We covered recently the Leaf-Node weakness and the different proposals that were discussed to address it.
Sergio Demian Lerner, from the RSK team and one of the participants in the discussion, went on presenting his idea for a new fix on his blog.
Recently a fix to the Bitcoin Merkle tree design weakness in the RSK’s bridge was built by making invalid SPV proofs whose internal hashes are valid Bitcoin transaction. While this solves the problem, it is by no means a “clean” solution: it creates false-negative cases (with very low probability) and it reduces verification efficiency.
64 bytetransaction, as inner nodes of the merkle tree are constructed by concatenation of two
32 byte SHA256'dhashes.