As I hope everybody is well-aware, re-using the key for CBC-MAC and CBC is a really bad idea.
Now, there's also the paradigm to prepend the message length to the CBC-MAC (as "associated data") to ensure CBC-MAC is resistant against length-extension attacks.
Now my simple question is:
Does this simple length prepending also fix the security vulnerability with re-using the key?
For clarity: The attack scenario is a standard CCA(2) scenario, the message length is feed into CBC-MAC but not CBC and there's no intent (by me) to use or even implement this.
From what I can tell, this "fix" should indeed fix the issue, because it relies on the ciphertexts being used for CBC-MAC and CBC to be the same (at the same positions) and so this sounds secure.