Conn. Gen. Stat. § 12-2e: Closing agreements.

(a) The Commissioner of Revenue Services may enter into an agreement, in writing, with any person relating to the liability of such person, or of the person or estate for whom such person acts, in respect of any tax payable to said commissioner for any taxable period.

(b) Such agreement shall be final and conclusive and, except upon a showing of fraud or malfeasance or misrepresentation of a material fact, (1) the case shall not be reopened as to the matters agreed upon or the agreement modified by any officer, employee or agent of the state of Connecticut, and (2) in any suit, action or proceeding, such agreement, or any determination, assessment, collection, payment abatement, refund or credit made in accordance therewith, shall not be annulled, modified, set aside or disregarded.