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Unrestricted SSH Access

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Risk Level: Medium (should be achieved)
Rule ID: EC2-002

Check your Amazon EC2 security groups for inbound rules that allow unrestricted access (i.e. 0.0.0.0/0 or ::/0) on TCP port 22 and restrict the access to trusted IP addresses or IP ranges only in order to implement the Principle of Least Privilege (POLP) and reduce the attack surface. TCP port 22 is used for secure remote login by connecting an SSH client application with an SSH server.

This rule can help you with the following compliance standards:

  • CISAWSF
  • PCI
  • APRA
  • MAS
  • NIST4

For further details on compliance standards supported by Conformity, see here.

This rule can help you work with the AWS Well-Architected Framework.

This rule resolution is part of the Conformity Security & Compliance tool for AWS.

Security

Exposing the Secure Shell (SSH) port 22 to the Internet can increase opportunities for malicious activities such as hacking, Man-In-The-Middle attacks (MITM), and brute-force attacks, therefore it is strongly recommended to configure your Amazon EC2 security group rules to limit inbound traffic on TCP port 22 to known and trusted IP addresses only.

Audit

To determine if your Amazon EC2 security groups allow unrestricted SSH access, perform the following actions:

Using AWS Console

01 Sign in to the AWS Management Console.

02 Navigate to Amazon EC2 console at https://console.aws.amazon.com/ec2/.

03 In the navigation panel, under Network & Security, choose Security Groups.

04 Click inside the Filter security groups box located under the console top menu and select the following options from the Properties dropdown menu:

  1. Choose Protocol and select TCP from the protocols list.
  2. Choose Port range and select SSH from the port ranges list.

05 Select the security group that you want to examine and choose the Inbound rules tab from the console bottom panel to access the inbound rules created for the selected group.

06 Check the configuration value available in the Source column for any inbound/ingress rules with the Port range set to 22. If one or more rules have the Source value set to 0.0.0.0/0 or ::/0(i.e. Anywhere), the selected Amazon EC2 security group allows unrestricted traffic on TCP port 22, therefore the SSH access to the associated EC2 instance(s) is not secured.

07 Repeat steps no. 5 and 6 for each EC2 security group returned as result at step no. 4.

08 Change the AWS cloud region from the navigation bar and repeat the audit process for other regions.

Using AWS CLI

01 Run describe-security-groups command (OSX/Linux/UNIX) with predefined and custom query filters to expose the ID of each Amazon EC2 security group that allows unrestricted inbound access (from anywhere) on TCP port 22 (SSH): 

aws ec2 describe-security-groups
  --region us-east-1
  --filters Name=ip-permission.from-port,Values=22 Name=ip-permission.to-port,Values=22 Name=ip-permission.cidr,Values='0.0.0.0/0'
  --output table
  --query 'SecurityGroups[*].GroupId'

02 The command output should return a table with the requested security group ID(s): 

--------------------------
| DescribeSecurityGroups |
+------------------------+
|  sg-01234abcd1234abcd  |
|  sg-0abcd1234abcd1234  |
+------------------------+

If the describe-security-groups command does not produce an output, there are no security groups that allow unrestricted inbound access on TCP port 22 in the selected AWS region. If the command output returns a table with one or more security group IDs, those Amazon EC2 security groups allow unrestricted traffic on TCP port 22, therefore the SSH access to the associated EC2 instance(s) is not secured.

03 Change the AWS cloud region by updating the --region command parameter value and repeat steps no. 1 and 2 to perform the audit process for other regions.

Remediation / Resolution

To update the inbound rule configuration for your Amazon EC2 security groups in order to restrict SSH access to trusted entities only (i.e. authorized IP addresses and IP ranges, or other security groups), perform the following actions:

Using AWS CloudFormation

01 CloudFormation template (JSON): 

{
	"AWSTemplateFormatVersion": "2010-09-09",
	"Description": "Configure security group to restrict inbound SSH access to trusted entities only",
	"Resources": {
		"EC2SecurityGroup": {
			"Type": "AWS::EC2::SecurityGroup",
			"Properties": {
				"GroupName": "cc-instance-security-group",
				"GroupDescription": "Allow SSH access",
				"VpcId": "vpc-01234abcd1234abcd",
				"SecurityGroupIngress": [
					{
						"Description": "Allow inbound SSH traffic",
						"IpProtocol": "tcp",
						"FromPort": 22,
						"ToPort": 22,
						"CidrIp": "0.0.0.0/0"
						"CidrIp": "10.0.0.15/32"
					}
				],
				"SecurityGroupEgress": [
					{
						"Description": "Allow all outbound traffic",
						"IpProtocol": "-1",
						"FromPort": 0,
						"ToPort": 65535,
						"CidrIp": "0.0.0.0/0"
					}
				]
			}
		}
	}
}

