X0508 4 2025-03-18 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) 2025-03-18 4 A 0 1225 0 A Common Stock 1225 2655.373 D Deferred Stock (Deferred Compensation) 2025-03-20 4 A 0 344.9465 57.98 A Common Stock 344.9465 1715.0923 D The reporting person has received deferred stock units under the Community Financial System, Inc. 2022 Long-Term Incentive Plan, as amended (the "Plan"). 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 13.032, 11.2154, and 11.1256 units of phantom stock acquired as dividend equivalents on July 10, 2024, October 10, 2024, and January 10, 2025, respectively, under the Plan. 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 will be settled in Community Financial System, Inc. common stock at future dates selected by the reporting person. Includes 8.1166 units of phantom stock acquired as dividend equivalents on January 10, 2025 under the Deferred Compensation Plan. /s/ Danielle M. Cima, pursuant to Confirming Statement executed by Michele Sullivan 2025-03-20