https://blog.positive.com/rewriting-history-a-brief-introduction-to-long-range-attacks-54e473acdba9

Proof of Stake protocols are in the spotlight as more and more high-profile blockchains attempt to switch over from Proof of Work protocols. Many of them explore the option of hybrid systems (PoW/PoS) while others aim for a pure PoS implementations.

One of the greatest threats against Proof of Stake protocols is Long Range attacks. Due to the existence of Weak Subjectivity and Costless Simulation, these attacks are more dangerous than in Proof of Work protocols.