Can a Double-Spend Attack Be Launched on a PoS Network?

Yes, a double-spend attack is possible, but it is executed differently. In PoS, an attacker would need to acquire a majority (e.g.

2/3rds) of the staked currency to finalize a fraudulent chain. This is called a "long-range attack" or "nothing-at-stake" attack.

However, PoS mechanisms often include "slashing" penalties to make such an attempt economically ruinous.