How to use a VPC default security group as a peer for another security group?

Question

Using CDK (Typescript) I am trying to create a security group which can be accessed from my VPCs default security group.

I can't figure out the correct syntax:

    this.dbSecurityGroup = new SecurityGroup(this, 'db-security-group', {
      vpc: props.vpc,
      allowAllOutbound: false,
      securityGroupName: 'dbSecurityGroup',
    });

    this.dbSecurityGroup.connections.allowFrom(props.vpc.vpcDefaultSecurityGroup, Port.tcp(5432));

I get an error saying:

Argument of type 'string' is not assignable to parameter of type 'IConnectable'

This is because the props.vpc.vpcDefaultSecurityGroup is a string and not an IConnectable type.

What is the correct way to access/use the vpcDefaultSecurity value to access another security group?

$ cdk --version
1.42.0 (build 3b64241)

Answer 1

Turns out it easy to convert a security group id (string) to a security group type using:

    const vpcDefaultSecurityGroup = SecurityGroup.fromSecurityGroupId(this, "SG", props.vpc.vpcDefaultSecurityGroup);
    this.dbSecurityGroup.connections.allowFrom(vpcDefaultSecurityGroup, Port.tcp(5432));