The Second Department gives a black-letter lecture on the common interest privilege, in a legal malpractice setting.  Saint Annes Dev. Co. v Russ  2018 NY Slip Op 00451 [157 AD3d 919]
January 24, 2018  Appellate Division, Second Department holds that:

“The common-interest privilege is an exception to the traditional rule that the presence of a third party waives the attorney-client privilege (see Hyatt v State of Cal. Franchise Tax Bd., 105 AD3d 186, 205 [2013]; Aetna Cas. & Sur. Co. v Certain Underwriters at Lloyd’s, London, 176 Misc 2d 605, 611 [Sup Ct, NY County 1998], affd 263 AD2d 367 [1999]; In re Quigley Co., 2009 WL 9034027, *2-3, 2009 Bankr LEXIS 1352, *7-8 [SD NY, Apr. 24, 2009, No. 04-15739 (SMB)]). To fall within that exception, the privileged communication must be for the purpose of furthering a legal, as opposed to a commercial, interest common to the client and the third party (see Hyatt v State of Cal. Franchise Tax Bd., 105 AD3d at 205; Delta Fin. Corp. v Morrison, 69 AD3d 669 [2010]; U.S. Bank N.A. v APP Intl. Fin. Co., 33 AD3d 430, 431 [2006]). “The legal interest that those parties have in common must be identical (or nearly identical), as opposed to merely similar” (Hyatt v State of Cal. Franchise Tax Bd., 105 AD3d at 205; see United States v Doe, 429 F3d 450, 453 [3d Cir 2005]; F.D.I.C. v Ogden Corp., 202 F3d 454, 461 [1st Cir 2000]). Moreover, the communication must “relate to litigation, either pending or anticipated, in order for the exception to apply” (Ambac Assur. Corp. v Countrywide Home Loans, Inc., 27 NY3d 616, 620 [2016]; see Hyatt v State of Cal. Franchise Tax Bd., 105 AD3d at 205).”