Function app has suddenly started returning 401 unauthorized

Question

Hi,

one of our Function Apps has suddenly started rejecting all incoming HTTP requests with a 401 Unauthorized response.

No deployment was done, no manual config change was done - the App access keys have not changed on the calling applications or on the function app.

Tried restarting, redeploying.

I've checked the other similar questions here, but removing the access key authentication is not an option as that's the main/only security feature on Function Apps.

Kind regards,
Martin

Answer 1

Hi @(SPUD) Martin Weiss ,

Thanks for the confirmation. Glad the issue is resolved.

I am summarizing the discussion and posting as answer.

Solution:

The issue appears to have resolved after completely tearing down and redeploying the Function App from scratch. After the redeployment, incoming HTTP requests were accepted and 401 Unauthorized responses no longer occurred.

In such cases, a sudden 401 on a Function App without any deployment or configuration changes can sometimes be caused by internal platform-level issues for example, corrupted configuration state or transient issues with app keys validation. Redeploying the Function App effectively resets the configuration state, which can resolve such issues.

If the issue reoccurs, you can you try adding the app setting ASPNETCORE_FORWARDEDHEADERS_ENABLED=true or

MICROSOFT_ASPNETCORE_HTTPOVERRIDES_IGNORE_UNKNOWN_PROXIES_WITHOUT_FOR=1 in the function app and run the function again.

Hope it helps!

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If you have any other questions, let me know in the "comments" and I would be happy to help you.

Answer 2

Hi Martin,

It sounds like you already resolved the issue by redeploying the Function App. From experience, sudden 401 errors on Function Apps when the keys haven’t changed are often caused by transient platform or key validation issues. Other things that can help prevent or troubleshoot this include:

Verify the X-Functions-Key in your request header.

Check Authentication/Authorization settings in the Function App.

Enable App Service logs or Application Insights to capture any silent failures.

Restarting or redeploying the Function App often resolves temporary platform glitches.

Since your redeployment fixed the issue, monitoring for recurrence is key. If it happens again, you can also test the function directly using a simple curl/Postman request with the function key to isolate the problem.