X0508 4 2025-12-18 0000723188 COMMUNITY FINANCIAL SYSTEM, INC. CBU 0002009605 Sullivan Michele C/O COMMUNITY FINANCIAL SYSTEM, INC. 5790 WIDEWATERS PARKWAY DEWITT NY 13214 1 0 Phantom Stock (Deferred Stock Units) Common Stock 2711.5953 2711.5953 D Deferred Stock (Deferred Compensation) 2025-12-18 4 A 0 409.8698 60.69 A Common Stock 409.8698 3062.6708 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 12.7836, 20.6277, and 22.811 units of phantom stock acquired as dividend equivalents on April 10, 2025, July 10, 2025, and October 10, 2025, respectively, 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 13.3543, and 18.8141 units of phantom stock acquired on July 10, 2025, and October 10, 2025, respectively, under the DCP for Directors. /s/ Danielle M. Cima, pursuant to Confirming Statement executed by Michele Sullivan 2025-12-19