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Unrestricted Telnet Access

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Last updated: 13 April 2018
Security

Risk level: Medium (should be achieved)

Check your EC2 security groups for inbound rules that allow unrestricted access (i.e. 0.0.0.0/0 or ::/0) to TCP port 23 and restrict access to only those IP addresses that require it in order to implement the principle of least privilege and reduce the possibility of a breach. TCP port 23 is used by the Telnet server application (telnetd). Telnet is usually used to check whether a client is able to make TCP/IP connections to a particular service: https://goo.gl/r4tbwi

This rule resolution is part of the Cloud Conformity Base Auditing Package

Allowing unrestricted Telnet access can increase opportunities for malicious activity such as IP address spoofing, man-in-the-middle attacks (MITM) and brute-force attacks.

Audit

To determine if your EC2 security groups allow unrestricted Telnet access, perform the following:

Using AWS Console

01 Sign in to the AWS Management Console.

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

03 In the left navigation panel, under NETWORK & SECURITY section, choose Security Groups.

04 Click inside the attributes filter box located under the dashboard top menu and select the following options from the dropdown list:

  1. Choose Protocol and select TCP from the protocols list.
  2. Choose Port Range, type 23 as input for the port number then press Enter.

05 Select an EC2 security group returned as result.

06 Select the Inbound tab from the dashboard bottom panel.

07 Verify the value available in the Source column for any inbound/ingress rules with the Port Range set to 23. If one or more rules have the source set to 0.0.0.0/0 or ::/0 (Anywhere), the selected security group allows unrestricted traffic on port 23, therefore the Telnet access to the associated EC2 instance(s) is not secured.

08 Repeat steps no. 5 – 7 to verify the rest of the EC2 security groups returned as result at step no. 4.

09 Change the AWS 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) using the necessary filters to expose the security groups that have ingress rules that allow Telnet traffic (TCP port 23) from all addresses (0.0.0.0/0 or ::/0):

aws ec2 describe-security-groups
	--region us-east-1
	--filters Name=ip-permission.from-port,Values=23 Name=ip-permission.to-port,Values=23 Name=ip-permission.cidr,Values='0.0.0.0/0'
	--query 'SecurityGroups[*].{Name:GroupName}'
aws ec2 describe-security-groups
	--region us-east-1
	--filters Name=ip-permission.from-port,Values=23 Name=ip-permission.to-port,Values=23 Name=ip-permission.ipv6-cidr,Values='::/0'
	--query 'SecurityGroups[*].{Name:GroupName}'

02 The command output should return an array with the requested information. If the command does not return any output, there are no EC2 security groups that allow unrestricted Telnet access, otherwise it should return the name of the security group(s) that match filter criteria, as shown in the following example:

[
    {
        "Name": "MyWebEC2SecurityGroup"
    },
    {
        "Name": "EC2DBServerSecurityGroup"
    }
]

03 Repeat step no. 1 and 2 to perform the audit process for other AWS regions.

Remediation / Resolution

To update your security groups inbound/ingress configuration in order to restrict Telnet access to specific entities (IP addresses, IP ranges, etc), perform the following:

Using AWS Console

01 Sign in to the AWS Management Console.

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

03 In the navigation panel, under NETWORK & SECURITY section, choose Security Groups.

04 Select the appropriate security group (see Audit section to identify the right one(s)).

05 Select the Inbound tab from the dashboard bottom panel and click the Edit button.

06 In the Edit inbound rules dialog box, change the traffic Source for any inbound rules that allow unrestricted access through TCP port 23 by performing one of the following actions:

  1. Select My IP from the Source dropdown list to allow inbound traffic only from your machine (from your 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/Elastic IP address of the permitted host with the suffix set to /32, e.g. 54.164.53.109/32.
    • The IP address range of the permitted hosts in CIDR notation, for example 54.164.53.109/24.
    • The name or ID of another security group available in the same AWS region.

07 Click Save to apply the changes.

08 Repeat steps no. 4 – 7 to update other EC2 security groups that allow unrestricted Telnet access.

09 Change the AWS region from the navigation bar and repeat the process for other regions.

Using AWS CLI

01 First, run revoke-security-group-ingress command (OSX/Linux/UNIX) to remove the inbound rule(s) that allow unrestricted access through TCP port 23, from the selected EC2 security group (the command does not return an output):

aws ec2 revoke-security-group-ingress
	--region us-east-1
	--group-name MyWebEC2SecurityGroup
	--protocol tcp
	--port 23
	--cidr 0.0.0.0/0

02 Run authorize-security-group-ingress command (OSX/Linux/UNIX) to add the inbound rules removed at the previous step with a different set of parameters in order to restrict Telnet access to specific entities. To add custom inbound/ingress rules to the selected security group, use one of the following options (the command does not produce an output):

  1. Add an inbound rule that allows Telnet access to the specific static IP/Elastic IP address of the permitted host via port 23:
    aws ec2 authorize-security-group-ingress
    	--region us-east-1
    	--group-name MyWebEC2SecurityGroup
    	--protocol tcp
    	--port 23
    	--cidr 54.164.53.109/32
    
  2. Add an inbound rule that allows Telnet access to the IP address range of the permitted hosts via port 23:
    aws ec2 authorize-security-group-ingress
    	--region us-east-1
    	--group-name MyWebEC2SecurityGroup
    	--protocol tcp
    	--port 23
    	--cidr 54.164.53.109/24
    
  3. Add an inbound rule that allows Telnet access to another EC2 security group in the same AWS region via port 23:
    aws ec2 authorize-security-group-ingress
    	--region us-east-1
    	--group-name MyWebEC2SecurityGroup
    	--protocol tcp
    	--port 23
    	--source-group MyWebAppSecurityGroup
    

03 Repeat step no. 1 and 2 to update other EC2 security groups that allow unrestricted Telnet access using AWS CLI.

04 Repeat steps no. 1 - 3 to implement the entire process for other AWS regions.

References

Publication date Jun 19, 2016