Exceptions

Exceptions in MSAL.NET

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

Client exceptions have an error code which you can use to handle the exception. The error codes are explained in the MsalError class.

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.

2. HTTP errors

MSAL uses HttpClient to make web calls. HttpClient can fail with exception when the network is unavailable. Otherwise, HttpClient calls do not fail. Public Client apps are encouraged check if an internet connection is available before calling AcquireToken* methods.

In cases where an response other than 200 OK is received, MSAL will throw an MsalServiceException and you can check the HttpStatuScode for details.

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 code might contain a code which you can find in Authentication and authorization error codes

MsalUiRequiredException

The "Ui Required" is proposed as 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.

The remediation is to call AcquireTokenInteractive

try
{ 
 app.AcquireTokenXXX(scopes, account)
   .WithYYYY(...)
   .ExecuteAsync()
}
catch(MsalUiRequiredException ex)
{
 app.AcquireTokenInteractive(scopes)
    .WithAccount(account)
    .WithClaims(ex.Claims)
    .ExcecuteAsync();
}

HTTP Exceptions

Developers are expected to implement their own retry policies when calling MSAL. MSAL makes HTTP calls to the AAD 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.

MSAL does not rethrow HTTP exceptions as MsalException.

See also Simple retry for errors with HTTP error codes 500-600 and Http 429 (Retry After)

Handling Claim challenge exceptions in MSAL.NET

In some cases, when the Azure AD 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 .WithClaim() method of PublicClientApplicationBuilder class as shown above.