Is one who describes a person as a liar, hypocrite, and possessing bad characteristics, and attacks him in front of those present in his absence, subject to sin, and is this considered backbiting?

Backbiting is speaking about your brother in a way he dislikes, and it is forbidden except in specific circumstances. Among these is warning against an innovator and clarifying his innovation, out of sincere advice for the sake of Allah, His Messenger, and the believers, and as a warning to Muslims. If the speech about an innovator is for this purpose and within these limits, there is no sin in it, and it is not considered part of forbidden backbiting. Rather, it is part of sincere advice, clarifying the truth, and warning against falling into innovation, without exceeding what the necessity calls for. Those who call themselves "Qur'aniyyun" (Qur'anists) are among the people of innovation, against whom and their innovation one must warn.