What is the ruling on divulging a secret that one swore by Allah Almighty to keep, in order to prevent harm from befalling another person, and is expiation for breaking an oath (kaffarat yameen) required by fasting three days?

One must keep the secret entrusted to them. However, if divulging the secret prevents harm from others, there is no sin in revealing it with the intention of advising and warning, not slandering. If one has sworn to conceal the secret, then an expiation for breaking an oath (kaffarat yamin) is obligatory upon them.