Introduction Today we're covering some basic but very important elements of AWS Security -- Security Groups and Network ACLs. Security Groups Security Groups are a fundamental security feature in AWS. They help protect EC2 instances, Lambda scripts, Load Balancers, containers (Amazon ECS), AWS Transfer Family, and much more. They are the "last line" of defense in a lot of cases as well, so they're important to understand and get right. Security Groups operate similarly to a firewall by limiting the flow of traffic. Security Groups do this to and from whatever they are attached to (technically they attach to network interfaces but those are attached to the things in AWS you're looking to protect). By default, Security Groups allow all outbound traffic and deny all inbound traffic. You then create or delete rules, but any rules created are Allow rules. In other words, there are no Deny rules. Access is granted to a specific CIDR range (e.g. 192.168.1.10/32, 192.168.