What Is the Difference between a Soft Fork and a Hard Fork in Response to a Chain Reorganization?

A soft fork is a backward-compatible change to the protocol, meaning non-upgraded nodes still recognize the new blocks as valid, though they may not fully enforce the new rules. It is often used to implement minor security fixes.

A hard fork is a permanent, non-backward-compatible change that requires all nodes to upgrade. If a severe 51 percent attack causes a major chain reorganization, a community may choose to execute a hard fork to roll back the fraudulent transactions, effectively creating a new, separate chain that ignores the attacker's history.