Is it possible to customize authentication flow in Azure?

I need to customize standard authentication flow e.g. introduce validation and transformation of client_assertion form field. Here's how I authenticate:

POST https://some-url.com/oauth/token
Content-Type: application/x-www-form-urlencoded; charset=UTF-8

grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&
client_id=id
client_secret=secret
client_assertion=lots-of-chars-here

I know I can create an HTTP function triggered by TokenIssuanceStart event, however, there I cannot access client_assertion, so my question is whether there is an event fired on authentication request received or other means to intercept authentication and include my own logic into it?

Note that: It is not possible to customize authentication flow in Azure.

I agree with @juunas, it is not possible to access client_assertion elsewhere.

• It is handled by Microsoft Entra ID.
• And even the headers cannot be handled elsewhere.

As you noted, the TokenIssuanceStart event can be triggered to execute an HTTP function, but it does not provide access to the client_assertion. Unfortunately, there is no direct event that gives you access to the raw authentication request data.

• Microsoft Entra ID does not allow access to modify the authentication request, you can implement a separate API that the client calls before making the token request.

POST https://some-url.com/oauth/token
Content-Type: application/x-www-form-urlencoded; charset=UTF-8

grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer
client_id=id
client_secret=secret
client_assertion=lots-of-chars-here