What Is a “Long-Range Attack” and Is It Unique to PoS Systems?

A long-range attack is a type of attack where an attacker attempts to create an alternative, valid chain from the very beginning of the blockchain's history. It is a major concern primarily for Proof-of-Stake (PoS) systems.

In PoS, the cost to create this long chain is low because the attacker only needs the private keys of validators from the past, who may have long since spent their stake. In contrast, a PoW long-range attack is nearly impossible because it would require re-mining the entire chain's history, which is economically infeasible due to the massive energy cost.