API Security Best Practices: How to Catch Shadow and Zombie APIs Before Attackers Do

APIs are the lifeblood of modern applications, powering service-to-service communication, mobile integrations, and backend workflows. But every new API increases the attack surface, especially when endpoints aren’t tracked. Shadow and zombie APIs are among the most dangerous blind spots: endpoints that slip outside centralized API gateways or remain routable long after their intended retirement.

These overlooked endpoints may still accept traffic, return sensitive data, or rely on outdated authentication logic, making them prime targets for exploitation.

For DevOps engineers, this means API security best practices must extend far beyond linting known endpoints. You need runtime-aware inventory systems, integrated SAST analysis for code-level API enumeration, and DAST to validate security posture in live environments. Full coverage requires combining static and dynamic analysis in pipelines, ensuring documented, undocumented, live, or deprecated APIs are continuously detected, validated, and controlled.

Understanding Shadow and Zombie APIs

Before we dig into how to protect against them, let’s define what we’re dealing with from a more technical perspective:

• Shadow APIs are undocumented or hidden endpoints that may be exposed via ad hoc routes, unregistered microservices, or bypassed API gateways. They often appear during rapid prototyping, test harness creation, or temporary integrations. Without API gateway registration or schema documentation, they bypass centralized authentication and logging.
• Zombie APIs are legacy endpoints left after refactors, microservice decomposition, or version migrations. They may still respond to requests due to DNS persistence, load balancer configs, or reverse proxy rules. Even when business logic is deprecated, their handlers, controllers, or lambda functions may still execute code and return sensitive payloads.

From a security standpoint, both shadow and zombie APIs expand the exposed attack surface. Both can expose sensitive data, use outdated JWT or TLS configurations, or contain unpatched vulnerabilities. They often evade CI/CD governance, making early detection critical. Industry research shows API-related incidents frequently trace back to unmanaged endpoints that evade standard CI/CD checks, gateway governance, and runtime monitoring. 

As discussed in our knowledge hub article on Shadow and Zombie APIs, these unmanaged APIs create blind spots that require proactive detection and testing to close security gaps.

Sidebar: 

Real-World Attack Scenario – Zombie API Exploit

Scenario: A legacy v1 /customer-data endpoint remains active months after being replaced by v2. The old endpoint lacks modern authentication and still queries sensitive customer details.

• Attacker Action: Scans exposed endpoints, locates /customer-data v1, and sends crafted requests exploiting weak auth to extract records.
• Impact: Unauthorized access to sensitive PII without triggering alerts because v1 wasn’t in the API gateway inventory.
• Checkmarx Prevention: API discovery detects the zombie v1 endpoint during code scan, alerts in CI/CD. DAST tests confirm exposure, and remediation instructions remove v1 routes before deployment.

Why API Security Best Practices Matter

Relying on a static API inventory is risky because service endpoints evolve with each sprint, commit, and deployment. API gateway configurations, service discovery registries, and reverse proxies may introduce or expose endpoints outside the approved schema.

Without continuously scanning codebases, parsing OpenAPI definitions, inspecting route handlers, and validating runtime configurations, the environment can quickly become a patchwork of known and unknown exposure points.

This means treating API inventory as a living artifact tied to CI/CD, not a one-time document, enabling greater:

• Visibility: You can’t secure what you can’t see. Automated API discovery during code scanning should enumerate all routes, controllers, and OpenAPI paths and compare them to the API gateway registry. This enables you to detect undocumented shadow APIs early.
• Risk reduction: Identifying and deprecating zombie APIs involves scanning service mesh and ingress controller configurations for inactive routes, detecting leaked api keys, verifying DNS and load balancer rules are removed, and ensuring old handlers are not deployed in container images.
• Compliance: Many frameworks now require active security validation of all APIs—documented or not—which means integrating schema validation, endpoint authentication checks, and runtime posture verification into CI/CD workflows.

For DevOps teams, these protocols aren’t just best practices; they’re operational requirements. Checkmarx supports this technical rigor by embedding API discovery, schema comparison, and runtime validation directly into build pipelines. 

These API security best practices require integrated platforms that combine static and dynamic analysis. Checkmarx’s integrated SAST and DAST engines continually reconcile detected endpoints against approved definitions. This closes the feedback loop, ensuring API inventories stay synchronized with live deployments and no shadow or zombie endpoint goes unnoticed.

Integrated AppSec: SAST + DAST for API Security

Point solutions create blind spots between code analysis and runtime testing. The most effective way to address shadow and zombie APIs is through an integrated AppSec strategy that combines static and dynamic analysis, each embedded at key stages of the DevOps pipeline:

• Static Application Security Testing (SAST): Beyond basic vulnerability scanning, SAST can enumerate API endpoints by parsing routing logic, annotations, and OpenAPI specifications. It can flag undocumented endpoints, weak authentication logic, and improper input validation directly in the code. For microservices, it should integrate with build scripts to capture ephemeral endpoints before they’re deployed.
• Dynamic Application Security Testing (DAST): DAST validates API security by exercising endpoints with authenticated and unauthenticated requests, probing CORS configurations, rate limiting, TLS setup, and verifying responses against expected security controls. Operating on deployed artifacts in realistic conditions, these runtime tests can include fuzzing and business logic abuse simulations to expose flaws invisible in static scans.

