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My typical cloud server hardening includes blocking ports at the UFW level on the server as well as at the AWS security group level.

I recently learned that Docker overwrites your IPTables. Given that, I had previously blocked a port x with UFW, but also published port x with Docker, which I believe negates my UFW status.

Did my AWS security group settings still block this port x from being exposed to the world?

2 Answers 2

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The AWS security groups are independent from your server settings. If you had IPTables open to everything but a AWS security group blocking port 80, port 80 will be blocked.

Therefor, the AWS security groups will still block port x if configured to do so even if the IPTable rules allow the same port.

I recommend you read here about the differences.

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AWS Security Group is a Allowed List based host firewall (attached to ENI). Therefore, if you have not added a firewall rule to allow traffic to this specific port (+ from specific IP range) in your AWS Security Group(s) attached to the relevant resource, AWS SG will block that port.

From https://docs.aws.amazon.com/vpc/latest/userguide/VPC_SecurityGroups.html

"Characteristics of security group rules

  • You can specify allow rules, but not deny rules.

...

  • When you associate multiple security groups with a resource, the rules from each security group are aggregated to form a single set of rules that are used to determine whether to allow access."

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