There hasn't been a secure decentralized PoS protocol. Mostly they rely on one or more trusted validators. Cardano's Ouroboros is apparently the first secure decentralized PoS, that is mathematically proven to be as secure as Bitcoin. I don't know if that's true, but that's what they say.[0]
I can't really understand their papers, but they've been published in peer-reviewed journals, so they're at least somewhat legit. And if it's a scam then someone should be able to attack it.
[0] Ouroboros - A Provably Secure Proof-of-Stake Blockchain Protocol https://eprint.iacr.org/2016/889.pdf