Secrets

A person is required to conceal secrets that his friend revealed to him in confidence, even if it is not slander, because revealing a secret can cause damage to its owner and canundermine his plans ... Furthermore, one who reveals secrets violates standards of privacy and the trust of the secret's owner and King Shlomo stated (Mishlei 20:19) "One who reveals secrets goes and slanders." This is to say that someone who cannot control himself from revealing secrets will become a slanderer.
Sha'arei Teshuva 3:225