API Security Testing Checklist for Developers

APIs are the backbone of modern applications, facilitating seamless communication and data exchange. However, this ubiquity makes them prime targets for cyberattacks. As developers, building robust and secure APIs isn't just a best practice; it's a critical responsibility. 

This blog post provides a comprehensive API security testing checklist to help you identify and mitigate API vulnerabilities, ensuring your APIs are fortified against evolving threats.

What is API security testing?

API security testing involves analyzing your application programming interfaces (APIs) for vulnerabilities, misconfigurations, and weaknesses that could be exploited by attackers.

Unlike general application security testing, it’s about understanding how your APIs handle identity, data, and trust—then stress-testing those assumptions under real-world conditions.

Unlike traditional appsec, which often focuses on the surface of web or mobile interfaces, API security testing dives into the connective tissue: how data flows, how permissions are enforced, and how business logic stands up to creative abuse. API testing ensures that only authorized users and applications can access sensitive data and perform permitted actions.

When should you perform API security testing?

All the time. API testing should be carried out during every sprint, every release, and every integration because your app’s attack surface is never static.

That is to say, API security testing should be integrated throughout the software development lifecycle (SDLC)—from initial development and staging to production deployments and ongoing updates. Regular testing is crucial because APIs are constantly evolving, and new vulnerabilities can emerge with each change or integration.

Why API security testing is non-negotiable

APIs are a prime target for attackers because they often handle sensitive data and critical business operations. Testing helps prevent data breaches, service disruptions, financial losses, reputational damage, and regulatory penalties by identifying and fixing security gaps before they can be exploited.

The headlines are full of examples:

Optus data breach (2022)

A massive data breach affecting nearly 10 million customers in Australia was attributed to a reportedly misconfigured and publicly exposed API that lacked authentication. This allowed an attacker to query customer records directly, highlighting the critical failure in authentication and security misconfiguration.

T-Mobile data breaches (Multiple)

T-Mobile has experienced multiple breaches, some of which involved API vulnerabilities. In one instance, an attacker gained access to customer data (including names, addresses, and phone numbers) by exploiting an API that was not properly secured. This underscores the importance of authentication and authorization in preventing unauthorized access.

Capital One data breach (2019)

This breach, impacting over 100 million customers, involved a misconfigured web application firewall (WAF) that was vulnerable to a Server-Side Request Forgery (SSRF) attack. The WAF had excessive permissions, allowing the attacker to access sensitive data stored in S3 buckets. This case dramatically illustrates the dangers of broken access control (specifically, the principle of least privilege) and security misconfiguration.

What are the top API security risks?

Here are the most critical API vulnerabilities, based on the OWASP API Top 10:

Broken object-level authorization

Imagine a user changing a single digit in an API call and suddenly accessing another customer’s data. These happen more often than you think. Attackers manipulate object IDs to access data they shouldn’t, due to missing or weak access controls.

Broken authentication

Insecure or improperly implemented authentication lets attackers impersonate users or access protected resources. 

Broken object property-level authorization

Sensitive properties are exposed or modifiable due to insufficient property-level access controls.

Unrestricted resource consumption

APIs that don’t limit requests can be weaponized for denial-of-service or to rack up cloud bills.

Broken function-level authorization

Privileged operations, when left unguarded, enable regular users to perform admin actions. This results in escalation, as APIs fail to restrict access to sensitive functions.

Unrestricted access to sensitive business flows

Critical business logics and operations are left unprotected, allowing attackers to disrupt or abuse workflows.

Server-Side Request Forgery (SSRF)

APIs fetch remote resources without proper validation, allowing attackers to pivot deeper into your infrastructure, ultimately leading to unauthorized requests.

Security misconfiguration

Default settings, verbose error messages, or unnecessary features give attackers an easy way in.

Improper inventory management

Shadow and outdated APIs lead to breaches as they are endpoints that linger undocumented and unprotected, waiting to be discovered..

Unsafe consumption of APIs

Trusting data from third-party APIs without validation can introduce vulnerabilities into your system.

