API Security Testing: How to test your API security

What is API security?

API security describes the collection of processes, practices, and products that prevent malicious attacks on or misuse of APIs.

In the past decade, APIs have become essential not only to modern software development but to modern software companies. Many applications make APIs central to their functionalities, such as social login, which uses an authentication API offered by the social network. Entire companies, such as communications service company Twilio and payment processors Stripe, are essentially providers of APIs as a service. 

4 Examples of common API security breaches

APIs are compelling and increasingly ubiquitous for the same reason API security is important: APIs enable seamless access to and usage of backend data and processes. A secure API can create immense business value, just as an insecure API can risk destroying that value. 

Various attacks target insecure APIs, such as:

• Man-in-the-middle attacks:, A man-in-the-middle attack occurs when an adversary intercepts and manipulates traffic between the API client and server. Without proper encryption and authentication, the attacker can eavesdrop, alter data in transit, or inject malicious payloads, often leading to data theft, session hijacking, or financial fraud.

• Injection attacks: Injection vulnerabilities occur when APIs fail to properly validate or sanitize user-supplied input, allowing attackers to insert malicious code into backend systems. Common variants include SQL injection, command injection, and NoSQL injection, each exploiting weak input handling to manipulate database queries or system commands. Once executed, these attacks can lead to data exfiltration, data corruption, or full system compromise. To mitigate injection risks, all input—whether from parameters, headers, or payloads—should undergo rigorous validation and sanitization. Developers should also use parameterized queries or prepared statements when interacting with databases, and enforce strict schema validation for JSON or XML inputs. Incorporating automated security testing tools into CI/CD pipelines can help detect injection vulnerabilities before deployment.

• and DdDistributed denial of service (DDoS) attacks:. In a denial-of-service attack, an adversary overwhelms an API with excessive requests, consuming system resources until legitimate users experience service degradation or complete downtime. Distributed versions of these attacks (DDoS) can involve thousands of coordinated clients or bots, amplifying the volume of malicious traffic. APIs without proper rate limiting, throttling, or quota management are particularly vulnerable. Effective mitigation requires using API gateways or load balancers with built-in rate control and anomaly detection.

• Brute-force attacks:. A brute-force attack involves systematically and programmatically guessing authentication credentials—such as passwords, API keys, or tokens—to gain unauthorized access. Attackers often automate these attempts using scripts or botnets to accelerate credential guessing across large numbers of accounts or endpoints.

Given the number of possible attacks, API security needs to weave together a variety of security measures, but API security testing is chief among them.

Six things to keep in mind when performing API security tests

As with other security methods, the difference between API security testing done superficially and done effectively can be significant. If you keep the following six things in mind, you can better ensure your API security tests will be as effective as possible:

1. Deployment: How are you deploying your tests? Are they running in your CI/CD pipelines?

2. Configuration: Are your tests relying on HTML spiders? Or are your tests crawling your application to identify API routes? 

3. Support: Are the APIs you run supported by the test suite you’ve adopted?

4. Accuracy: Are your tests finding actual vulnerabilities? Are your tests minimizing false positives?

5. Customization: Can your tests be customized to suit specific use cases and situations that emerge over time?

6. Developer experience: Are your tests easy to use? Do they fit well into already existing developer workflows?

With these guiding questions, you can ensure your API security testing process is effective and intuitive. 

Why is API security testing important?

API security testing is important because the more often you test, and the better you test, the more likely you are to prevent API-based attacks. These kinds of attacks are especially dangerous because APIs grant significant levels of access to often sensitive data – access that is beneficial for legitimate users and dangerous for malicious users. 

Regular API testing not only supports compliance with frameworks such as OWASP API Security Top 10, PCI DSS, and GDPR but also strengthens an organization’s overall security posture by ensuring APIs maintain confidentiality, integrity, and availability as they evolve through development and deployment cycles.

API security testing & DevSecOps

Effective API security testing is not a one-time effort—it’s an ongoing practice embedded throughout the API lifecycle. By integrating automated testing, manual penetration assessments, and continuous monitoring into a DevSecOps workflow, organizations can enforce consistent security standards, quickly remediate vulnerabilities, and maintain visibility into API risks. This continuous approach ensures that as APIs evolve, security remains an integral part of development, deployment, and operational governance—reducing the attack surface while supporting rapid innovation.

What aAre the types of API security tests?

API security testing comes in different forms, each best suited for different contexts and different types of security risks.

