A soft fork is some modification made in a cryptocurrency’s source code that doesn’t lead to the division of blockchain, as happens with a hard fork. After a soft fork, only the previous transactions and blocks become invalid. Nevertheless, if users don’t want they don’t need to upgrade all the system because old nodes will continue working. There are two kinds of soft forks, such as a miner-activated soft fork (MASF), which means that the new rules are accepted when the majority of miners’ upgrade, and a user-activated soft fork (UASF), when the appliance of new rules happens without the miners’ support. The recently-made protocol upgrade in Bitcoin is a great example of a soft fork. Also, one of the most popular examples is the Ethereum soft fork that was made to reset all transactions that had been made by a hacker in order to get the money back.