California Code, Family Code - FAM § 3665

(a) A copy of the prior year's federal and state personal income tax returns shall be attached to the income and expense declaration of each party.

(b) A party shall not disclose the contents or provide copies of the other party's tax returns to anyone except the court, the party's attorney, the party's accountant, or other financial consultant assisting with matters relating to the proceeding, or any other person permitted by the court.

(c) The tax returns shall be controlled by the court as provided in Section 3552.