What was the vulnerability of pNetwork? Hacker has stolen $12M today!

As you know, pNetwork is cross-chain protocol network.
https://p.network/
https://dapp.ptokens.io/
Today's breaking news: "Cross-Chain Protocol pNetwork Loses $12M to Hack"

What was the vulnerability that hacker found?
Let's discuss about this question to prevent our blockchain network from such malicious behaviors.
I think it is necessary to examine security of contracts in big market.