API 03:2023 – Broken Object Property Level Authorization


API endpoints can be vulnerable to attacks based on their data: either they may expose more data than is required for their business purposes (excessive information exposure), or they may inadvertently accept and process more data than they should (mass assignment).ย 

Use case

  • The API returns full data objects as they are stored in the backend database.
  • The client application filters the responses and only shows the data that the users really need to see.
  • Attackers call the API directly and retrieve sensitive data that the UI would filter out.
  • The API works with the data structures without proper filtering.
  • Received payload is blindly transformed into an object and stored.
    • NodeJS:
      var user = new User(req.body);
    • Rails:
      @user = User.new(params[:user])
  • Attackers can guess the fields by looking at the GET request data.

How to prevent


  • Never rely on the client to filter data!
  • Review all API responses and adapt them to match what the API consumers really need.
  • Carefully define schemas for all the API responses.
  • Do not forget about error responses, define proper schemas as well.
  • Identify all the sensitive data or Personally Identifiable Information (PII), and justify its use using a robust data governance process.


  • Do not automatically bind incoming data to internal objects.
  • Explicitly define all the parameters and payloads you are expecting.
  • Use the readOnly property set to true in object schemas for all properties that can be retrieved through APIs but should never be modified.
  • Precisely define the schemas, types, and patterns you will accept in requests at design time and enforce them at runtime.

OWASP API Security Top 10 2023 list