API Security Review

What an API security review evaluates

An api security review is a structured examination of how an API is exposed, who can reach it, what data it returns, and how it behaves under normal use and abuse. It focuses on the control points that decide whether the interface is resilient or easy to misuse.

The review is not limited to authentication alone. It typically checks request paths, object access, authorization logic, input handling, rate limiting, sensitive data exposure, logging, and how errors or edge cases may reveal more than intended.

Because APIs often sit between services, mobile apps, partners, and internal systems, the review is as much about trust boundaries as it is about code quality. A technically correct endpoint can still be insecure if it exposes excessive data, accepts unsafe assumptions, or trusts client-side enforcement.

Core security checks in an API review

The most important checks usually map to how the API verifies identity, enforces privilege, and constrains data access. That includes authentication quality, token handling, object-level authorization, function-level authorization, and whether the API accepts requests only from approved paths and callers.

Reviewers also look for issues that create unsafe exposure at the transport and application layers, such as weak input validation, unsafe deserialization, verbose error messages, missing schema enforcement, and unrestricted access to business actions. These issues can lead to broken authorisation, information disclosure, or unintended state changes even when the API appears stable in normal testing.

Operational controls matter too. Logging, monitoring, and abuse detection help distinguish ordinary traffic from probing, scraping, replay attempts, and automated exploitation. A review should treat rate limits, pagination, and resource consumption as part of security because they affect how easily an API can be enumerated or overwhelmed.

For interfaces that rely on secrets or machine-to-machine credentials, the review should also consider whether credentials are overexposed, long-lived, or reused across environments. NHIMG’s Ultimate Guide to Non-Human Identities notes that 96% of organisations store secrets outside secrets managers in vulnerable locations, and 79% have experienced secrets leaks, which makes API credential handling a material review concern.

Where API reviews fail in practice

API weaknesses usually emerge where developers assume the client will behave honestly, or where separate controls are applied inconsistently across endpoints. One common failure is object-level exposure, where an authenticated caller can access another user’s records simply by changing an identifier. Another is function-level exposure, where privileged actions are reachable without a strong server-side check.

Security reviews also fail when teams test only the happy path. APIs often break under malformed input, unexpected sequence changes, concurrent requests, or repeated calls that bypass intended workflow controls. If those edge cases are not exercised, the review can miss conditions that lead to data leakage, account abuse, or service disruption.

External dependencies can increase the blast radius. If an API trusts third-party data, federated callers, or shared secrets without clear boundaries, the review should examine whether that trust can be abused to pivot into higher-value systems. The result may be less about a single coding flaw and more about a trust model that is too broad for the data and actions being exposed.

The T-Mobile breach page in NHIMG’s collection shows how API exposure can become a large-scale data and credential problem when access controls are weak and privileges are excessive, making it a useful companion reference for this topic.

Why API security reviews matter

APIs are now a primary control plane for applications, partner integrations, and automation. That makes them a high-value target because a single flaw can expose many records or high-impact actions at once. A review helps identify whether the API’s design matches the sensitivity of the functions it performs.

From a security perspective, the main value is reducing unauthorized access before release and reducing ambiguity after deployment. A well-run review can reveal whether the API’s data model, authorization model, and telemetry are aligned, or whether the interface is quietly relying on assumptions that attackers can test and exploit.

From an engineering perspective, the review also exposes whether security is built into the interface or bolted on later. If authentication, authorization, schema validation, and logging are inconsistent across endpoints, the API is usually easier to misuse than the surrounding application suggests.

OWASP’s OWASP API Security Top 10 is the clearest reference for the recurring failure patterns an API review should cover, while the OWASP Web Security Testing Guide provides a broader testing methodology that helps validate those controls systematically.

Risk and Threat Considerations

API security reviews are exposed to both design risk and adversarial abuse. Weak authorization, excessive data exposure, and missing abuse controls can let an attacker enumerate objects, exfiltrate records, automate abuse, or use a legitimate session to reach functionality that should not be available.

Failure mechanism: The API trusts the caller, the object identifier, or the workflow state more than it should, so a valid request can be repurposed into unauthorised access or unintended action.

Impact: The result can be customer-data exposure, privilege misuse, business-flow abuse, service degradation, or a wider compromise path if the API is tied to privileged back-end functions.

Practitioner Guidance

What to watch for: Focus review effort on endpoints that return sensitive objects, perform privileged actions, or accept high-value tokens. These are the places where broken authorisation and unsafe assumptions usually become visible first.

Governance implication: Treat API security review as a release gate for material interfaces, not as an optional code-quality exercise. The review should have named ownership for authentication, authorisation, logging, and secret handling so failures do not fall between teams.

Related resources from NHI Mgmt Group

• What is the difference between runtime API testing and traditional static security review?
• How should security teams conduct an API security review to find weaknesses before attackers do?
• How should security teams use AI assistants to improve API security testing without replacing human review?
• How should security teams test persisted GraphQL queries as part of API security review?