Revealing secrets is prohibited

Sanhedrin (31a) The mishna teaches: And from where is it derived that when the judge leaves the courtroom, he should not say: I deemed you exempt and my colleagues deemed you liable, but what can I do, as my colleagues outnumbered me and consequently you were deemed liable? About this it is stated: “You shall not go as a talebearer among your people” (Leviticus 19:16), and it says: “One who goes about as a talebearer reveals secrets, but one who is of a faithful spirit conceals a matter” (Proverbs 11:13). The Sages taught in a baraita: From where is it derived that when the judge leaves he should not say: I deemed you exempt and my colleagues deemed you liable, but what can I do, as my colleagues outnumbered me and consequently you were deemed liable? The verse states: “You shall not go as a talebearer among your people” (Leviticus 19:16), and it says: “One who goes about as a talebearer reveals secrets” (Proverbs 11:13)

The Gemara relates: There was a certain student, about whom a rumor emerged that he revealed a statement that was stated in the study hall and should have been kept secret, and the rumor emerged twenty-two years after the time the statement was revealed. Rav Ami removed him from the study hall as a punishment. Rav Ami said: This is a revealer of secrets and he cannot be trusted.

Rambam (Sanhedrin 22:7)אָסוּר לְאֶחָד מִן הַדַּיָּנִים כְּשֶׁיֵּצֵא מִבֵּית דִּין לוֹמַר אֲנִי הוּא הַמְזַכֶּה אוֹ הַמְחַיֵּב וַחֲבֵרַי חוֹלְקִין עָלַי אֲבָל מָה אֶעֱשֶׂה שֶׁהֵם רַבּוּ עָלַי וְאִם אָמַר כֵּן הֲרֵי הוּא בִּכְלַל הוֹלֵךְ רָכִיל מְגַלֶּה סוֹד. וּמַעֲשֶׂה בְּתַלְמִיד אֶחָד שֶׁהוֹצִיא דְּבָרִים שֶׁנֶּאֶמְרוּ בְּבֵית הַמִּדְרָשׁ לְאַחַר שְׁתַּיִם וְעֶשְׂרִים שָׁנָה וְהוֹצִיאוּהוּ בֵּית דִּין מִבֵּית הַמִּדְרָשׁ וְהִכְרִיזוּ עָלָיו זֶה מְגַלֶּה סוֹד הוּא:

Chofetz Chaim, (Introduction to the Laws of the Prohibition of Lashon Hara and Rechilut, Opening Comments 1) One who bears tales against his friend transgresses a negative commandment, viz. (Vayikra 19:16): "Do not go talebearing among your people." What is talebearing? "Loading oneself" with words and going from one to another, saying: "This is what ploni [so and so] said about you"; "This and this is what I heard ploni did to you." Although what he says may be true, it destroys the world. And there is a sin much greater than this — lashon hara, which is included in this negative commandment. And that is speaking disparagingly of one's friend, even if what is said is true. But one who speaks falsely [about his friend] is referred to as a "motzi shem ra" [one who spreads an evil report].