Submitted by kirk-baird
A merkleRoot may only be used once in createSplit() since it is used as salt to the deployment of a SplitProxy.
The result is an attacker may front-run any createSplit() transaction in the mem pool and create another createSplit() transaction with a higher gas price that uses the same merkleRoot but changes the other fields such as the _collectionContract or _splitAsset().  The original transaction will revert and the user will not be able to send any more transaction with this merkleRoot.
The user would therefore have to generate a new merkle tree with different address, different allocations or a different order of leaves in the tree to create a new merkle root. However, the attack is repeateable and there is no guarantee this new merkle root will be successfully added to a split without the attacker front-running the transaction again.
The excerpt from createSplitProxy() shows the merkleRoot() being used as a salt.
As seems to be the case here if the transaction address does NOT need to be known ahead of time consider removing the salt parameter from the contract deployment.
Otherwise, if the transaction address does need to be known ahead of time then consider concatenating msg.sender to the merkleRoot. e.g.
sofianeOuafir (Joyn) confirmed and commented:
