Link
The if block requires the data to be greater than equal to 4 bytes, equal to the onMessageInvocation selector and last but not the least for the target address to be a contract.
What an attacker could do to bypass this expected spec is to pre-compute an address for the destination chain and pass it in _message.to. He can pass gasLimit = 0 from source to only allow him to process the message on the destination. 
On the destination chain, the attacker can deploy his pre-computed contract address and call processMessage() with it from the constructor. For a chain (L2s/L3s) with no invocation delays, the proving + executing of the message data would go through in one single call.
When we arrive at the isContract check below on the _message.to address, we evaluate to false since the size of the contract during construction is 0. Due to this, the attacker can validly bypass the onMessageInvocation selector that is a requirement/single source of tx origination by the protocol for all transactions occurring from the bridge contract. This breaks a core invariant of the protocol.
