A network security group is a service that allows the cloud administrator to filter and control the network traffic between different resources in a cloud environment. A network security group contains security rules that specify the source, destination, protocol, port, and direction of the traffic, and whether to allow or deny it. A network security group can be associated with a subnet or a network interface in a virtual machine, and it can apply to inbound or outbound traffic. A network security group would be the best way to achieve the objective of controlling the connections between a group of web servers and database servers as part of the financial application security review, as it can provide granular and flexible control over the network access and security of the servers.