You are not quite right. The fact that the child is born in marriage proves paternity of your ex-wife’s husband. According to Art. 48 part 2 of the Family Code of the Russian Federation in case a child is born in marriage, mother’s husband is considered to be child’s father unless otherwise proved. Paternity of child’s mother’s husband is identified by marriage registration record.

Your ex-wife can register you as child’s father only in case she conceals a certificate of her new marriage and presents the divorce certificate. Such situation is unlikely and easily contested as you know about her new marriage.

Art. 52 part 2 of the Family Code of the Russian Federation is not applicable to you as child’s father in this case will be registered not in accordance with Art. 51 part 2 when child’s parents are not married and information about the father shall be entered on the basis of parent’s mutual application.

And finally, according to Art. 17 part 1 of the Law of the Russian Federation “About Certificates of Civil Statutes”, married mother and father are registered as parents on the basis of application of any of them. Information about child’s mother is entered on the basis of medical documents, and information about child’s father is entered on the basis of a marriage certificate.

Thus I do not see any problem here.