API Security Checklist: What You Need To Know

In the past few years, APIs have quietly become the primary target for cyberattacks. IBM Security reports that over 70% of all cloud breaches in 2024 involved vulnerable or exposed APIs. The Optus data breach, T-Mobile’s repeated leaks, and several fintech compromises all had one common root cause: misconfigured or forgotten API endpoints.

Every modern business now depends on APIs. They power mobile apps, integrations, partner ecosystems, and even internal systems. But as companies scale, visibility often lags behind growth. Many organizations have hundreds of active APIs, some untracked, others outdated, and many lacking consistent authentication.

The result is a sprawling attack surface that traditional firewalls and vulnerability scans can’t fully cover. That’s why this guide brings together the latest OWASP API Security Top 10 best practices, real-world case studies, and automation insights from APIsec.ai.

You’ll find eight actionable sections that form a complete framework from automated discovery to business logic validation and continuous testing, all designed to help you detect risks before attackers do.

1. Start with Visibility: Automated API Discovery

The biggest risk to any organization isn’t a known vulnerability, but it’s an API you didn’t even know existed. Most security teams underestimate their actual API count by more than 40%. Between staging builds, microservices, and legacy systems, untracked APIs multiply quickly.

Automated API discovery tools such as APIsec and Pynt continuously scan repositories, gateways, and network logs to map every live endpoint. They create an accurate API inventory, tagging each by purpose, owner, and sensitivity.

This process isn’t a one-time scan. Discovery should run continuously, especially after code merges or infrastructure updates. By integrating it directly into CI/CD workflows, new APIs are automatically cataloged the moment they go live.

A complete API inventory becomes your foundation for every security control that follows authentication, rate limiting, and vulnerability testing, and all depend on knowing exactly what’s running.

To understand how visibility drives prevention, the post Why APIs Are Your Biggest Security Risk dives deeper into real-world incidents.

2. Eliminate Hidden Threats: Shadow and Zombie APIs

Every organization has its shadow and zombie APIs quietly operating in the background. Shadow APIs are built outside governance, often for testing or short-term use. Zombie APIs are retired versions that never got fully decommissioned but still respond to requests.

Both create a perfect backdoor for attackers. In one major retail breach, a deprecated API used during a 2019 campaign was still accessible years later and leaked customer data from new systems.

The solution is to treat APIs as living assets that move through a defined lifecycle: design → build → release → deprecate → retire. Automated tools should compare active network traffic against your documented APIs to highlight endpoints that don’t belong.

Once identified:

• Disable unused or deprecated endpoints immediately.
• Route legacy APIs through an authentication proxy until decommissioned.
• Configure alerts for requests targeting outdated paths.
• Maintain a single source of truth for your API catalog.

When continuous discovery is paired with disciplined API retirement, the concept of “zombie endpoints” disappears entirely.

3. Secure Access: Authentication and Authorization the Modern Way

Weak authentication remains the most exploited API vulnerability in the world. Attackers rarely “hack” into systems they log in to using stolen or unprotected credentials.

Modern APIs should adopt OAuth2.1 or OpenID Connect to control user and app access. Tokens should be short-lived, ideally expiring every 10 to 15 minutes, and signed using asymmetric algorithms like RS256 or ES256.

Key best practices:

• Validate JWT claims issuer, audience, and expiration at every request.
• Rotate API keys and signing certificates regularly.
• Store credentials in secret vaults, never hard-coded.
• Implement scope-based permissions that limit what each token can access.

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This setup prevents common OWASP issues like Broken Object Level Authorization (API1) and Excessive Data Exposure (API3). The APIsec resource What Is OAuth and Why It Matters explains these frameworks in greater detail.

4. Encrypt Everything: TLS and mTLS in Practice

Encryption isn’t optional anymore; it’s the backbone of secure communication. Every API call should happen over HTTPS (TLS 1.3) to protect data in transit from interception.

For high-stakes environments like banking or healthcare, implement mutual TLS (mTLS), where both the client and server authenticate each other using digital certificates. This two-way verification ensures that even if an attacker spoofs a connection, they can’t decrypt or impersonate the legitimate service.

Quick implementation tips:

• Enforce TLS 1.3 as the default.
• Rotate certificates regularly and revoke compromised ones.
• Never send tokens or credentials in URL query strings.
• Use HSTS headers to force secure connections.

Proper encryption prevents man-in-the-middle attacks, eavesdropping, and data tampering threats that can quietly undermine even well-designed APIs.

