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Put inexpensive request-shape checks at the API edge, but keep authoritative business and security validation in trusted application services. The goal is not to validate only once: define shared rules centrally where practical, then enforce them at every reachable trust boundary that needs them.
Why four independent copies of a rule can be worse than one shared rule
Imagine the browser, an API gateway, and two services each implementing the same field rules. If those copies drift, one path may accept a value another rejects, while a direct request may bypass the browser entirely. That can produce confusing errors and leave gaps. “Four checks in four places” is an illustration of this maintenance problem, not a measured count of how teams typically validate input.
Centralizing rule definitions can reduce that drift, but centralization does not mean there is only one enforcement point. OWASP recommends a centralized validation library or framework and also calls for validation on trusted systems. Inputs may arrive through REST requests, URL parameters, headers, cookies, files, databases, and external APIs—not only browser fields. See the OWASP Developer Guide on validating inputs and OWASP ASVS 5.0, V2.
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| Layer | Best fit | Important limit |
|---|---|---|
| Client or browser | Fast feedback and clear field-level errors for users. | It is not a security boundary; direct requests and system handoffs can bypass it. |
| API edge or gateway | Common request-shape checks, required fields, configured schema checks, and inexpensive rejection before backend integration. | Coverage depends on route, method, content type, and gateway configuration; gateway checks may not establish domain meaning. |
| Trusted application or service | Authoritative type, range, domain, ownership, authorization-dependent, cross-field, workflow, and business-limit checks. | Rules can drift if services each implement separate copies; internal handoffs and direct service access still need coverage. |
OWASP ASVS states: “While client-side validation improves usability and should be encouraged, it must not be relied upon as a security control.” The OWASP Web Security Testing Guide likewise emphasizes testing backend checks and data handoffs rather than trusting data simply because it entered the system.
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How an API gateway can reject malformed requests early
Amazon API Gateway’s REST API request validation illustrates the edge’s useful role. A configured request validator can check whether required URI, query-string, and header parameters are present and non-blank, and can validate a request body against a configured JSON Schema model. When validation fails, API Gateway can return a 400 response before invoking the integration; validation results can also be published to CloudWatch Logs. AWS describes the intent as: “API Gateway can perform the basic request validation, so that you can focus on app-specific validation in the backend.” See Amazon API Gateway request validation for REST APIs.
Know what the gateway check does not establish
- Required-parameter validation checks existence and non-blank values; AWS says it does not check parameter type or format.
- Body validation depends on a configured model and a matching content type. If no matching content type is found, request-body validation is not performed.
- Validation is associated with API methods, so the intended validator must be configured for each relevant route and method.
A JSON body can match its schema and still be invalid for the current account, workflow, permissions, inventory, or relationship between fields. Keep those decisions in the backend service that has the necessary state and policy context.
Separate well-formed input from valid business meaning
Syntax and structure
Structural checks ask whether input is shaped as expected: whether required fields exist, values have the expected types, and simple ranges or formats are acceptable. A schema or boundary check can handle many of these predictable conditions early.
Semantics and context
Business validation asks whether the value makes sense now and for this operation. Examples include whether a user owns the referenced resource, whether an account is active, whether a workflow transition is allowed, or whether two individually valid fields are consistent together. Such checks may require domain state or another system, so a gateway’s view of the request alone is insufficient. OWASP distinguishes boundary-value checks from logical validation that may depend on related state in its business logic data validation guidance.
Design a shared validation approach without trusting a single border
- Define reusable rules centrally. Maintain common formats, ranges, and schema constraints in a shared library, framework, or governed schema where practical. Document the rules so separate services do not quietly diverge.
- Give the client usability checks. Show prompt, field-specific feedback, but treat all client values as untrusted when they reach a server.
- Apply inexpensive edge checks to covered routes. Configure the gateway for request-shape checks it actually supports, and verify the configuration for each method and accepted content type.
- Enforce authoritative rules in trusted services. Check domain meaning, ownership, permissions, workflow state, cross-field constraints, and limits where the relevant state and policy are available.
- Recheck at trust-boundary handoffs. If a service can be invoked directly or receives data from another component, do not assume a prior layer always performed the necessary checks.
- Keep rejection handling safe and observable. Log validation failures as appropriate without exposing sensitive input in logs. OWASP’s REST Security Cheat Sheet recommends considering validation-failure logging and request-size limits.
Common failure modes to look for
- Only the form validates: an API caller can bypass browser-side checks with a direct request.
- The gateway checks shape, but not state: a structurally valid value may violate an account rule or workflow transition.
- Services disagree: copied rules can diverge on accepted lengths, ranges, or formats.
- A route or content type escapes the intended check: verify validator assignment per method and model matching for each supported content type.
- Previously accepted data is trusted forever: reassess what is needed when data crosses an internal boundary or enters a new operation.
- Validation is mistaken for injection prevention: it does not replace parameterized queries, correct output encoding, or context-specific sanitization.
For a review, map every request entry point, identify which routes have edge validators, and record exactly what each one checks. Then determine whether internal services are directly reachable, which rules depend on current business state, and where field combinations or workflow transitions are enforced. OWASP ASVS explicitly notes that validation does not eliminate the need for correct encoding, parameterization, or sanitization when data is used by another component or presented as output.
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