Bitcoin solves the biggest ever problem of digital currencies: "Double-spending"

However, there have been serious attempts to hack using double-spending ... as I read an article this summer confirming that cryptocurrency security firm ZenGo has identified exploiting a double-spending attack dubbed "BigSpender" to target multiple BTC wallets. BigSpender is exploiting a “Bitcoin Fee Exchange (RBF)” loophole to carry out the double-spending attack.

RBF is a standard way to allow users to undo a transaction that has not yet been confirmed, by sending another transaction that spends the same virtual currencies (but possibly a different destination) at a higher fee.