CfnSecurityGroup

Specifies a security group. You must specify ingress rules to allow inbound traffic. By default, no inbound traffic is allowed. When you create a security group, if you do not add egress rules, we add egress rules that allow all outbound IPv4 and IPv6 traffic. Otherwise, we do not add them. After the security group is created, if you remove all egress rules that you added, we do not add egress rules, so no outbound traffic is allowed. If you modify a rule, CloudFormation removes the existing rule and then adds a new rule. There is a brief period when neither the original rule or the new rule exists, so the corresponding traffic is dropped. This type supports updates. For more information about updating stacks, see [AWS CloudFormation Stacks Updates](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/using-cfn-updating-stacks.html) . > To cross-reference two security groups in the ingress and egress rules of those security groups, use the [AWS::EC2::SecurityGroupEgress](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-security-group-egress.html) and [AWS::EC2::SecurityGroupIngress](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-security-group-ingress.html) resources to define your rules. Do not use the embedded ingress and egress rules in the `AWS::EC2::SecurityGroup` . Doing so creates a circular dependency, which CloudFormation doesn't allow.