I'm not sure what you mean by fake transactions, but transactions are verified with public key cryptography, and a block with invalid transactions will be rejected out of hand by blockchain nodes.
There are two* major classes of attack which someone could do with the power to mint blocks at will (i.e. a 51% attack):
- discriminatory filtering, where valid transactions are
never validated
- double spends, where a transaction appears like it's
been validated for a time but is then rolled back and
becomes invalid
But no one can mint transactions without private keys.
* They can also monopolize the rewards of mining, but that's not nearly as bad as the other issues.
There are two* major classes of attack which someone could do with the power to mint blocks at will (i.e. a 51% attack):
But no one can mint transactions without private keys.* They can also monopolize the rewards of mining, but that's not nearly as bad as the other issues.