Ahmad b. Harun (ra) narrated to us. He said: Muhammad b. `Abdullah narrated to us from his father from `Abdullah b. Ja`far b. Jami` from Ahmad b. Muhammad al-Barqi from Harun b. al-Jahm from al-Sadiq Ja`far b. Muhammad (as). He said
If a depraved person (fasiq) publicly declares his debauchery, then he has no sanctity and backbiting [him is permissible].