Question: I have a security group assigned to an RDS instance which allows port 5432 traffic from our EC2 instances.

However, this security group has all outbound traffic enabled for all traffic for all IP's.

Is this a security risk? What should be the ideal outbound security rule?
In my perspective, the outbound traffic for the RDS security group should be limited to port 5432 to our EC2 instances, is this right?

Answer: 
By default, all Amazon EC2 security groups:

* Deny all **inbound** traffic
* Allow all **outbound** traffic

You must configure the security group to permit **inbound traffic**. Such configuration should be limited to the minimal possible scope. That is, the fewest protocols necessary and smallest IP address ranges necessary.

**Outbound access**, however, is traditionally kept open. The reason for this is that you would normally "trust" your own systems. If they wish to access external resources, let them do so.

You are always welcome to restrict Outbound access, especially for sensitive systems. However, determining which ports to keep open may be a challenge. For example, instances may want to download Operating System updates, access Amazon S3 or send emails.
When using **Security Goups** (as opposed to **ACL rules**) all inbound traffic is automatically allowed in outbound traffic so outbound rules may be empty in your case.

> 
> Is this a security risk? What should be the ideal outbound security
>  rule? In my perspective, the outbound traffic for the RDS security
>  group should be limited to port 5432 to our EC2 instances, is this
>  right?
> 
> 
> 

It's a risk only if you RDS is in a **public** subnet inside your VPC. 

Best practices recommend in your scenario to have a public subnet within your web server and a private subnet for all private resources (RDS, other private services, etc).

[![enter image description here](https://i.stack.imgur.com/A8Wny.png)](https://i.stack.imgur.com/A8Wny.png)

As you can see in the image, hosting your RDS inside a **private** subnet there is no way to access it from outside your VPC