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Exceptions in MSAL.NET

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Exceptions in MSAL.NET are intended for app developers to troubleshoot and not for displaying to end-users. Exception messages are not localized.

The different types of exceptions

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Exception Description
MsalException Base class for MSAL exceptions.
MsalClientException Errors which occur in the library itself, for example an incomplete configuration.
MsalServiceException Represents errors transmitted by the token provider (Microsoft Entra ID). See Microsoft Entra errors. Service unavailable errors (e.g. HTTP 500), indicating a problem with the service, have the error code service_not_available
MsalUiRequiredException Special Microsoft Entra error which indicates that the user must interactively login.

No other exception is caught by MSAL. Any network issues, cancellations etc. are bubbled up to the application.

MSAL throws MsalClientException for things that go wrong inside the library (e.g. bad configuration) and MsalServiceException for things that go wrong service side or in the broker (e.g. a secret expired).

Common exceptions

  1. User cancelled authentication (public client only)

When calling AcquireTokenInteractive, a browser or the broker is invoked to handle user interaction. If the user closes this process or if they hit the browser back button, MSAL generates an MsalClientException with the error code authentication_canceled (MsalError.AuthenticationCanceledError).

On Android, this exception can also occur if a browser with tabs is not available.

  1. HTTP Exceptions

Developers are expected to implement their own retry policies when calling MSAL. MSAL makes HTTP calls to the Microsoft Entra service, and occasional failures can occur, for example the network can go down or the server is overloaded. HTTP 5xx status code responses are retried once.

Exception types

When processing exceptions, you can use the exception type itself and the ErrorCode member to distinguish between exceptions. The values of ErrorCode are constants of MsalError.

You can also have a look at the fields of MsalClientException, MsalServiceException, MsalUiRequiredException.

In the case of MsalServiceException, the error might contain a code which you can find in Authentication and authorization error codes.

MsalUiRequiredException

The "UI Required" is a specialization of MsalServiceException named MsalUiRequiredException. This means you have attempted to use a non-interactive method of acquiring a token (e.g. AcquireTokenSilent), but MSAL could not do it silently. this can be because:

  • you need to sign-in
  • you need to consent
  • you need to go through a multi-factor authentication experience.

To remediate, call an AcquireToken* method that prompts the user, for example AcquireTokenInteractive in public clients, redirect the user to login in websites or respond with a 401 in a web API.

Continuous Access Evaluation

See How to use Continuous Access Evaluation enabled APIs in your applications.

Handling Claim challenge exceptions in MSAL.NET

In some cases, when the Microsoft Entra tenant admin has enabled conditional access policies, your application will need to handle claim challenge exceptions. This will appear as an MsalServiceException which Claims property won't be empty. For instance if the conditional access policy is to have a managed device (Intune) the error will be something like AADSTS53000: Your device is required to be managed to access this resource or something similar.

To handle the claim challenge, you will need to use the WithClaims(String) method.

Retry policies

See Retry-Policy