If it is in the course of litigation, it may constitute the crime of obstructing testimony and perjury.
If it is a fake star signature for sale, it should be an infringement of the right to name, and if the amount is particularly large, it can also be considered as fraud.
If someone else pretends to be my signature without my knowledge, I won't ratify it, I won't bear legal responsibility, and the signatory will bear the responsibility; If it is approved, I should bear the responsibility.
Signing on behalf of the agent is the behavior. If I don't know, I have no right to represent and I don't bear legal responsibility. However, if I know that someone else signed my name without denying it, or that I ratified it, the signature is binding on me and I should bear legal responsibility.
Legal basis: Article 172 of the Civil Law of People's Republic of China (PRC), where the actor has no power of agency, exceeds the power of agency or terminates the power of agency, and the counterpart has reason to believe that the actor has power of agency, the agency act is valid.