Unlike other cases of constructive purpose, protecting someone from possible harm is permissible even when no firsthand information is available. If one happens to hear that one Jew is plotting to harm another, it is permissible to advise the potential victim to beware of the suspected plotter. In such a case, where the suspicions are based on hearsay, the information must not be presented as fact. It should be clearly stated that the concerns stem from unconfirmed secondhand information that may or may not be true.
The Chofetz Chaim maintains that although one who fails to warn his felllow Jew of impending danger is violation of, “Do not stand aside while your fellow’s blood is being shed,” this does not apply when one is unable to personally verify that danger does, indeed, exist. Neverthless, it is proper to convey even secondhand information if this might spare someone harm.
Sefer Chofetz Chaim, 11 Kislev, Page 190