X0508 4 2025-12-01 0000723188 COMMUNITY FINANCIAL SYSTEM, INC. CBU 0001485739 Whipple John F. C/O COMMUNITY FINANCIAL SYSTEM, INC. 5790 WIDEWATERS PARKWAY DEWITT NY 13214 1 0 Common Stock 2025-12-01 4 M 0 1370 A 8506.0302 D Phantom Stock (Deferred Compensation) 2025-12-01 4 M 0 1370 D Common Stock 1370 5707.3915 D On December 1, 2025, the reporting person's phantom stock units were settled for 1,370 shares of Community Financial System, Inc. common stock. 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 53.6874, 61.6877, 53.8395, and 59.5376 units of phantom stock acquired on January 10, 2025, April 10, 2025, July 10, 2025, and October 10, 2025, respectively, as dividend equivalents under the Plan. /s/ Danielle M. Cima, pursuant to a Confirming Statement executed by John F. Whipple, Jr. 2025-12-03