On Wednesday, Solana, a popular blockchain and token, said that some wallets that held its assets had been breached. At least 7,700 such wallets are believed to be affected, the company said, while London-based blockchain-analysis firm Elliptic put the amount stolen at $5.2 million in crypto, which includes Solana tokens and the stablecoin known as USD Coin. The hack is believed to have taken hold on wallets such as Slope and Phantom. These are “hot wallets” — that is, wallets that allow for lightning-fast transactions because they are always connected to the internet, as opposed to “cold wallets,” which usually...