In an API, what is the difference between authentication and authorization?

Separate authentication and authorization in an API

Explains identity versus permission checks in API request handling and why both boundaries need server-side enforcement.

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Authentication proves who is calling the API, such as a user identified by a validated token. Authorization decides whether that authenticated caller can perform a specific action on a specific resource. A valid token is not enough by itself. The API still needs permission and ownership checks before returning or changing data.

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Identity, claims, roles, permissions, resource checks, 401 versus 403 behavior, and server-side enforcement.

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Authentication establishes who the caller is; authorization decides whether that caller may perform a particular action on a particular resource. Keeping those boundaries separate prevents a valid identity from becoming blanket access.
Authentication proves who is calling the API, such as a user identified by a validated token. Authorization decides whether that authenticated caller can perform a specific action on a specific resource. A valid token is not enough by itself. The API still needs permission and ownership checks before returning or changing data.
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I would map this to HTTP behavior. If the caller is not authenticated or the token is missing or invalid, the API usually returns 401. If the caller is authenticated but lacks the permission or resource access, the API usually returns . The authorization check should happen on the server even if the UI hides buttons, because clients can call endpoints directly.

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Treating a logged-in user as allowed to access every resource.
Relying on the frontend to enforce authorization.
Returning inconsistent 401 and 403 responses.
Putting business authorization rules only in controller comments or UI code.

In an API, what is the difference between authentication and authorization?

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