Continue Azure B2C user journey on authentication failure - azure-ad-b2c

Question

I am creating a custom user journey using Azure B2C Identity Experience Framework. My issue is that I want to continue the user journey when authentication fails. However, it appears that authentication failure is interpreted as an exception, which causes the journey to terminate.
This journey is intended to accommodate a just-in-time account migration process from a legacy idenity provider to B2C.
The flow that I am seeking to accomplish is:
Attempt authentication using a B2C sign in form
On authentication failure, query a REST API to determine if the user's email address exists in the legacy system
If email address exists, present user with a B2C signup form
Is this scenario even possible?

Answer 1

I am not sure if there is a way to continue on full authentication failure, but you might not need to do that if just checking the existence of the user's account is enough.
You can check if the entered username exists in B2C without attempting authentication. Setting the RaiseErrorIfClaimsPrincipalDoesNotExist metadata to false allows the B2C policy to continue if the user does not exist in the directory. You can then take the entered username and continue with other technical profiles.
I used the below snippet as a validation technical profile and if the object ID is found, I run the login-NonInteractive profile, if not, I run a custom authentication profile
<TechnicalProfile Id="AAD-UserReadUsingEmailAddress-NoError">
<Metadata>
<Item Key="Operation">Read</Item>
<Item Key="RaiseErrorIfClaimsPrincipalDoesNotExist">false</Item>
</Metadata>
<IncludeInSso>false</IncludeInSso>
<InputClaims>
<InputClaim ClaimTypeReferenceId="signInName" PartnerClaimType="signInNames.emailAddress" />
</InputClaims>
<OutputClaims>
<!-- Required claims -->
<OutputClaim ClaimTypeReferenceId="objectId" />
<OutputClaim ClaimTypeReferenceId="authenticationSource" DefaultValue="localAccountAuthentication" />
<OutputClaim ClaimTypeReferenceId="extension_isMigrated" DefaultValue="False" />
<OutputClaim ClaimTypeReferenceId="strongAuthenticationPhoneNumber" />
<!-- Optional claims -->
<OutputClaim ClaimTypeReferenceId="userPrincipalName" />
<OutputClaim ClaimTypeReferenceId="displayName" />
<OutputClaim ClaimTypeReferenceId="accountEnabled" />
<OutputClaim ClaimTypeReferenceId="otherMails" />
<OutputClaim ClaimTypeReferenceId="signInNames.emailAddress" />
<OutputClaim ClaimTypeReferenceId="email" PartnerClaimType="signInNames.emailAddress" />
</OutputClaims>
<IncludeTechnicalProfile ReferenceId="AAD-Common" />
</TechnicalProfile>