X0609 4 2026-06-18 0000723188 COMMUNITY FINANCIAL SYSTEM, INC. CBU 0002009605 Sullivan Michele false C/O COMMUNITY FINANCIAL SYSTEM, INC. 333 BUTTERNUT DRIVE SYRACUSE NY 13214 1 0 Phantom Stock (Deferred Stock Units) Common Stock 3973.0866 3973.0866 D Deferred Stock (Deferred Compensation) 2026-06-18 4 A 0 406.1888 61.24 A Common Stock 406.1888 3959.8489 D The reporting person has received deferred stock units under the Community Financial System, Inc. 2022 Long-Term Incentive Plan, as amended (the "LTIP"). Each phantom stock unit which represents a deferred stock unit is the economic equivalent of one share of Community Financial System, Inc. common stock and will be settled in common stock at a predetermined date. Includes 20.5647 units of phantom stock acquired as dividend equivalents on April 10, 2026 under the LTIP. Each unit of phantom stock is the economic equivalent of one share of Community Financial System, Inc. common stock. The units of phantom stock relating to the Deferred Compensation Plan for Directors (the "DCP for Directors"), will be settled in Community Financial System, Inc. common stock at future dates selected by the reporting person. Includes 23.2041 units of phantom stock acquired as dividend equivalents on April 10, 2026 under the DCP for Directors. /s/ Danielle M. Cima, pursuant to Confirming Statement executed by Michele Sullivan 2026-06-18