Securing a serverless API is not a single choice but a sequence of them. Native API Gateway authorization with AWS IAM, custom Lambda authorizers, and Web Application Firewalls each answer a different question: who is calling, what are they allowed to do, and is the traffic itself malicious. Understanding which question each mechanism answers—and where it sits in the request lifecycle—is what lets you pick the right tool, or more often, the right combination of tools.
Understanding Serverless API Security Layers
The three mechanisms are frequently discussed as if they were competing options, but they operate on different problems. Authorization mechanisms—whether IAM policies or Lambda authorizers—concern themselves with identity and permissions. A Web Application Firewall, by contrast, inspects the content of HTTP traffic itself, screening for known attack patterns before they ever reach your application logic.
Amazon API Gateway supports all three approaches natively. According to Solo.io's guide to the service, API Gateway provides authentication through AWS IAM policies, Amazon Cognito user pools, and Lambda authorizers, while also integrating with AWS WAF to shield APIs from common web-based exploits. That breadth is precisely why the decision can feel confusing: the options overlap in marketing language but not in function.
Native API Gateway Authorization (IAM)
IAM-based authorization is the most direct option when your callers are already inside the AWS ecosystem. Access is controlled through AWS IAM policies configured directly on the API Gateway, meaning only authorized AWS principals can invoke your API methods. This makes it a strong fit when the primary need is managing access for identities and roles that AWS already knows about.
The trade-off is scope. IAM policies evaluate AWS identities, so this mechanism is best suited to scenarios where your callers are AWS principals—internal services, cross-account workloads, or tooling that can sign requests with AWS credentials. If your API serves external users whose identities live in a third-party system, IAM alone will not carry the weight.
The practical appeal is simplicity: there is no custom code to write or deploy, and the authorization decision leverages identity infrastructure you already operate. The cost of that simplicity is rigidity—complex, dynamic authorization rules fall outside what policy documents can comfortably express.
Custom Lambda Authorizers
A Lambda authorizer moves the authorization decision into code you control. API Gateway invokes your Lambda function before the request reaches your backend, and that function returns an IAM policy plus the caller's principal identifier, as described in Amazon's API Gateway developer documentation. The function can also return a context object carrying additional information into the integration backend.
This is where flexibility becomes the selling point. Amazon's documentation notes that in production code you may need to authenticate the user before granting authorization, and you can add that authentication logic in the Lambda function by calling an external authentication provider. In other words, the authorizer is a natural integration point for identity systems that live outside AWS.
Two authorizer types shape the implementation. A TOKEN authorizer receives the caller's identity in a bearer token such as a JWT or OAuth token, with optional RegEx-based token validation. A REQUEST authorizer receives the request details directly. Caching matters here too: Amazon's documentation explains that changing cache key parts and redeploying the API discards the cached policy document, and that turning off caching for a REQUEST authorizer means API Gateway passes requests straight to the function—trading invocation cost and latency for fresher policy decisions.
The cost of this flexibility is operational: you are now writing, testing, and maintaining a Lambda function that sits on the critical path of every request. Its utility is highest when standard IAM policies are insufficient for your authorization requirements.
Web Application Firewalls (WAFs)
A WAF addresses a threat class the authorization layers do not touch: malicious payloads from otherwise legitimate-looking requests. AWS WAF is a service that protects web applications and APIs from cyber attacks, and it can specifically help protect API Gateway APIs from common web-based attacks such as SQL injection and cross-site scripting, as Solo.io's guide explains.
API7.ai's guidance on securing serverless APIs frames WAF integration as adding "another critical layer of defense," one that monitors and filters HTTP traffic between the application and the internet, protecting against common web exploits like SQL injection, XSS, and other OWASP Top 10 vulnerabilities. A WAF is essential for protecting public-facing APIs from known attack patterns.
Crucially, a WAF does not authenticate anyone. A perfectly valid, authenticated caller can still submit a SQL injection attempt, and a WAF is the layer designed to catch it. Conversely, a WAF will not decide whether user A may access resource B—that remains the job of your authorization layer.
Serverless API Security Mechanism Comparison
The table below classifies each mechanism by the security layer it provides and the class of threats it is designed to mitigate. Use it to map your specific threat model to the tool that addresses it.
| Mechanism | Security Focus | Implementation Context | Primary Use Case | Protects Against |
|---|---|---|---|---|
| Native API Gateway Authorization (IAM) | Identity and access management at the API Gateway level | Configured directly in API Gateway using AWS IAM policies; no custom code | Managing access for AWS principals, such as internal services and cross-account workloads | Unauthorized access to API endpoints by unauthenticated or unprivileged callers |
| Custom Lambda Authorizer | Custom authentication and authorization logic in a Lambda function | Write and deploy a Lambda function that API Gateway invokes; TOKEN or REQUEST types, with optional policy caching | Integrating external identity providers or enforcing fine-grained, dynamic authorization rules | Unauthorized access by users whose identities are managed outside AWS IAM |
| Web Application Firewall (WAF) | HTTP traffic inspection and filtering | Attached to API Gateway as an external service; rule-based, no application code changes | Shielding public-facing APIs from known attack patterns | Common web exploits such as SQL injection, XSS, and other OWASP Top 10 vulnerabilities |
Read horizontally, the table shows why these are complementary rather than competing choices: no single row covers both identity-based access control and payload-level attack filtering. Read vertically, it shows the implementation trade-off—IAM is configuration-only, a Lambda authorizer is code you own, and a WAF is a managed service you attach and configure.
Building a Layered Defense-in-Depth Strategy
Because each layer answers a different question, the strongest architectures combine them. In a layered request flow, the WAF inspects and filters incoming HTTP traffic first, blocking known exploit patterns before they reach your infrastructure. Requests that pass then hit API Gateway's authorization layer—either an IAM policy check or your Lambda authorizer—which verifies identity and permissions before the request is allowed through to your backend integration.
This ordering matters. A WAF filtering traffic upstream reduces the load and noise reaching your authorization logic, while the authorization layer catches the requests a WAF was never designed to evaluate: valid, well-formed calls from users who simply lack permission. A multi-layered approach enhances overall security by mitigating the risk that any single control fails.
Runtime protection deserves a place in this picture as well. Data Theorem's white paper on API security pillars describes runtime protection as security that watches live API traffic in production, protecting APIs during the window between a vulnerability being found and the fix being deployed. A WAF's rules and your authorizer's policies are controls you configure in advance; runtime monitoring is the safety net for the flaws nobody has found yet.
Architecting for Layered Serverless API Security
Architects and developers should implement a multi-layered security strategy for serverless APIs, combining native API Gateway authorization, custom Lambda authorizers, and a Web Application Firewall based on the specific threats each layer addresses. Start with the decision the table makes explicit: if your callers are AWS principals, IAM policies may suffice; if identity lives elsewhere or authorization rules are dynamic, add a Lambda authorizer; and if the API is public-facing, attach a WAF against known exploit patterns.
The measurable indicator of success is reduced incidents of unauthorized access—authentication and authorization failures—and common web exploits such as SQL injection and XSS, as reported in API Gateway logs and WAF metrics. Those two signals map directly onto the two halves of your defense: watch the authorization layer's rejection counts to confirm your access controls are working, and watch WAF metrics to confirm the exploit-filtering layer is doing its job.
