It's not perfect, but I believe Coinbase uses a combination of cold (offline) storage for most of its coins and insurance for the rest. Also, as you probably already know, people should not hold large sums on exchanges if they can use secure their own keys (and wallets) instead.

> "Coinbase prioritizes the security of our customer's digital currency through a combination of online “hot storage” and offline “cold” storage. Coinbase maintains 98% or more of customer digital currency in cold storage, with the remainder in secure hot servers as necessary to serve the liquidity needs of our customers. All digital currency that Coinbase holds in its online hot storage is insured. If Coinbase were to suffer a breach of its online hot storage, the insurance policy would pay out to cover any customer funds lost as a result."

https://help.coinbase.com/en/coinbase/other-topics/legal-pol...