Azure turns function 401 unauthorised HTTP response into 500 internal server error

Question

Azure turns function 401 unauthorised HTTP response into 500 internal server error

Alistair Young 56

I have an Azure function that calls an API and when running locally it works when the response from the API is 401 unauthorised. The same function when running in Azure doesn't get as far as handling the 401 as Azure takes over and raises 500 internal server error instead.

I can see this in application insights:

2023-09-01T11:01:59Z [Information] End processing HTTP request after 173.1307ms - 401

Running locally, the correct error response JSON from the function is displayed. Running in Azure nothing is displayed as Azure has taken over and thrown 500 internal server error instead of allowing the 401 to propagate to the handler code in the function.

Application insights shows this, which is not correct as it's 401 and not 500:

Response code 500 Successful request false

I checked and the HTTP status code is 401. Azure is throwing 500 internal server error instead of allowing the function deal with it.

Is this normal for Azure?

MayankBargali-MSFT 70,936 Reputation points Moderator

2023-09-07T13:53:01.0733333+00:00

Hi @Alistair Young

Following up to see if my answer and comment helps. Feel free to get back to me if you need any assistance.

Please "Accept Answer" if the answer is helpful so that it can help others in the community.
MayankBargali-MSFT 70,936 Reputation points Moderator

2023-09-11T04:46:10.6433333+00:00

Hi @Alistair Young

I hope the answer helps. Feel free to get back to me if you need any assistance.

Please "Accept Answer" if the answer is helpful so that it can help others in the community.

1 answer

Your answer

MayankBargali-MSFT 70,936 Reputation points Moderator

2023-09-07T13:53:01.0733333+00:00

Hi @Alistair Young

Following up to see if my answer and comment helps. Feel free to get back to me if you need any assistance.

Please "Accept Answer" if the answer is helpful so that it can help others in the community.
MayankBargali-MSFT 70,936 Reputation points Moderator

2023-09-11T04:46:10.6433333+00:00

Hi @Alistair Young

I hope the answer helps. Feel free to get back to me if you need any assistance.

Please "Accept Answer" if the answer is helpful so that it can help others in the community.

Answer 1

MayankBargali-MSFT 70,936 Moderator

@Alistair Young Thanks for reaching out.

I will suggest you to review the Diagnose and solve problems blade on your function app to know more details on the 500 error. Looks like there might be some dependency missing or your have configure some application setting in your local.setting.json which is not configured in your function app application setting blade which would cause this behavior. There could be multiple other reasons for the 500 error but reviewing the application insights logs/ diagnose and solve problem blade will give you more insights on the issue and the steps to resolve the issue if there are any.

In case if you need any assistance then please let me know.

Alistair Young 56 Reputation points

2023-09-01T11:21:23.23+00:00

when invoking the function directly azure throws 500 internal server error when it sees the 401 response from the external API.

Invoking the function through API Management works correctly and the error JSON is displayed.
Alistair Young 56 Reputation points

2023-09-01T11:53:54.3066667+00:00

thanks for that. I looked at the application insights and there was nothing to show why or where the 500 was being produced by Azure. I ran the function directly from Code/Test in the portal and saw the HTML response but Azure said it was 500 internal server error when the HTTP response code is 401.
Alistair Young 56 Reputation points

2023-09-01T12:20:46.1633333+00:00

it's the function error handler code that is transforming the 401 into a 500 and returning the error messages to the client. APIM allows this while Azure Functions does not. The Azure functions runtime intercepts this 500 and stops it reaching the client.
MayankBargali-MSFT 70,936 Reputation points Moderator

2023-09-04T14:29:58.2633333+00:00

@Alistair Young For your unhandled error/exception in your code the function should return 500 error. When you are not passing the authentication correctly to your Azure function app then function app return 401 error. But when you are calling any HTTP API (like APIM) then you need to return the response based on the API response. If you want 401 error to be returned, then you need to validate the StatusCode of your response and further return the status code from your function app as per your need. Can you share the same code that you are using? I can connect with your offline to review the logs as it the diagnosis and solve problem should give you full exception trace for the 500 error code. As it cannot be confirmed without reviewing the logs.

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Azure turns function 401 unauthorised HTTP response into 500 internal server error

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