What do you mean by "don't do proof of work validation".
They don't verify the has in the header is the hash of the previous block's header? They don't that the hash is sufficiently small? They don't calculate the difficulty correctly?
They don't verify the has in the header is the hash of the previous block's header? They don't that the hash is sufficiently small? They don't calculate the difficulty correctly?
Yes. It's possible to create a long chain at low difficulty and the original bitcoin client protects against that by computing the total "work" that has gone in a chain. However, Electrum just assumes that the longest chain will be the one which has most "work" in it which is wrong.