• Dynamic API security tests (DAST): Dynamic API security tests examine running APIs for security vulnerabilities and exploitable bugs. DAST tests take an “outside-in” approach by simulating attacks that a malicious attacker would make. It does not require access to the application's source code, which makes it a black box testing method.

• Static API security tests (SAST): Static API security tests examine source code to help developers find insecure coding patterns. 

• API fuzzing: This type of security testing automatically creates and inputs random, invalid, and otherwise unexpected data into an API so developers can see how and if the API crashes or if bugs emerge. 

• Pen testing: By simulating attacks on an API from internal or external sources and trying to exploit any vulnerabilities, security teams can identify and mitigate potential vulnerabilities in the API.

• API performance testing: Performance testing involves measuring how the API works under different load and stress conditions to determine how it handles scalability, concurrency, and availability issues. This helps security teams ensure that the API is still secure enough to cope with peak traffic and meet specified performance requirements.

• RASP: Runtime application self-protection (RASP) is not a testing tool but can help monitor the API when it’s running to detect attacks in real time by analyzing its behavior and blocking suspicious requests.

• WAF: A web application firewall is another tool that can help API security by blocking malicious web traffic before it reaches the API server.

Many API security testing tools are available, many of them open source. A few of the best open source tools include Apache JMeter, which developers can use for load and performance testing, Astra, which developers can use to create API tests easily, and Karate, which developers can use to script calls to API endpoints. 

Together, SAST, DAST, and manual penetration testing provide a layered approach—ensuring that both the code-level security and the runtime behavior of APIs are tested comprehensively. This multi-method strategy reduces the likelihood of exploitable vulnerabilities escaping into production and supports continuous improvement of an organization’s API security posture. 

In addition, fuzz testing is used to uncover unexpected behavior under malformed or random inputs. Additional checks may include validating rate limiting, encryption enforcement, and schema compliance to prevent common attack vectors like injection, replay, or parameter tampering.

4 most common API security risks

Though even a good API security testing regimen isn’t foolproof, effective testing can prevent or at least reduce the likelihood of numerous attacks, including:

• Unauthorized access: API security testing can ensure that only permitted users have access to the API. Three ways to do this are role-based access control, resource-based access control, and field-level access control.

• Data leakage: API security testing can prevent the leakage of sensitive data by ensuring malicious actors either don’t have access to an API they shouldn’t have access to or can’t use an API in an unintended way. 

• Injection vulnerabilities: API security testing can prevent numerous kinds of injection vulnerabilities, including SQL injection, OS command injection, and XML injection. Broadly, injections are performed by what appear to be user data but are actually manipulative commands. 

• Parameter tampering: API security testing can prevent parameter tampering, an attack that uses API requests to exploit backend validation errors. 

This is not an exhaustive list, so it’s worth checking out additional resources, such as OWASP’s API security project.

Benefits of using API security testing

API security testing provides proactive vulnerability detection, improved compliance, and enhanced trust. By integrating security testing into the development lifecycle, organizations can identify and remediate vulnerabilities before they reach production, reducing the risk of costly breaches. Strong API security testing also helps you meet regulatory requirements, protect sensitive data, and build customer confidence in your applications.

5 best practices for API security Testing

Security testing is most effective when it’s proactive, continuous, and comprehensive. The following best practices help ensure your API testing strategy addresses both code-level vulnerabilities and real-world threat scenarios.

1. Integrate security testing early and continuously (“shift left”)

Security should be built into the development pipeline, not bolted on afterward. Integrating static (SAST) and dynamic (DAST) security testing tools within your CI/CD workflows allows teams to detect misconfigurations, insecure dependencies, and logic flaws before APIs are released. This “shift left” approach reduces remediation costs, shortens development cycles, and ensures every build is validated against organizational security standards.

2.Test for business logic and authorization flaws

Automated scanners excel at identifying technical vulnerabilities, but they often miss contextual or business logic issues—such as improper access control, role confusion, or multi-step privilege escalation. Conduct manual exploratory testing and scenario-based assessments to evaluate how APIs enforce authorization and role-based restrictions. 

Mapping user roles to access scopes and verifying endpoint isolation are critical steps to prevent broken object-level authorization (BOLA) and related OWASP API Top 10 risks.

3. Simulate real-world attack scenarios

API testing should include fuzzing and penetration testing to simulate actual attack conditions. Fuzzing sends malformed or random data to endpoints to uncover unexpected crashes or security exceptions, while penetration testing replicates adversarial behavior to identify chained vulnerabilities. These tests reveal how an API behaves under stress and validate that rate limiting, authentication, and input handling mechanisms can withstand real-world exploitation attempts.

