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Security group does not allow ingress from the Internet to TCP port 23 (Telnet)

ec2_securitygroup_allow_ingress_from_internet_to_tcp_port_telnet_23
Service: ec2Severity: high
OfficialBuilt-inVerified
Categories:internet-exposed
Resource Type:AwsEc2SecurityGroup

EC2 security groups are evaluated for rules that allow inbound Telnet on TCP 23 from the Internet (0.0.0.0/0 or ::/0).

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Risk

Public Telnet exposes cleartext credentials and remote shell access.

  • Brute-force and credential interception enable account takeover
  • Command execution enables data theft and lateral movement

This threatens confidentiality and integrity and can degrade availability through misuse.

Run this check with Prowler

Run in Prowler Cloud
Run
prowler aws --checks ec2_securitygroup_allow_ingress_from_internet_to_tcp_port_telnet_23

Recommendation

Remove rules permitting Internet access to TCP 23 from 0.0.0.0/0 or ::/0. Disable Telnet on hosts. Prefer SSH or SSM and apply least privilege network rules. Restrict admin access to trusted IPs, VPN, or private endpoints, and use defense in depth with NACLs and logging.

Remediation

CLI

aws ec2 revoke-security-group-ingress --group-id <SECURITY_GROUP_ID> --ip-permissions '[{"IpProtocol":"tcp","FromPort":23,"ToPort":23,"IpRanges":[{"CidrIp":"0.0.0.0/0"}],"Ipv6Ranges":[{"CidrIpv6":"::/0"}]}]'

Terraform

resource "aws_security_group" "<example_resource_name>" {
  vpc_id = "<example_resource_id>"

  ingress {
    from_port   = 23
    to_port     = 23
    protocol    = "tcp"
    cidr_blocks = ["10.0.0.0/8"] # CRITICAL: Restricts Telnet (23); do not use 0.0.0.0/0 or ::/0
  }
}

NativeIaC

# CloudFormation: restrict Telnet (port 23) from Internet
Resources:
  <example_resource_name>:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Restrict Telnet
      VpcId: <example_resource_id>
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 23
          ToPort: 23
          CidrIp: 10.0.0.0/8  # CRITICAL: Restricts Telnet (23) to internal range; removes Internet-wide (0.0.0.0/0) access

Other

1In the AWS console, go to VPC > Security Groups
2Select the affected security group and open Inbound rules
3Click Edit inbound rules
4Find any rule allowing TCP port 23 (Telnet) from 0.0.0.0/0 or ::/0
5Delete the rule or change Source to a specific trusted CIDR
6Save rules

Check Code

from prowler.lib.check.models import Check, Check_Report_AWS
from prowler.providers.aws.services.ec2.ec2_client import ec2_client
from prowler.providers.aws.services.ec2.ec2_securitygroup_allow_ingress_from_internet_to_all_ports import (
    ec2_securitygroup_allow_ingress_from_internet_to_all_ports,
)
from prowler.providers.aws.services.ec2.lib.security_groups import check_security_group
from prowler.providers.aws.services.vpc.vpc_client import vpc_client


class ec2_securitygroup_allow_ingress_from_internet_to_tcp_port_telnet_23(Check):
    def execute(self):
        findings = []
        check_ports = [23]
        for security_group_arn, security_group in ec2_client.security_groups.items():
            # Check if ignoring flag is set and if the VPC and the SG is in use
            if ec2_client.provider.scan_unused_services or (
                security_group.vpc_id in vpc_client.vpcs
                and vpc_client.vpcs[security_group.vpc_id].in_use
                and len(security_group.network_interfaces) > 0
            ):
                report = Check_Report_AWS(
                    metadata=self.metadata(), resource=security_group
                )
                report.resource_details = security_group.name
                report.status = "PASS"
                report.status_extended = f"Security group {security_group.name} ({security_group.id}) does not have Telnet port 23 open to the Internet."
                # only proceed if check "..._to_all_ports" did not run or did not FAIL to avoid to report open ports twice
                if not ec2_client.is_failed_check(
                    ec2_securitygroup_allow_ingress_from_internet_to_all_ports.__name__,
                    security_group_arn,
                ):
                    # Loop through every security group's ingress rule and check it
                    for ingress_rule in security_group.ingress_rules:
                        if check_security_group(
                            ingress_rule, "tcp", check_ports, any_address=True
                        ):
                            report.status = "FAIL"
                            report.status_extended = f"Security group {security_group.name} ({security_group.id}) has Telnet port 23 open to the Internet."
                            break
                else:
                    report.status_extended = f"Security group {security_group.name} ({security_group.id}) has all ports open to the Internet and therefore was not checked against the specific Telnet port 23."

                findings.append(report)

        return findings

References