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A firewall or web application firewall (WAF) can filter suspicious traffic, but it cannot decide whether a particular caller is allowed to read a particular record, change a specific field, or trigger a sensitive business action. Securing an API requires controls that understand its operations and data, plus checks that cover development, deployment, and runtime—not just the network edge.
What a firewall can—and cannot—protect
A firewall filters network traffic according to configured rules. A WAF can inspect web requests for patterns associated with attacks, such as a payload that resembles SQL injection. These are useful layers, but they do not automatically understand the permissions and rules built into a specific API.
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NIST illustrates the difference with a field-validation example: a WAF might detect a SQL-injection-like payload, but it cannot establish that an API’s name field must be a string shorter than 100 characters. That constraint needs application-aware schema or business-rule validation. Similarly, a request passing an edge filter does not prove that its caller may access the requested account, change a particular property, or perform a particular action.
Use the firewall or WAF as one part of a layered design. The API and the services behind it must enforce the rules that depend on identity, data, and business context.
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Where API security risks arise
The OWASP API Security Top 10 for 2023 is a useful assessment prompt. Its categories span authorization, authentication, abuse, configuration, inventory, and dependencies; the order is not a measured probability ranking.
| OWASP 2023 category | What to examine |
|---|---|
| API1: Broken Object Level Authorization | Whether every operation that uses a caller-supplied object ID checks that caller’s permission to access that specific object. |
| API2: Broken Authentication | Whether the API reliably establishes the caller’s identity and handles authentication credentials and flows securely. |
| API3: Broken Object Property Level Authorization | Whether callers can read or change only the properties they are permitted to access. |
| API4: Unrestricted Resource Consumption | Whether requests can consume excessive compute, memory, bandwidth, or other limited resources. |
| API5: Broken Function Level Authorization | Whether access to each operation or function is limited to the callers permitted to use it. |
| API6: Unrestricted Access to Sensitive Business Flows | Whether important workflows can be automated or abused in ways that bypass intended business protections. |
| API7: Server Side Request Forgery | Whether attacker-influenced requests can cause the server to reach unintended destinations or resources. |
| API8: Security Misconfiguration | Whether API-facing components and services have insecure or unintended settings. |
| API9: Improper Inventory Management | Whether teams know which endpoints and versions are deployed, including obsolete or undocumented ones. |
| API10: Unsafe Consumption of APIs | Whether data and responses from upstream or third-party APIs are treated as untrusted and validated appropriately. |
OWASP’s 2023 list is awareness guidance, not a measured league table. Its release notes say that edition did not use contributed data; the categories were assembled from project-team experience, specialist review, and community feedback. OWASP’s methodology also explains that its risk ratings reflect team consensus and do not determine the details or impact of risk in a particular organization. Use the list to prompt a local assessment, not to infer which issue is most likely in your system.
Authentication is not authorization
Authentication answers, “Who is calling?” Authorization answers, “What may this caller do here?” An authenticated session or valid token is not blanket permission to every record, field, or operation.
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Check every object identified by the caller
If a request contains an object ID—such as an account, invoice, or document—the server must verify that the authenticated caller has permission to access that particular object. OWASP’s API Security Project says: “Object level authorization checks should be considered in every function that accesses a data source using an ID from the user.” A difficult-to-guess ID or a request that passes a WAF is not a substitute for that check.
Restrict properties and functions separately
Permission to view an object does not necessarily mean permission to view every field or modify any field. Likewise, permission to use one API operation does not automatically authorize another. Define and enforce access rules at the property and function level, including on updates—not only when a user first signs in.
Protect against abuse, not just unauthorized access
An API can be abused by a caller who is authenticated and technically authorized. Expensive requests can exhaust resources, while repeated use of a sensitive workflow can cause harm even if each request is valid on its own.
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- Set resource limits: Bound request sizes and other costly inputs, and apply suitable limits to operations that consume substantial resources.
- Protect sensitive flows: Identify workflows that need controls against automated or excessive use; ordinary request authentication alone may not address that risk.
- Observe usage: Monitor for resource pressure and unusual patterns so teams can investigate and respond.
The appropriate limits and workflow safeguards depend on what the API does. A single generic request-rate threshold should not be treated as a complete abuse-control strategy.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Inventory and configuration are part of the attack surface
Defenses can miss endpoints that teams do not know are deployed. Keep an inventory of API endpoints and versions, and make it possible to distinguish current services from obsolete or undocumented ones. Review API-facing components and their configuration deliberately; an intended control cannot protect a service if the service is misconfigured or outside the control’s coverage.
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Also treat responses and data from upstream APIs as untrusted input. An external service’s output does not become safe merely because it came from another API.
Apply controls across development and runtime
NIST SP 800-228 provides a lifecycle frame for API risk in cloud-native systems. It analyzes development and runtime concerns and recommends both pre-runtime and runtime protections, with basic and advanced measures that support incremental, risk-based adoption. The publication was first issued in June 2025 and its final record was updated on March 13, 2026, adding appendices that list API risks by category and recommended controls by lifecycle stage.
For a practical starting point, use the following assessment questions. They synthesize OWASP’s risk categories and NIST’s lifecycle framing; they are not a checklist quoted from either source.
- Inventory: Can the team identify deployed endpoints and distinguish current versions from obsolete or undocumented ones?
- Identity and authorization: For every operation, does the server check the caller’s right to the requested object, properties, and function?
- Input and output: Are accepted field names, types, and sizes constrained, and are returned properties limited to what the caller needs?
- Abuse resistance: Are resource-intensive operations and sensitive business workflows protected with appropriate limits and monitoring?
- Configuration and dependencies: Are API-facing components configured deliberately, and are upstream API responses treated as untrusted?
- Lifecycle ownership: Are protections checked before release and during runtime, with a clear owner for follow-up?
Prioritize gaps according to the API’s actual data, operations, exposure, and business impact. A perimeter control can reduce some traffic risks, but it cannot supply application rules that the API itself has not defined and enforced.
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