Can you explain malleability issue, please?
This is what I read here: https://qz.com/175565/why-nobody-can-withdraw-bitcoins-from-one-of-the-currencys-largest-exchanges/
"When a transaction record is sent out to the network of bitcoin miners, it’s possible for a hacker to tweak it enough to change the hash, but not enough to make the transfer invalid. One of the ways the hacker can do this is by slightly modifying the digital signature in a way that it is still recognized as the right signature, but, again, results in a completely different hash."
I can't understand how, say, Bob can change Alice's signature in a Alice->Bob transaction if that signature doesn't belong to him in the first place.