Suggested read: The Ultimate Guide to OWASP API Top 10

Best practices to secure your APIs: Comprehensive API testing checklist for developers

1. Authentication and Authorization

Authentication is the first line of defense, verifying the identity of the user or application accessing your API, while authorization determines what resources they can access and what actions they can perform.

Implement strong authentication mechanisms

• • OAuth 2.0 & OpenID Connect: Opt for these industry-standard protocols for delegated authorization and authentication.
  • API keys: If using API keys, ensure they are treated as secrets, rotated regularly, and have restricted permissions. Avoid embedding them directly in client-side code.
  • Multi-Factor Authentication (MFA): For sensitive operations, consider implementing MFA for user accounts accessing your API.

Secure credential handling

• • Never transmit credentials in URL parameters. Use request bodies or HTTP headers.
  • Hash and salt passwords: Store user passwords securely using strong hashing algorithms (e.g., bcrypt, scrypt) and unique salts.

Implement access control

• • Role-Based Access Control (RBAC): Assign roles to users (e.g., admin, user, guest) and define permissions based on these roles.
  • Attribute-Based Access Control (ABAC): For more complex scenarios, use attributes (e.g., department, location) to define access policies.

Check authorization on every request

• • Don't trust client-side authorization checks. Always validate authorization on the server side for every API endpoint.
  • Prevent Insecure Direct Object References (IDOR): Ensure users cannot access or manipulate resources they are not authorized to by simply changing an ID in the URL or request body. For example, if a user can access api/orders/123, ensure they cannot access api/orders/124 if 124 belongs to another user.

Real-world impact

Optus data breach (2022): The Optus breach was a stark reminder of what happens when authentication is completely bypassed. A public-facing API endpoint lacked any authentication, allowing anyone who discovered it to query and extract customer data directly. 

Had proper authentication (such as API keys or OAuth) and subsequent authorization checks been enforced, this massive data leak could have been prevented. This highlights a critical failure in both authentication and the absence of any authorization to restrict data access.

[Ref: https://www.upguard.com/blog/how-did-the-optus-data-breach-happen]

2. Input validation: Trust no input

Malicious input is a common attack vector. Thoroughly validate all incoming data to prevent injection attacks and unexpected behavior.

Validate all input

• • Data types: Ensure input matches expected data types (e.g., integer, string, boolean).
    
  • Length constraints: Enforce minimum and maximum length limits for strings.
  • Format validation: Validate formats such as email addresses, phone numbers, and dates using regular expressions or specialized validators.
  • Whitelisting vs. blacklisting: Prefer whitelisting (allowing only known good input) over blacklisting (trying to block known bad input), which is often incomplete.

Prevent injection attacks

• • SQL injection: Use parameterized queries or ORMs to prevent SQL injection.
    
  • NoSQL injection: Sanitize input for NoSQL databases.
  • Command injection: Avoid executing user-supplied input as system commands.
  • Cross-Site Scripting (XSS): Sanitize output before rendering it in client-side applications to prevent XSS.

Real-world impact

Equifax data breach (2017 - general web application vulnerability): While not purely an API breach, the Equifax breach was famously caused by an Apache Struts vulnerability that allowed for arbitrary code execution due to improper input validation. This led to the exposure of sensitive personal information of millions of customers. 

The data breach directly illustrates how neglecting robust input validation can open doors to severe injection attacks and remote code execution, impacting APIs that process user-supplied data. A seemingly innocuous input field can become a critical vulnerability if not rigorously validated.

3. Rate limiting: Preventing abuse

Rate limiting protects your API from abuse, denial-of-service (DoS) attacks, and brute-force attempts.

Implement appropriate rate limits

• • Set limits per IP address, user, or API key to control the number of requests within a given timeframe.
    
  • Implement burst limits to handle temporary spikes in traffic while preventing sustained abuse.
  • Return clear error messages (e.g., HTTP 429 Too Many Requests) when limits are exceeded.

Consider different limits for different endpoints

• • More sensitive or resource-intensive endpoints might require stricter rate limits.

