API Security Checklist

Checklist of the most important security countermeasures when designing, testing, and releasing your API.
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Table Of Contents

• API Security Checklist
  • Design
  • Input Validation
  • Access Control
    • Rate Limiting
  • Authentication and Authorization
    • JSON Web Token
    • OAuth
  • Security Configuration
  • Logging
  • Build
  • Security Testing
  • Processing
  • Output
  • CI and CD

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Design

• Make sure that all the RESTful APIs have Swagger definitions (e.g. format, maximum, minimum, minItems, maxItems) See Swagger documentation for more information
• Make sure that the Swagger definitions contain input validation and all expected header parameters
• Make sure that the definitions contains authentication and authorisaiton requirements
• Make sure that the API definitions contain the expected Content-Type
• Use an API gateway to expose APIs

Input Validation

• Validate all user-supplied input in the headers and in the body before they are processed
• Validate user input to avoid common vulnerabilities (e.g. XSS, SQL-Injection, Remote Code Execution, etc.).
• Use an API Gateway service to enable caching, Rate Limit policies (e.g. Quota, Spike Arrest, or Concurrent Rate Limit) and deploy APIs resources dynamically.
• Don't use any sensitive data (credentials, Passwords, security tokens, or API keys) in the URL, but use standard Authorization header.
• Validate the type (e.g. integer, string, floating point number), size (e.g. minimum string lengths, minimum abd maximum values for numbers) and format the composition of expected inputs
• Validate the sizes of the JSON arrays and number of child elements in XML requests
• Use built-in libraries or annotations for input validation as much as possible, build custom validators if built-in functionality is inadequate
• Validate the size of the request body and request headers (at preferably at the API gateway)
• Validate that the content-type in the request header matches the expected content type
• Do not attempt to sanitise input (e.g. remove certain bad characters or strings)
• Make sure that if input validation fail, the request is rejected with an appropriate error HTTP response.

Access Control

Rate Limiting

• Make sure that rate limiting/throttling is applied to each API based on either per-session or per-IP or based on other properties that's relevant
• Limit requests (Throttling) to avoid DDoS / brute-force attacks.
• For private APIs, only allow access from whitelisted IPs/hosts.

Authentication and Authorization

• Avoid developing custom authentication protocols as much as possible
• Use an identity provider (IDP) and use open standards and frameworks such as OpenID Connect for user identity and OAuth to delegate authorisation to API resources
• Avoid HTTP basic authentication or OAuth 2.0 Client Credentials Grant for user authorisaion
• Use Authorizaion Code grant with PKCE (Proof Key for Code Exchange) instead of the Implicit Grant for Single Page Applications (SPAs)
• Don't use Basic Auth. Use standard authentication instead (e.g. JWT, OAuth).
• Don't reinvent the wheel in Authentication, token generation, password storage. Use the standards.
• Use Max Retry and jail features in Login.

JSON Web Token

• Use a random complicated key (JWT Secret) to make brute forcing the token very hard.
• Don't extract the algorithm from the header. Force the algorithm in the backend (HS256 or RS256).
• Make token expiration (TTL, RTTL) as short as possible.
• Don't store sensitive data in the JWT payload, it can be decoded easily.

OAuth

• Always validate redirect_uri server-side to allow only whitelisted URLs.
• Always try to exchange for code and not tokens (don't allow response_type=token).
• Use state parameter with a random hash to prevent CSRF on the OAuth authentication process.
• Define the default scope, and validate scope parameters for each application.

Security Configuration

• Use HTTPS on server side to avoid MITM (Man in the Middle Attack).
• Use HSTS header with SSL to avoid SSL Strip attack.
• Make sure that APIs are exposed through secure channels such as TLS
• Use encryption on all sensitive data.
• Make sure that debug logging or error messages are disabled in production deployments
• Make sure that monitoring and diagnostic endpoints provided by frameworks (e.g. Spring Boot Actuator) are either disabled or secured (HTTPS) and the exposure is controlled

Logging

• Do not log entire the HTTP request or the HTTP headers or the entire request body as they can potentially contain sensitive information
• Do not log user and system credentials
• Do not log user session information (Cookies, JWT tokens, etc)

Build

• Use third-party components that do not have vulnerabilities
• Make sure to incorporate security testing into CI/CD processes

Security Testing

• Make sure that static application security testing (SAST) is performed
• Make sure that software composition analysis (SCA) scanning is performed
• Make sure that dynamic application security testing (DAST) is performed

Processing

• Check if all the endpoints are protected behind authentication to avoid broken authentication process.
• User own resource ID should be avoided. Use /me/orders instead of /user/654321/orders.
• Don't auto-increment IDs. Use UUID instead.
• If you are parsing XML files, make sure entity parsing is not enabled to avoid XXE (XML external entity attack).
• If you are parsing XML files, make sure entity expansion is not enabled to avoid Billion Laughs/XML bomb via exponential entity expansion attack.
• Use a CDN for file uploads.
• If you are dealing with huge amount of data, use Workers and Queues to process as much as possible in background and return response fast to avoid HTTP Blocking.
• Do not forget to turn the DEBUG mode OFF.

Output

• Send X-Content-Type-Options: nosniff header.
• Send X-Frame-Options: deny header.
• Send Content-Security-Policy: default-src 'none' header.
• Remove fingerprinting headers - X-Powered-By, Server, X-AspNet-Version, etc.
• Force content-type for your response. If you return application/json, then your content-type response is application/json.
• Don't return sensitive data like credentials, Passwords, or security tokens.
• Return the proper status code according to the operation completed. (e.g. 200 OK, 400 Bad Request, 401 Unauthorized, 405 Method Not Allowed, etc.).

CI and CD

• Audit your design and implementation with unit/integration tests coverage.
• Use a code review process and disregard self-approval.
• Ensure that all components of your services are statically scanned by AV software before pushing to production, including vendor libraries and other dependencies.
• Design a rollback solution for deployments.