5. Adopt Zero Trust and Role-Based Access

Gone are the days when “internal” meant “safe.” APIs now connect microservices, mobile apps, and third parties, each one of them a potential breach point.

A Zero Trust approach means every request, even internal ones, must prove its legitimacy. Combine that with Role-Based Access Control (RBAC) or Attribute-Based Access Control (ABAC) to ensure least privilege access.

Here’s how it works:

• Every user and microservice must authenticate, no exceptions.
• API gateways continuously validate permissions.
• Logs record every request for anomaly detection.
• Access between internal services is restricted to verified scopes.

This model reduces insider risk and lateral movement within your environment. It also aligns directly with OWASP API2: Broken Authentication and API5: Broken Function Level Authorization, both common causes of modern breaches.

6. Detect and Prevent Business Logic Flaws

This is where most security programs fall short. Even with perfect authentication, an API can fail if its logic allows misuse. Business logic flaws occur when an attacker uses valid functions in unintended ways, changing order amounts, reusing expired tokens, or bypassing verification steps.

These vulnerabilities correspond to API7 in the OWASP API Security Top 10 and are among the hardest to catch because they mimic normal user behavior.

Examples of logic abuse:

• Submitting the same refund request multiple times.
• Triggering payment APIs without proper sequence validation.
• Using an older API version to skip new business rules.

Best practices for logic validation:

1. Simulate real workflows, not just technical inputs. Tools like APIsec.ai replicate user journeys to detect improper sequences.
2. Enforce strict step ordering; for instance, a “confirm” call should only succeed if preceded by “create order.”
3. Apply time-based controls to prevent replay attacks.
4. Validate roles and permissions at every state transition.
5. Add server-side logic to confirm that actions occur only once per session.

When security is tied to real business behavior, your APIs protect not just data but revenue, reputation, and user trust.
For a practical breakdown of real-world examples, APIsec’s post on How to Tackle Business Logic Flaws During Application Design is an essential next read.

7. Shift Left: Build Security into CI/CD

In today’s continuous delivery cycles, security can’t wait until production. The Shift Left philosophy embeds security testing directly into development pipelines.

Each new commit triggers automated API tests for authentication, input validation, and logic flow. Platforms like APIsec and Pynt plug into Jenkins, GitHub Actions, or GitLab CI to test endpoints during every build, ensuring vulnerabilities never reach production.

Benefits include:

• Early detection of misconfigurations and logic errors.
• Reduced remediation costs by fixing during build, which costs 1/10th of post-release patches.
• Stronger collaboration between dev and security teams.

Shifting left turns API testing into a daily routine rather than a quarterly audit, a principle that supports OWASP’s recommendation for continuous validation across every stage of development.

You can explore implementation details in Shift-Left Security: Bringing Testing Early in the Pipeline.

8. Continuously Test, Monitor, and Improve

Even the most secure API today can become a risk tomorrow. Continuous monitoring ensures that new vulnerabilities are detected before attackers find them.

Automated scans should run daily on staging and production environments, comparing current behavior against OWASP Top 10 benchmarks. Real-time alerts from your API gateway or SIEM solution can detect anomalies like sudden spikes in failed logins or unexpected data transfers.

Continuous monitoring transforms security from a one-time checklist into an always-on safety net.

For guidance on tool selection, Best API Security Testing Tools compares leading platforms used by enterprise DevSecOps teams.

Download the Complete Checklist and Visual Guide

To help teams put this framework into action, APIsec offers a free downloadable PDF checklist summarizing all eight sections along with a visual infographic for easy reference during code reviews or compliance audits.

The resource covers every stage from discovery and authentication to logic validation and continuous testing, so development and security teams can work from the same trusted playbook.

Partnering with APIsec for Continuous API Protection

The reality is that manual testing can’t keep up with modern development velocity. That’s where APIsec.ai makes the difference.

Its automated testing platform continuously scans APIs for vulnerabilities, configuration flaws, and logic errors, aligning directly with the OWASP API Security Top 10. It detects issues early, tests every deployment automatically, and provides clear remediation steps, all without slowing your delivery pipeline.

You can request a free vulnerability scan and download the full API Security Checklist 2025 at APIsec.ai.

Key Takeaways

• APIs now account for the majority of cloud breaches, mostly due to shadow and misconfigured endpoints.
• Automated discovery gives full visibility across environments.
• OAuth2, short-lived JWTs, and mTLS keep access secure.
• Zero Trust models prevent insider and cross-service abuse.
• Business logic validation is just as important as vulnerability testing.
• Continuous scanning and shift-left integration protect APIs as they evolve.

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