4. Validate authentication and session management

Authentication and session handling are among the most frequently targeted components in API attacks. Security tests should verify the robustness of token-based authentication (e.g., OAuth 2.0, JWT), session expiration policies, and revocation mechanisms. Tests should also confirm that credentials are not exposed in URLs, logs, or response headers, and that replay attacks are mitigated using nonce values or timestamps. Ensuring strict session management prevents unauthorized access even if credentials are compromised.

5. Continuously monitor and retest after changes

APIs evolve constantly—new versions, integrations, and dependencies can introduce fresh vulnerabilities. Continuous monitoring and post-deployment testing are essential to maintain visibility into the API’s security posture. Implement runtime application self-protection (RASP) or API security gateways to detect anomalies in live traffic, and retest after every major code or configuration change. A “test once, monitor always” mindset ensures that security keeps pace with the speed of modern API development.

API Security and Application Security

In the past, API security testing was often a series of tools supported but mostly manual processes. Security teams would perform penetration testing or scan their APIs for vulnerabilities by hand. Eventually, security teams would be able to pass on any vulnerabilities they found to engineering teams, who could then fix them. The process was slow and error-prone—neither of which are defects that companies should allow in a security context.

Increasingly, especially given the rise of DevSecOps, companies are integrating API security tests into the development and operations pipelines. As with other security processes, companies are looking to shift left to test for vulnerabilities earlier in the software development lifecycle and catch potential security issues earlier. The earlier companies can catch issues, the sooner they can fix them – meaning breaches are less likely and fixes are cheaper. 

API security testing consists of various tests, but the general goal is to examine your APIs and match them against known vulnerabilities – either known insecure coding practices or dependencies with known vulnerabilities

Why API security differs from traditional application security

APIs extend beyond traditional applications by exposing critical data and functionality across systems, platforms, and third-party services—creating a broader and more dynamic attack surface. Unlike web interfaces, APIs are machine-consumable, making them prime targets for automated attacks like brute-force, fuzzing, and injection.

Modern APIs often orchestrate multiple microservices and AI-driven components, which can amplify the impact of a single vulnerability. Flaws in business logic, authorization, or data handling can cascade across systems, potentially compromising sensitive AI models, training datasets, or automated workflows. Continuous testing, real-time monitoring, and AI-enhanced threat detection are therefore essential to maintain API integrity and prevent exploitation in fast-evolving, intelligent application environments.

The status of API Security

What does SpaceX Inc. have in common with Alphabet CEO Sundar Pichai? Among 200 million others, both had their data captured and sold as part of a massive data leak from social media company Twitter. Now, Twitter is subject to an inquiry from Ireland’s privacy regulator as well as being the latest case study in API security. They’re not the only ones. US Senate and House members and 170,000 others have also been affected by the recent hack of DC Health Link. 

Despite the size of these leaks, API security issues are not uncommon, and API security testing appears to be an industry-wide weakness. Salt Security research reveals some disturbing results:

• 94% of companies have experienced API security issues in production APIs.

• 47% of companies have found vulnerabilities in production APIs.

• Malicious API attack traffic grew 117% through last year (July 2021 to July 2022), from an average of 12.22 million malicious calls per month to an average of 26.46 million.

Worse still, almost one-fifth of companies have experienced a data breach resulting from poor API security testing and practices.

Companies need to rethink API security from the basics to the best practices. Snyk's application security solution can help you test your APIs with vulnerability scanning and continuous monitoring to ensure peace of mind.

Snyk API security testing tools and solutions

One of the primary reasons API security flaws are on the rise, despite widespread recognition of the issue, is that many companies struggle to identify which teams are in charge of the company’s APIs. The very centrality of APIs means that they tend to sit at the intersection of numerous teams and endure rapid iteration as the ways these teams use the APIs evolve. 

The result is that APIs can remain insecure despite a company’s or a team’s best intentions. Forward-thinking companies are fighting against this tendency by empowering developers to take ownership of an application’s security. A developer-first approach – complemented by developer-first tooling and guidance – is essential to building more secure APIs. 

At Snyk, we believe in building tools that help companies empower developers and helping those developers use those tools to the greatest effect. With Snyk, developers can tune security automation functions to their specific workflows, ensuring consistency across:

• Source code via Snyk Code.

• Security configurations via Snyk IaC.

• Package security issues via the Snyk API.

• Running applications via DAST.

To learn more about our API security product vision and the benefits of using the Snyk platform, read about extensibility and the Snyk API.