So after some searching, I kinda know the attack procedure of altering bit by bit. But I'm still confused how this is possible. If I understand it correctly, the attacker never has the key, or tries to iterate keys. So since the oracle can check the padding, I guess itself has the encryption key?
What is a real life oracle example I can see? I know this should be well patched nowadays, but what were affected before patching? I read that some kinds of web servers were impacted most, but what part of the server has this kind of oracle?