Is there a way to find out what the updates are changing so I can fix
the actual problem?
Not really. Microsoft does not provide detailed change logs for their updates that I am aware of. If you read the appropriate KBs and/or Security Bulletins they will often give you enough detail for you to make decisions on how the update may or may not effect your applications.
In this case you can read the CVEs that the update fixes which might give you more insight into why your authentication system in your application stopped working.
A couple of suggestions going forward:
- Make sure you are adequately testing Windows Updates before applying them to your Production systems. Make sure your Testing systems mirror your Production systems, both in terms of configuration and in code-base. Virtualization has lowered the barrier to setting up a tier-ed development environment so much. Just do it, you will thank yourself later.
- Don't just install updates willy-nilly. Make sure your operations team is not synchronizing everything under the sun, that they are talking to you about what updates they are planning on deploying and that there is some underlying business logic and risk decision about what updates you are installing where.
- Contact Microsoft Support. They are generally very good and very interested about diagnosing regression faults. This will be the quickest way forward.