Real-world impact

Brute-force attacks on login APIs: Many account takeover attempts leverage brute-force attacks against API login endpoints. Without rate limiting, attackers can try thousands or millions of password combinations in a short period. 

For example, if a banking API didn't rate limit login attempts, an attacker could continuously try passwords until they guessed a valid one, leading to account compromise. The T-Mobile breach, while complex, involved attackers using brute-force techniques to gain access to internal systems after an initial entry into testing environments, highlighting the importance of limiting attempts across all access points.

[Ref: https://www.strongdm.com/what-is/t-mobile-data-breach]

4. Misconfiguration

Misconfiguration refers to insecure default configurations, incomplete or unpatched systems, open cloud storage, unnecessary features, and other configuration mistakes that leave APIs vulnerable.

Secure default configurations

• • Always change default credentials and settings.
  • Disable unnecessary services, ports, and features.

Patch management

• • Regularly update all components (operating systems, libraries, frameworks, API gateways) to the latest secure versions.

Cloud security

• • Properly configure cloud storage buckets (e.g., S3, Azure Blob Storage) to prevent public access unless explicitly required.
  • Implement strict network security groups and firewalls to ensure optimal security.

Error handling

• • Return generic error messages instead of detailed ones that could reveal internal system information (e.g., stack traces, database schema details).
  • Log detailed errors internally, but do not expose them to the client.

Real-world impact

Capital One data breach (2019): This breach, impacting over 100 million customers, was largely attributed to a misconfigured web application firewall (WAF). The WAF had overly permissive access to Amazon S3 buckets, allowing the attacker to retrieve sensitive customer data by exploiting a Server-Side Request Forgery (SSRF) vulnerability. 

This is a classic example of how a seemingly minor configuration error or oversight in permission management can lead to catastrophic data loss. Similarly, the Optus Data Breach also involved a misconfigured API endpoint that was publicly accessible without authentication.

5. Data in transit and at rest: Encryption is key

Ensuring data is encrypted both when it's moving between systems and when it's stored is fundamental to API security.

Encrypt data in transit (HTTPS/TLS)

• • Always use HTTPS/TLS for all API communication. This encrypts data between the client and server, preventing eavesdropping and tampering.
  • Enforce strict TLS configurations (e.g., TLS 1.2 or higher, strong ciphers) and HSTS (HTTP Strict Transport Security).

Encrypt data at rest

• • Encrypt sensitive data stored in databases (e.g., PII, financial data) using disk encryption, database encryption features, or application-level encryption.
  • Securely manage encryption keys.

Real-world impact

Man-in-the-Middle (MITM) attacks on unencrypted APIs / data exposure from unencrypted databases: If an API transmits sensitive data (like credentials or personal information) over plain HTTP (without TLS/SSL), it's vulnerable to a Man-in-the-Middle (MITM) attack. An attacker on the same network can intercept the unencrypted traffic, steal the data, and even modify it before it reaches the server. 

While high-profile breaches often involve more sophisticated attacks, a lack of HTTPS is a foundational flaw that still exists in some systems. Similarly, if sensitive data in a database is not encrypted at rest, a breach of the database (even if the API is secure) can directly expose all customer data, as seen in many incidents where unencrypted data was stolen.

6. Security testing

Proactive and continuous security testing is paramount to identify vulnerabilities before attackers do.

Comprehensive testing strategy

• • Static Application Security Testing (SAST): Analyze source code for security flaws during development.
  • Dynamic Application Security Testing (DAST): Test the running API by simulating attacks (e.g., OWASP ZAP, Burp Suite).
  • API penetration testing: Engages security experts to manually and automatically probe your API for vulnerabilities, simulating real-world attacker techniques.
  • Fuzz testing: Send malformed or unexpected inputs to uncover crashes or vulnerabilities.

Regular audits and reviews

• • Periodically review your API security configurations, access controls, and code for potential weaknesses.
  • Stay updated on the latest OWASP API Security Checklist and common attack vectors.

