Start by inventorying endpoints, then test authentication, authorization, input validation, error handling, rate limits, and abuse paths. A useful review maps sensitive functions and verifies that only the right users can reach them, that data is validated and encoded, and that failures do not leak information. The goal is to reduce exploitability early, before small bugs become reliable attack paths.
How to Structure an API Security Review
Start with an endpoint inventory, then review the API as a set of reachable functions, data flows, and trust decisions. The most useful finding usually comes from tracing where sensitive actions are exposed, who can call them, what input they accept, and what the service reveals when something fails. That makes the review practical instead of purely checklist-driven.
For a structured method, the OWASP Web Security Testing Guide is a strong companion because it gives teams a repeatable way to test access control, input handling, and error behaviour. It pairs well with the OWASP API Security Top 10, which helps reviewers focus on the API-specific failure modes that matter most in practice.
One useful way to organise the work is to move from discovery to control verification. First map the endpoints and their business purpose, then validate authentication and authorization on every sensitive path, then probe input validation, output encoding, and error handling, and finally test rate limits and abuse paths. That sequence helps teams find the weaknesses attackers normally chain together.
Where API Weaknesses Usually Hide
The highest-value checks are usually not the obvious public endpoints, but the sensitive functions hidden behind seemingly routine calls. Reviewers should look for broken authorization, excessive data exposure, missing object-level checks, and functions that behave differently when parameters are changed. These issues often survive basic testing because the API is reachable and functional, just not properly constrained.
Input validation matters because APIs are often trusted to pass structured data between systems. If values are accepted without strict validation, normalization, and encoding, the result can be injection, malformed requests, privilege abuse, or unsafe downstream processing. Error handling is equally important: verbose responses, stack traces, and inconsistent failure messages can reveal internal logic, schema details, or even account and object existence.
Rate limiting and abuse testing should be treated as part of the security review, not as an optional performance check. Attackers commonly use APIs for enumeration, credential stuffing, scraping, or automated abuse, so teams should confirm that throttles, quotas, and anomaly controls apply consistently to authentication, search, export, and bulk-update paths.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP Non-Human Identity Top 10, OWASP Agentic AI Top 10 and MITRE ATT&CK address the attack and risk surface, while CIS Controls v8 and NIST CSF 2.0 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| OWASP Non-Human Identity Top 10 | NHI-01 — Secrets and Credential Management | APIs often expose keys, tokens, and service credentials. |
| NHI-03 — Excessive Privilege | API reviews must catch overprivileged non-human access paths. | |
| NHI-07 — Monitoring and Detection | API abuse paths require visibility into anomalous access and misuse. | |
| Recommendation — Inventory and rotate API secrets with strict ownership and expiry. Enforce least privilege for API clients and service accounts. Log sensitive API access and alert on suspicious usage patterns. | ||
| OWASP Agentic AI Top 10 | A1 — Input and Prompt Injection | API reviews should test untrusted input handling and abuse paths. |
| Recommendation — Validate all API inputs and reject malformed or unexpected content. | ||
| CIS Controls v8 | 6 — Access Control Management | API security reviews hinge on verifying who can reach sensitive functions. |
| 13 — Network Monitoring and Defense | API abuse detection depends on logging and monitoring for anomalous use. | |
| Recommendation — Review and remove unnecessary API access paths and privileges. Monitor API traffic for brute force, enumeration, and abnormal volume. | ||
| MITRE ATT&CK | T1190 — Exploit Public-Facing Application | Public APIs are common initial access targets for attackers. |
| T1110 — Brute Force | APIs are often abused with repeated authentication attempts and enumeration. | |
| Recommendation — Test exposed API paths for exploitable weaknesses before release. Rate-limit and detect repeated authentication abuse against API endpoints. | ||
| NIST CSF 2.0 | PR.AC-4 — Access Permissions and Authorizations | API reviews must verify correct authorization on sensitive functions and objects. |
| PR.DS-1 — Data-at-Rest Protection | Sensitive API data exposure depends on how protected data is handled and returned. | |
| Recommendation — Validate that API callers can access only the resources and actions they are authorized for. Protect sensitive API data and limit unnecessary disclosure in responses. | ||
Practitioner Guidance
What to prioritise: Start with endpoints that move money, expose records, change permissions, or return bulk data. If a call can reveal, alter, or export something sensitive, verify the control path before spending time on low-impact routes.
What to verify: Confirm that authorization is enforced at the object and action level, not just at the API gateway or initial login. A common failure is to protect the wrapper while leaving individual resources or functions reachable through modified IDs, scopes, or parameters.
Common mistake: Teams often test the happy path and a few obvious failures, then stop. A stronger review tries the same request with alternate roles, missing fields, oversized inputs, repeated calls, and malformed values to see whether the API degrades safely or exposes a reliable attack path.
Practitioner takeaway: The goal is not to prove the API works, it is to prove that each sensitive function fails closed, resists abuse, and reveals as little as possible when it is misused.
Related resources from NHI Mgmt Group
- How should security teams identify hidden API risk in cloud-native environments before attackers do?
- How should security teams use exposure management to reduce the impact of hidden external assets before attackers find them?
- How should security teams secure publicly exposed MLOps platforms before attackers find them?
- How should security teams harden exposed PLCs in water and wastewater environments before attackers find them?
