Zombie APIs vs Shadow APIs: What’s the Difference?

As engineering teams scale, it’s common for new API endpoints to appear, some officially tracked, others hidden or unmaintained. This proliferation, known as API sprawl, introduces two critical blind spots: shadow APIs and zombie APIs.

In this article, I will define each, highlight the key differences between Shadow API vs Zombie API, and explain why addressing both is essential for security, compliance, and operational integrity.

What Is a Shadow API?

A Shadow API refers to an API that operates outside the purview of an organization's IT and security teams. These APIs are often created without formal approval and lack proper documentation, making them invisible to standard monitoring tools. Common examples include:

• Development or testing endpoints that were inadvertently exposed to the public.

• APIs integrated by individual teams without centralized oversight.

• Legacy APIs that remain active but are no longer maintained.

In September 2022, telecom major Optus' data breach exposed nearly 10 million customer records, not due to a public vulnerability, but because of an undocumented, unauthenticated API endpoint that wasn't on security’s radar. This shadow API lacked rate limits, authentication, or monitoring, providing easy access for anyone who discovered it.

What Is a Zombie API?

A zombie API is a deprecated or abandoned endpoint that remains operational in your production environment, unused and unmaintained, but still functioning. Common examples of Zombie APIs include:

• Temporary test or staging APIs, often created for troubleshooting or experimentation, are accidentally deployed to production and remain active long after their original purpose is served.

• The old endpoints that aren’t turned off after upgrading to a newer API version (e.g., SOAP → REST)

In 2023, a deprecated SOAP-based patient data API in St. Luke’s Health System exposed 450,000 records because it was never fully decommissioned, even after the organization moved to REST services; the endpoint remained live, unpatched, and undetected for months.

Shadow APIs vs Zombie APIs: Key Differences

Here are the key differences between Shadow APIs vs Zombie APIs:

Why You Need to Detect and Manage Both

Understanding shadow APIs vs. zombie APIs isn’t about closing off hidden weak spots in your system. Here’s what makes both of them significant:

1. Growing Your Attack Surface

Shadow APIs are live endpoints you didn’t track; they skip security checks and monitoring, making them prime targets.

Zombie APIs are deprecated services that continue to run. They often use old libraries or insecure protocols and lack patches.

2. Security and Compliance Blind Spots

Neither shadow nor zombie APIs get routed through gateways or logging. That means data can flow out, unmonitored and unchecked, potentially violating GDPR, CCPA, HIPAA, etc.

3. Broken API Lifecycle & Governance

Shadow APIs bypass the approval and documentation process. Zombie APIs may have been approved once, but are never properly retired. That leads to drift in your API catalog and audit failures.

4. Detection After the Fact is Expensive

These APIs often show up on the radar only after a breach or during a costly audit, by then, you’re in reactive mode. Early detection through spec comparison, traffic monitoring, or inventory scans is more efficient and cost-effective.

How to Detect Shadow and Zombie APIs

1. Use Observability Platforms

Modern observability tools continuously monitor live API traffic, comparing actual calls against documented specs or schemas. They signal undocumented or inactive endpoints, commonly known as shadow APIs (undocumented but active) and zombie APIs (obsolete but still exposed). This real-time visibility is critical for effective API governance and risk management.

2. Use Treblle’s API Intelligence Approach

Treblle is an API intelligence platform that helps you make sense of your observability data with its targeted capabilities to uncover and manage shadow and zombie APIs efficiently:

1. Dashboard

Treblle’s dashboard gives a centralized view of API activity, total requests, endpoint counts, compliance scores, recent request logs, performance trends, and threat levels, allowing you to identify unexpected endpoints.

2. Automated Discoverability & Cataloging

Treblle’s API Catalog helps automatically discover and aggregate all active endpoints in one place. It enables teams to identify undocumented ("shadow") or unused ("zombie") APIs and categorize them by status or function.

3. API Score

Each API is scored across AI readiness, performance, quality, security, and compliance. APIs that never register a score or suddenly drop out of the score panel signal potential shadow or zombie risk.

4. Traceability & Search Tools

With advanced trace IDs and real-time search filters, teams can trace request flows across microservices and filter by endpoint, client, or location, which is ideal for confirming suspected ghost APIs.

5. AI Assistant "Alfred"

Alfred is an intelligent assistant that helps teams discover and evaluate unknown endpoints. Alfred surfaces usage anomalies and flags endpoints with no historical documentation, reducing the time to detection.

6. Custom Alerts & Governance Checks

You can configure alerts for unexpected endpoint patterns, compliance deviations, or silent endpoints. Treblle also provides built‑in governance rules and automated compliance checks that help detect unused or risky APIs before they cause damage.

Best Practices to Avoid Shadow and Zombie APIs

1. Automate API Documentation via CI/CD

Prevent shadow endpoints with fully automated documentation as part of your CI/CD pipeline. By generating OpenAPI specs during builds and deployments, you ensure every new or changed API is captured and documented in real time, a proven way to eliminate undocumented APIs.

2. Enforce Formal API Lifecycle Governance

Define a clear API lifecycle, from design to decommission. This includes versioning, deprecation schedules, and retirement processes. Require business justification for every live endpoint, and verify its documentation and governance status before deployment.

3. Maintain a Continuous, Centralized API Inventory

Use tools or platforms to auto-discover every live endpoint across internal, external, and third-party environments. Monitor usage, documentation, versions, and ownership centrally. Regularly audit this inventory for undocumented (shadow) or unused/deprecated (zombie) APIs.

4. Conduct Regular API Usage Audits

Analyze traffic patterns: endpoints with zero or extremely low usage over time may be zombie APIs. Shadow APIs often surface via anomalous traffic coming from unexpected clients or environments. Integrate this analysis into annual or quarterly security reviews.

5. Deploy Discovery, Observability & Reporting Tools

Implement platforms that continuously scan your environment and compare runtime traffic against documented specs. Integrate these with governance dashboards, alerting, and compliance workflows to proactively flag new or inactive API endpoints.

6. Use an API Gateway for Central Traffic Control

All API requests should go through an API gateway, ensuring visibility, routing control, security scanning, and logging. Gateways help prevent shadow APIs by owning all external access and enforcing authentication and version policies.

7. Set Up Alerts and Automated Enforcement

Establish alerts for suspicious API behavior, such as unknown endpoints appearing, unused APIs lingering, or endpoints skipping governance. Couple this with automated lifecycle checks to detect and block endpoints that lack documentation or fail compliance criteria.

8. Foster Cross-Functional Collaboration & Training

Educate developers, DevOps, and security teams about the dangers of shadow/zombie APIs. Encourage accountability, clearly assign API “ownership,” and make it part of dev workflows to document and retire endpoints systematically.

Conclusion

Shadow APIs are undocumented yet active endpoints that operate under the radar, while zombie APIs are outdated services that remain exposed and forgotten.

To avoid these hidden risks, you should embed automated documentation and discovery into your CI/CD pipeline, enforce lifecycle policies for deprecation and retirement, and maintain real‑time monitoring for unexpected or unused endpoints.

Treblle simplifies this process by automatically discovering all live APIs, scoring them on performance and security, and alerting you to undocumented or inactive services through its unified dashboard and discoverability features. Adopting these practices, supported by Treblle, ensures your API landscape remains clean, secure, and manageable now and is ready for future scale.