In this post, we’ll cover the key indicators of a secure API, outline effective testing methods, and provide a structured checklist for security assessment. We’ll also answer frequently asked questions to help you approach API security with clarity and confidence.

Why securing APIs is critical

APIs are the backbone of modern applications, connecting mobile apps, web front ends, third-party integrations, and cloud services. But that connectivity comes with risk. If an API is left exposed or improperly secured, it can become a direct entry point for attackers. Consequences of insecure APIs may include:

• Data breaches: Sensitive user data, financial records, or personal information can be accessed or exfiltrated.
• Privilege escalation: Attackers can abuse flaws to impersonate users or access admin-level functionality.
• Service disruption: Poorly protected APIs can be exploited to perform denial-of-service attacks or trigger cascading failures.
• Compliance violations: Unsecured APIs can lead to non-compliance with standards like GDPR, HIPAA, or PCI-DSS, resulting in fines or legal consequences.

Because APIs often operate behind the scenes, their vulnerabilities can remain hidden, making them a favorite target for attackers. That’s why securing APIs must be a core part of your application security strategy.

API security checklist

While no system is ever perfectly secure, well-designed APIs share common features that reduce exposure and enforce proper access boundaries.

Require authentication

A secure API nearly always requires authentication, even for read-only endpoints (unless you’re deliberately running a public service). Any API that allows unauthorized access to sensitive data or business logic should be a clear red flag. Authentication helps ensure that only authorized users or services can access API functionality. Common methods include OAuth 2.0, JWTs (JSON Web Tokens), and API keys with scoped permissions. Whenever tokens or keys are used, they require secure storage and management.

Use HTTPS only

Transporting traffic over HTTPS with TLS (Transport Layer Security) is a baseline security requirement for APIs and applications. Without HTTPS, sensitive data, potentially including credentials, tokens, and user input, could be intercepted in transit and compromised. Wherever feasible, secure APIs should use HSTS to enforce HTTPS at all endpoints and reject non-encrypted requests outright to eliminate protocol downgrade risks.

Implement rate limiting

Rate limiting is an essential runtime defense against brute force attacks, credential stuffing, and abuse of API functionality. Secure APIs should define request thresholds per user or token and apply exponential backoff or lockout policies when limits are exceeded. This helps to maintain service availability and protect against denial-of-service (DoS) attacks.

Maintain audit logs

A secure API ecosystem logs authentication attempts, resource access, and other relevant events. Audit logs help detect suspicious behavior and provide a forensic trail in case of a breach. For sensitive APIs, logs should be immutable, timestamped, and centrally stored with proper access controls.

How to test your API security

Even if you’re following secure coding practices to minimize the risk of API vulnerabilities and using runtime protection to mitigate attack attempts, testing is the only way to really validate whether API security mechanisms are actually working as intended. API security testing should be continuous and layered to catch both common vulnerabilities and nuanced design flaws.

Use automated API security testing tools

By their very nature, APIs act as abstraction layers that provide unified access to backend systems and applications, which makes dynamic testing the go-to security testing approach. To make testing an inherent part of API and application development, start by incorporating dynamic application security testing (DAST) tools into your DevOps pipeline. 

Advanced DAST tools like Invicti can scan live APIs in runtime environments and realistically simulate attacker behaviors against APIs and their underlying systems, including attempts at authenticated API abuse. Invicti supports OpenAPI/Swagger definitions, several authentication methods (including OAuth), custom headers, and token-based workflows, making it suitable even for complex enterprise environments at scale.

Perform manual penetration testing

Penetration testing goes deeper than automated scanning to simulate real-world attacks and discover logic flaws. A skilled pen tester can uncover issues such as insecure direct object references (IDOR), broken function-level authorization, or privilege escalation paths that automated tools cannot detect. This is especially important for APIs, where roles, permissions, and business logic from several systems can overlap and interact in unexpected ways, sometimes leading to complex vulnerabilities that require human expertise and creativity to figure out and exploit. 

Review access controls

Every API endpoint should be mapped to an access control rule. Check your rules to ensure that:

• Only authenticated users can access protected endpoints
• Users can only perform actions allowed by their role or scope
• Sensitive operations are protected against CSRF and replay attacks
• Access decisions are enforced server-side, not in the client

Access control testing should include both positive and negative test cases—attempting actions as users with different roles, or without credentials, to see how the API responds.

Security assessment checkpoints

When evaluating API security, a consistent checklist helps ensure no layer is overlooked.

Transport Layer Security

• Enforce HTTPS on all endpoints
• Disable support for deprecated TLS versions (anything older than 1.2)
• Use strong cipher suites and valid certificates
• Implement HSTS (HTTP Strict Transport Security) to prevent protocol downgrade attacks

Data validation layers

• Sanitize input on the server side (never trust the client)
• Use strict content-type checks (for example, enforce application/json when only JSON is expected)
• Validate parameter formats, lengths, and data types
• Use schema validation (like JSON Schema) where possible

Error message obfuscation

• Avoid detailed error messages that reveal internal structures (e.g., stack traces, DB schema)
• Use generic error responses (e.g., 401 Unauthorized, 403 Forbidden) without revealing endpoint logic
• Monitor for verbose logging or debug modes accidentally exposed in production

Final thoughts

Knowing whether your API is secure means looking beyond surface-level indicators and validating real-world behavior through comprehensive testing. Starting with secure design, proper authentication, transport security, access controls, and auditing in place, you can then add regular DAST scanning and manual testing to significantly reduce the risk of compromise.

Legacy tooling limitations have long seen APIs treated as a separate security concern, but modern DAST-first application security solutions have made API discovery and vulnerability scanning an integral part of securing the entire application attack surface. As the leader in DAST-first application and API security, Invicti brings discovery, testing, and vulnerability management together on a single platform that covers multiple testing approaches and integrates into your existing development and AppSec workflows—with proof-based scanning at the core, so your teams can focus on exploitable vulnerabilities and reduce risk quickly.

Learn how Invicti can help you keep your APIs secure