How can I implement least‑privilege Basic auth in Elm while refreshing expired credentials using OAuth 2.0?

Question

When building an Elm front‑end that calls a protected API, I want to use HTTP Basic authentication to convey only the minimal credentials required for each request, following a least‑privilege approach. However, Basic credentials can expire, and the API expects clients to obtain fresh tokens via an OAuth 2.0 refresh‑token flow before retrying the request.

I am uncertain how to detect a 401 response indicating expired credentials within Elm’s Http API, securely trigger the refresh process without exposing more privilege than necessary, and then retry the original request with the updated Authorization header. How should I structure this interaction in Elm?

Tasadduq Burney · Answer

To call a protected API from an Elm front‑end while keeping credentials minimal and handling expired tokens, follow these steps:

Send the request with Basic auth. Build the Authorization: Basic <base64(user:pass)> header for each outgoing request. This follows the least‑privilege principle because only the username/password needed for that call are transmitted.
Detect a 401 response. In the Http.Error case, check if the status code is 401. According to MDN, "The server responds to a client with a 401 (Unauthorized) response status and provides information on how to authorize with a WWW-Authenticate response header containing at least one challenge"【1†L1-L3】.
Trigger the OAuth 2.0 refresh‑token flow. From Elm, issue a POST to the token endpoint (client_id, grant_type=refresh_token, refresh_token, etc.) over HTTPS. The response yields a new access token (and possibly a new refresh token). MDN notes that apps "can also request new ID and access tokens for previously authenticated entities by using a refresh mechanism"【3†L1-L3】.
Update stored credentials. Replace the stored Basic auth username/password (or derive a new token‑based credential) with the freshly obtained access token. If the API expects Basic auth using a token as the username (e.g., token:x), encode accordingly.
Retry the original request. Resend the failed request with the updated Authorization header. Because the Authorization header is "usually, but not always, sent after the user agent first attempts to request a protected resource without credentials"【2†L1-L3】, retrying after a 401 is the standard pattern.
Handle repeated failures. If the refresh also fails (e.g., 400 or 401 from the token endpoint), surface an authentication error to the user and clear stored credentials to force a fresh login.

Verification: after each retry, confirm a 2xx status or the expected payload. Rollback: if the refresh fails, discard the new token and revert to the previous credential state, prompting the user to re‑authenticate.

How can I implement least‑privilege Basic auth in Elm while refreshing expired credentials using OAuth 2.0?

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