Explaining the OAuth2 Client ID

The OAuth2 Client ID is a unique identifier assigned to an application when it is registered with an OAuth2 authorization server. It is used to identify the application making the authorization request.

Key Purposes of Client ID

  1. Identifies the Application -
    • It tells the authorization server which app is requesting access.
    • It helps the server apply the right policies (e.g., scopes, rate limits).
  2. Public Identifier -
    • The Client ID is public and can be shared (e.g., in URLs or frontend code).
    • It is not a secret, unlike the Client Secret.
  3. Ties Authorization Requests to a Registered App -
    • It ensures that authorization requests come from a registered source.
    • It redirects users to a pre-registered redirect URI after authorization.

Example of a Client ID

1234567890-abcdefghijklmnopqrstuvwxyz.apps.example.com

Where It Is Used

  • In the Authorization Request: 
    https://auth.example.com/oauth2/authorize?
response_type=code&
client_id=YOUR_CLIENT_ID&
redirect_uri=https%3A%2F%2Fyourapp.com%2Fcallback&
scope=read_profile+write_data
  • In API Requests to identify the calling application.
  • In Mobile and Web Apps where a public identifier is needed.

    • Important Points

    • It is publicly exposed, so it should not be used to secure data.
    • In public clients (e.g., JavaScript or mobile apps), the Client ID is sufficient, but in confidential clients (e.g., server-side apps), a Client Secret is also required.
    • Each registered application gets a unique Client ID.

    Security Note

    • Since the Client ID is public, always secure your Client Secret (used only in confidential clients).