02 CloudFormation template (YAML): 

AWSTemplateFormatVersion: '2010-09-09'
Description: Configure security group to restrict inbound SSH access to trusted entities only
Resources:
	EC2SecurityGroup:
	Type: AWS::EC2::SecurityGroup
	Properties:
		GroupName: cc-instance-security-group
		GroupDescription: Allow SSH access
		VpcId: vpc-01234abcd1234abcd
		SecurityGroupIngress:
		- Description: Allow inbound SSH traffic
			IpProtocol: tcp
			FromPort: 22
			ToPort: 22
			CidrIp: '0.0.0.0/0'
			CidrIp: '10.0.0.15/32'
		SecurityGroupEgress:
		- Description: Allow all outbound traffic
			IpProtocol: '-1'
			FromPort: 0
			ToPort: 65535
			CidrIp: '0.0.0.0/0'

Using Terraform (AWS Provider)

01 Terraform configuration file (.tf): 

terraform {
	required_providers {
		aws = {
			source  = "hashicorp/aws"
			version = "~> 4.0"
		}
	}
	required_version = ">= 0.14.9"
}
provider "aws" {
	profile = "default"
	region  = "us-east-1"
}
resource "aws_security_group" "ec2-security-group" {
	name        = "cc-instance-security-group"
	description = "Allow SSH access"
	vpc_id      = "vpc-01234abcd1234abcd"
	ingress {
		description      = "Allow inbound SSH traffic"
		from_port        = 22
		to_port          = 22
		protocol         = "tcp"
		cidr_blocks      = ["0.0.0.0/0"]
		cidr_blocks      = ["10.0.0.15/32"]
	}
	egress {
		description      = "Allow all outbound traffic"
		from_port        = 0
		to_port          = 0
		protocol         = "-1"
		cidr_blocks      = ["0.0.0.0/0"]
	}
}

Using AWS Console

01 Sign in to the AWS Management Console.

02 Navigate to Amazon EC2 console at https://console.aws.amazon.com/ec2.

03 In the navigation panel, under Network & Security, choose Security Groups.

04 Select the Amazon EC2 security group that you want to reconfigure (see Audit section part I to identify the right resource).

05 Select the Inbound rules tab from the console bottom panel and choose Edit inbound rules.

06 On the Edit inbound rules configuration page, change the traffic source for the inbound rule that allows unrestricted access through TCP port 22, by performing one of the following operations:

  1. Select My IP from the Source dropdown list to allow inbound traffic only from your current IP address.
  2. Select Custom from the Source dropdown list and enter one of the following options based on your access requirements:
    • The static IP address of the permitted host in CIDR notation (e.g. 10.0.0.5/32).
    • The IP address range of the permitted network/subnetwork in CIDR notation, for example 10.0.5.0/24.
    • The name or ID of another security group available in the same AWS cloud region.
  3. Choose Save rules to apply the configuration changes.

07 Repeat steps no. 4 – 6 to reconfigure other EC2 security groups that allow unrestricted SSH access.

08 Change the AWS cloud region from the navigation bar and repeat the remediation process for other regions.

Using AWS CLI

01 Run revoke-security-group-ingress command (OSX/Linux/UNIX) using the ID of the Amazon EC2 security group that you want to reconfigure as the identifier parameter (see Audit section part II to identify the right resource), to remove the inbound rules that allow unrestricted access on TCP port 22 (Secure Shell – SSH): 

aws ec2 revoke-security-group-ingress
  --region us-east-1
  --group-id sg-01234abcd1234abcd
  --ip-permissions IpProtocol=tcp,FromPort=22,ToPort=22,IpRanges=[{CidrIp="0.0.0.0/0"}],Ipv6Ranges=[{CidrIpv6="::/0"}]
  --query 'Return'

02 The command output should return true if the request succeeds. Otherwise, it should return an error: 

true

03 Run authorize-security-group-ingress command (OSX/Linux/UNIX) to add the inbound rule removed at the previous step with a different set of parameters in order to restrict access on TCP port 22 to trusted entities only (IP addresses, IP ranges, or security groups). To create and attach custom inbound/ingress rules to the selected Amazon EC2 security group based on your access requirements, use one of the following options (the command does not produce an output):

  1. Add an inbound rule that allows traffic from an authorized static IP address via TCP port 22, using CIDR notation (e.g. 10.0.0.5/32): 
    aws ec2 authorize-security-group-ingress
  --region us-east-1
  --group-id sg-01234abcd1234abcd
  --protocol tcp
  --port 22
  --cidr 10.0.0.5/32
  2. Add an inbound/ingress rule that allows traffic from a trusted IP address range via TCP port 22, using CIDR notation (for example, 10.0.5.0/24): 
    aws ec2 authorize-security-group-ingress
  --region us-east-1
  --group-id sg-01234abcd1234abcd
  --protocol tcp
  --port 22
  --cidr 10.0.5.0/24
  3. Add an inbound rule that allows traffic from another security group (e.g. sg-01234123412341234) available in the same AWS cloud region via TCP port 22: 
    aws ec2 authorize-security-group-ingress
  --region us-east-1
  --group-id sg-01234abcd1234abcd
  --protocol tcp
  --port 22
  --source-group sg-01234123412341234

04 Repeat steps no. 1 – 3 to reconfigure other EC2 security groups that allow unrestricted SSH access.

05 Change the AWS cloud region by updating the --region command parameter value and repeat steps no. 1 – 4 to perform the remediation process for other regions.

References

Publication date Jun 19, 2016

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