How Does a “Long Range Attack” Specifically Target Older PoS Systems?

A Long Range Attack exploits the fact that in some older PoS systems, the entire history of the blockchain is not economically secured. An attacker who sold their stake long ago can use their old keys to sign a fraudulent, alternative history of the chain starting from a very old block.

Since the network only tracks current stakers, it might accept this longer, old chain. Newer PoS systems mitigate this with "checkpointing" or "economic finality," which prevents reorgs beyond a certain, recent point in time.