Real-world impact

Undiscovered vulnerabilities leading to breaches (e.g., Facebook API Bugs): Many companies, including major tech giants like Facebook, have faced incidents due to undiscovered API vulnerabilities that were later exploited. 

For instance, in 2018, a bug in Facebook's "View As" feature allowed attackers to steal access tokens for nearly 50 million accounts. This wasn't a "misconfiguration" in the traditional sense, but rather a complex flaw that security testing should ideally have uncovered. 

Consistent, thorough security testing, including both automated scans and manual penetration testing, is crucial to find these subtle yet critical flaws before they are exploited by malicious actors.

Automating API security testing with Appknox

Manual API security testing can’t keep pace with today’s rapid DevOps cycles. Every now and then, teams struggle to catch every vulnerability before release, resulting in costly delays or, worse, breaches.

Appknox changes this by automating API security testing right inside your CI/CD pipeline. Every pull request triggers a deep scan that hunts for authentication flaws, injection points, and business logic weaknesses long before code hits production.

But what really sets Appknox apart?

It runs automated tests on real devices, simulating real attacks with precision and delivering results with less than 1% false positives. This means your developers spend time fixing real issues, not chasing noise.

The platform also provides clear, prioritized remediation guidance tailored for developers, helping teams close vulnerabilities faster and smarter. Compliance mapping to OWASP API Top 10, PCI-DSS, GDPR, and other standards provides security and compliance teams with confidence without additional overhead.

Case in point: A global retail giant reduced its average remediation time from two weeks to just a few hours after integrating Appknox into its workflow. 

That’s the power of shifting security left: catching issues early, reducing manual effort, and accelerating secure releases.

Suggested read: See how Appknox helped this iconic British retail giant transform their app security by reducing vulnerability detection time by 70% and simplifying compliance audits by 80%.

Read more

Key benefits of Appknox

• Seamless CI/CD integration with automated scans on every build
• Uncover hidden and shadow APIs to eliminate blind spots
• Comprehensive coverage of OWASP API Top 10 and misconfigurations
• Developer-friendly, actionable remediation reports
• Minimal false positives to keep teams focused

Appknox doesn’t just automate testing—it transforms security from a bottleneck into a growth enabler.

Detect vulnerabilities in minutes with deep, automated scans.

Start your free trial with Appknox today and escape security blind spots in your application portfolio.

Try Appknox for free

API security best practices: At a glance

Key takeaways for developers

• Authenticate every request.
• Validate all input—never trust the client.
• Limit and monitor API usage.
• Harden configs and encrypt everything.
• Test, test, and test again—automate where possible.

Shift to a proactive API security approach from the get-go

API security is an ongoing process, not a one-time task. 

By diligently following this API security testing checklist and embracing API security best practices, you can significantly reduce the attack surface of your APIs, protect your data, and build trust with your users. 

Regular testing, continuous monitoring, and staying updated on the latest security threats are paramount in this dynamic landscape. Make security an integral part of your development lifecycle, and you'll be well on your way to building robust and resilient APIs.

Frequently Asked Questions (FAQs)

1. How often should you test your APIs?

APIs should be tested:

• At every major release, 
• After significant code changes, and
• Ideally, as part of every build or pull request in your CI/CD pipeline. 

Many organizations also conduct periodic penetration testing; however, continuous automated testing is recommended to stay ahead of evolving threats.

2. What’s the difference between manual and automated API security testing?

Manual API security testing involves human testers exploring APIs, using their judgment to find complex or business logic flaws. It is flexible but time-consuming and can miss routine issues. 

Automated API security testing utilizes tools to execute predefined tests quickly and consistently, providing comprehensive coverage and prompt feedback. The best approach combines both: automation for speed and coverage, manual penetration testing for nuanced, context-driven vulnerabilities.

3. What are the most effective tools for API security testing in mobile apps?

The top free mobile API testing tool is OWASP ZAP. For enterprise-grade mobile app testing, you can opt for Appknox, NowSecure, Checkmarx, etc. These tools assist with SAST, DAST, API testing, SBOM, automated scanning, and other related tasks.

To learn more about the top 10 API security testing tools, check out this blog.