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API security checklist

A checklist for securing an API across authentication, access, input, processing and output.

Adapted from Shieldfy (API Security Checklist)4 min readPlaybook

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Use this checklist at design time, before each release, and when reviewing a pull request that adds an endpoint. Tick an item only when you have checked it in the code or the running service. If an item does not apply, write why next to it instead of leaving it blank.

1Authentication

  • Consider not using Basic Auth. Use standard authentication (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.
  • Use encryption on all sensitive data.

1.1JWT (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 payload. 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.

1.2OAuth

  • 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.

2Access

  • Limit requests (Throttling) to avoid DDoS / brute-force attacks.
  • Use HTTPS on server side to avoid MITM (Man in the Middle Attack).
  • Use HSTS header with SSL to avoid SSL Strip attack.

3Input

  • Use the proper HTTP method according to the operation: GET (read), POST (create), PUT/PATCH (replace/update), and DELETE (to delete a record), and respond with 405 Method Not Allowed if the requested method isn't appropriate for the requested resource.
  • Validate content-type on request Accept header (Content Negotiation) to allow only your supported format (e.g. application/xml, application/json, etc.) and respond with 406 Not Acceptable response if not matched.
  • Validate content-type of posted data as you accept (e.g. application/x-www-form-urlencoded, multipart/form-data, application/json, etc.).
  • Validate user input to avoid common vulnerabilities (e.g. XSS, SQL-Injection, Remote Code Execution, etc.).
  • Don't use any sensitive data (credentials, Passwords, security tokens, or API keys) in the URL, but use standard Authorization header.
  • 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.

4Processing

  • 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.

5Output

  • 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 response content-type 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.).

Source: API security checklist

6Example: baseline response headers

The output section asks for a few headers on every response. A response from a JSON API can look like this:

HTTP/1.1 200 OK
Content-Type: application/json
X-Content-Type-Options: nosniff
X-Frame-Options: deny
Content-Security-Policy: default-src 'none'
Strict-Transport-Security: max-age=31536000; includeSubDomains

Check them against a running service with curl -s -D - -o /dev/null https://api.example.com/v1/health, and confirm that Server and X-Powered-By are absent.