Chafetz Chaim, Part One, The Prohibition Against Lashon Hara, Principle 10
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וְכָל זֶה, (כה) כְּשֶׁאֵין עָלָיו אֵימָה מִן הָאִישׁ הַהוּא, שֶׁמְדַבֵּר עָלָיו, אֲבָל אִם יֵשׁ עָלָיו אֵימָה, שֶׁיֵּשׁ לְאֵל יָדוֹ לְהָרַע עִמּוֹ, אֶפְשָׁר שֶׁיֵּשׁ לְהָקֵל לְסַפֵּר עָלָיו הָעַוְלָה, שֶׁעָשָׂה לַחֲבֵרוֹ, אֲפִלּוּ שֶׁלֹּא בְּאַפֵּי תְּלָתָא.
Chofetz Chaim, Silverstein translation
And all this, when he is not in fear of the man he talks about; but if he is in fear of him, it being in his power to hurt him, it is possible that leniency may be exercised [to permit him] to relate the wrong that he [the accused] did to his friend, even not before three.