack these protections are vulnerable to brute force, credential stuffing, and denial-of-service attacks. Security engineers test how APIs handle high-volume or repetitive requests. They ensure that attack is recognized, blocked, and logged appropriately. This provides operational stability and threat resilience.

Penetration Testing

Penetration testing checks API security by imitating attacker behavior and identifying exploitable vulnerabilities. It uses both automated and manual scanning techniques to identify authentication weaknesses, access control issues, and logic vulnerabilities. It helps to evaluate the effectiveness of current defenses and identify security gaps.

How to Perform API Security Testing

API security testing involves a planned strategy that combines automated and manual tests, including API Scanning to detect vulnerabilities and ensure secure communication between systems. Here's how to conduct API security testing.

Discover All APIs

Start by identifying all active APIs, like internal, external, and undocumented (shadow) APIs. Use automated discovery tools or API gateways to collect traffic and build an inventory list. Make sure that outdated or legacy APIs are included. Updated inventory and accurate discovery are essential to perform effective testing and reduce blind spots.

Check API Documentation

Understand endpoint behavior by checking OpenAPI specs, Postman collections, or internal documentation. Documentation helps to find expected inputs, outputs, types of authentication, and data models. Identify discrepancies or undocumented parameters and check that schemas apply the right data types and restrictions. This ensures that test cases accurately represent actual API usage.

Validate Authentication and Access Controls

Check for missing or weak authentication mechanisms. Check access restrictions using different token types, expired sessions, and changing credentials. Enforce role-based access to ensure that users can only access permitted endpoints. Check for escalated privileges and IDOR (Insecure Direct Object Reference) vulnerabilities, because efficient access control is essential for data security.

Test Input Validation and Injection Points

Malicious payloads can be sent as parameters, headers, or request bodies. Security engineers should check for injection attacks like SQL, NoSQL, command, and LDAP injection. Check that input has been cleaned, validated, and logged correctly. To increase coverage, combine automatic scanners with tailored payloads. Secure input processing helps to protect against various common attacks.

Analyze Business Logic

Test API flows manually for broken processes, lost opportunities, and inconsistent state changes. Check if processes like payment, verification, or approvals are prevented or changed to challenge the assumptions of the API on client behavior. Logical errors are often identified by automated techniques that require a solid understanding of program behavior.

Report and Remediation

Document vulnerabilities, like severity, impact, reproduction steps, and solutions. Work with technical teams to prioritize fixes and test patched endpoints. Integrate security testing into CI/CD pipelines to ensure ongoing validation. Maintain the traceability of tests, findings, and repair efforts to promote long-term security maturity.

API Security Testing Methodology

A structured process ensures that API security testing is complete, consistent, and aligned with the organization's risk priorities. Here is the security testing methodology security engineers should follow:

API Inventory and Discovery

Start by identifying all APIs used in the organization, like internal, partner-facing, and undocumented shadow APIs. Use automated API discovery tools that combine with gateways, traffic logs, and code repositories. Maintain a live inventory to ensure that no endpoint is left untested. Accurate discovery will help to define the scope of effective testing.

Documentation Review

Analyze OpenAPI specs, internal docs, and interface contracts. Check the integrity of documented and performed behavior and find any undocumented parameters or endpoints. Analyze schemas to figure out field types, limitations, and data exposure.

Threat Modeling

Plan API processes, data flows, and integration points to detect possible attack vectors in the ecosystem. Map user roles, assets, security boundaries, and abuse scenarios to identify design flaws and insecure access channels. Analyze authentication flows, service usage, and data exposure across the API lifecycle. Prioritize risks based on their possibility and impact for performing targeted security testing.

Authentication Testing

Check how APIs verify identity, manage sessions, and remove credentials. Validate token formats, expiration management, token reuse, and misconfigurations. Ensure that session termination is reliable and secure. Ensure that OAuth, JWT, and API keys follow best practices.

Authorization

Verify role-based access, object-level access, and endpoint-specific limits and perform unauthorized behavior in valid and invalid user scenarios. Check IDOR, privilege escalation, and horizontal access issues. Enforce a clear boundary between user roles and resource scopes.

Reporting and Retesting

Document vulnerabilities like explanations, risk checks, and solutions. Retesting helps security engineers to maintain records for audit and follow compliance needs. They can add reports to security dashboards to offer continuous visibility. Security engineers should continuously bring improvement based on repeated testing and feedback loops.

Final Thoughts

APIs are an essential route in modern attack surfaces, requiring deep and ongoing security testing. Security engineers should add API testing to their CI/CD workflows and threat modeling activities. To solve logical and contextual issues, organizations will need an API security tool that is a combination of automation and human expertise.

Akto is an Agentic AI-powered API security platform that allows security engineers to perform continuous, automated API security testing at all stages of development. Akto connects easily with CI/CD pipelines and API gateways and offers various test cases that match the OWASP API Security Top 10 and configurable test flows. It provides continuous data and actionable insights that help to prevent security risks. Schedule a demo with Akto to learn how your organization can improve API security testing.