Tampering with Contents Before MAC Is there a way to modify the contents of data being sent between two parties in such a way that the MAC verification algorithm will not detect the tampering?
Not if


a secure MAC is chosen (e.g., HMAC-SHA256, CBC-MAC)
the MAC is used correctly (e.g., not reusing a CBC-MAC key for encryption), and
the secret MAC key is unknown by the attacker


That's literally the entire point of a MAC.