The Flow network experienced a significant security breach on December 27, resulting in a $3.9 million loss due to a vulnerability in its execution layer. Despite the attack, user balances and deposits were not affected. Validators were unable to prevent the unauthorized transfers before they occurred. In response, a protocol fix has been implemented, and node operators are currently deploying a network upgrade. The network plans to roll back to a pre-exploit checkpoint to eliminate the malicious transactions, ensuring the integrity of the system moving forward.