You have to know these about ripple wallet and secret key
TL;DR: It's possible to find out others' secret keys but it's so improbable that it's almost impossible. Make sure you have a good entropy source when generating your address/secret key. Use multi-sign https://ripple.com/build/how-to-multi-sign/ See the discussion here: https://www.xrpchat.com/topic/6054-questions-about-secret-keysripple-wallet-addresses/