Modern AppSec platforms like Checkmarx bring these capabilities together, enabling teams to:

• Automate API discovery from codebases and compare detected endpoints against declared API gateway definitions, ensuring discrepancies trigger alerts in CI/CD dashboards for immediate remediation.
• Validate runtime security of every endpoint, ensuring parity between code intent and deployed behavior, with automated tests that check for response integrity, authentication bypass, and misconfigured headers.
• Enforce consistent application of authentication and authorization handling across the API lifecycle, using policy-as-code controls and integrating security-as-code templates to prevent drift between environments.

While standalone tools force teams to manually coordinate SAST findings with DAST results, integrated platforms automatically map static discoveries to dynamic validation.

By leveraging Checkmarx’s integrated SAST/DAST engine, teams can detect hidden routes in code, confirm they aren’t exposed unintentionally in production, and maintain a continuous feedback loop that keeps security aligned with rapid deployments.

Automated API Discovery: Closing the Blind Spots

Shadow APIs don’t announce themselves. That’s why automated API discovery is essential. By integrating automated discovery into DevOps workflows, security teams can analyze source code repositories, OpenAPI specifications, service registries, and runtime configurations to:

• Map all documented and undocumented endpoints: Use static analysis to enumerate routes, handlers, and controller classes, and reconcile them against API gateway or service mesh definitions. Tools can detect mismatches or orphaned endpoints that aren’t in the official spec.
• Identify deprecated or redundant APIs: Scan for versioned endpoints or routes flagged as deprecated in code comments, commit histories, or schema annotations. Cross-reference against deployment manifests and ingress rules to ensure old versions are removed from production.
• Pinpoint areas of high risk for testing and monitoring: Apply risk scoring based on authentication method, data sensitivity, exposure to external traffic, and past vulnerability history. Feed these into dynamic tests for targeted fuzzing, broken object-level authorization (BOLA) checks, and rate-limiting validation.

This process ensures nothing is left to chance and prevents zombie APIs from lingering unnoticed. Integrating automated discovery with runtime validation allows DevOps teams to keep their API inventory synchronized with production realities, sealing up vulnerabilities before attackers can exploit them

Continuous Monitoring in DevOps Pipelines

Modern DevOps environments are dynamic and event-driven. New APIs, versioned routes, and service endpoints may be deployed several times a day as part of continuous delivery. This creates constant drift between approved API definitions and what is actually deployed. Incorporating API security best practices directly into CI/CD pipelines is crucial to maintain control.

This includes:

• Automated scans at build time: Use SAST tools to parse code, route handlers, and OpenAPI specs during each build, flagging new or modified endpoints. Integrate schema diff checks against the API gateway registry to catch shadow endpoints. Employ automated API key scanning to scan code repositories, configuration files, logs, and cloud storage for exposed credentials.
• Runtime validation: Deploy staging builds to test harnesses running DAST, which sends authenticated and unauthenticated requests, fuzzing parameters, and validating that only approved endpoints respond and that responses match expected security controls.
• Alerts and remediation steps: Configure pipeline stages to fail builds or block deployments if noncompliant endpoints are detected. Alerts should integrate with Slack, Jira, or GitOps workflows to open remediation tickets and trigger automated API deprecation steps.

This aligns with the shift-left philosophy, ensuring that API issues are detected and addressed before production release, with continuous validation at each pipeline stage.

Actionable Steps for Securing APIs

To catch shadow and zombie APIs before attackers do, security teams should:

1. Inventory everything: Maintain a live map of all APIs by integrating automated discovery into CI/CD. Use tools to reconcile source-level API definitions with gateway configurations, flagging discrepancies for review. Regularly export inventories to a central dashboard that supports drift detection.
2. Integrate SAST and DAST: Validate security in both code and runtime. SAST enumerates endpoints in code, checking for vulnerabilities and authentication logic flaws; DAST runs runtime tests such as fuzzing, broken object level authorization checks, and rate limit validation against deployed APIs.
3. Deprecate safely: Remove zombie APIs and ensure they’re unreachable after decommissioning by updating DNS, removing load balancer rules, and verifying through negative runtime tests that deprecated endpoints no longer respond.
4. Monitor continuously: Integrate API discovery and testing into the CI/CD pipeline with recurring SAST scans, schema drift detection, and DAST runtime sweeps. Feed monitoring results into centralized alerting (Slack, Jira) for rapid triage.

A disciplined application of these steps builds a self-reinforcing API security loop. This ensures that as code evolves, every change is continuously scanned, validated, and monitored through Checkmarx’s integrated SAST, DAST, and automated API discovery, leaving no endpoint untracked or untested.

Checkmarx’s Role in API Security

We help organizations operationalize API security across the software lifecycle. The Checkmarx platform integrates SAST, DAST, and advanced API discovery capabilities to:

• Automatically discover shadow and zombie APIs in code by scanning source repositories, parsing routing logic, and reconciling detected endpoints with API gateway definitions.
• Test endpoints in production-like environments using Checkmarx DAST with authenticated and unauthenticated request scenarios, fuzzing payloads, and verifying security controls such as authentication, rate limiting, and CORS.
• Maintain continuous security alignment by embedding these capabilities directly into CI/CD pipelines, ensuring that any deviation from approved API definitions is identified and remediated before production.

By embedding these capabilities into development workflows, teams gain the visibility and control needed to keep APIs from becoming hidden vulnerabilities.

Protect Your APIs Before Attackers Find Them

Shadow and zombie APIs are more than just housekeeping issues. They’re real attack vectors that can bypass security controls and expose sensitive data. By adopting API security best practices that combine SAST, DAST, and continuous monitoring, you can keep these hidden risks in check.

Security isn’t just about knowing what’s in production today. It’s about being confident that there’s nothing lurking in the shadows tomorrow.