AWS Security Group Restricts Inter-SG Traffic

AnalysisType: policy
Filename: aws_security_group_restricts_inter_security_group_traffic.py
PolicyID: "AWS.SecurityGroup.RestrictsInterSecurityGroupTraffic"
DisplayName: "AWS Security Group Restricts Inter-SG Traffic"
Enabled: false
ResourceTypes:
  - AWS.EC2.SecurityGroup
Tags:
  - AWS
  - PCI
  - Lateral Movement:Exploitation of Remote Services
Reports:
  PCI:
    - 1.2.1
  MITRE ATT&CK:
    - TA0008:T1210
Severity: Low
Description: >
  This policy validates that Security Groups have restrictions on inter Security Group traffic. Administrators may assume there is an implicit level of trust between Security Groups in the same account, but this is not always a good assumption in cases one Security Group contains far more sensitive data that another.
Runbook: >
  Add appropriate restrictions to the Security Group peering connection.
Reference: https://docs.aws.amazon.com/cli/latest/userguide/cli-services-ec2-sg.html
Tests:
  - Name: Security Group Restricts Inter Security Group Traffic
    ExpectedResult: true
    Resource:
      {
        "Description": "example VPC security group",
        "GroupId": "sg-abc123",
        "GroupName": "default",
        "IpPermissions":
          [
            {
              "FromPort": 80,
              "IpProtocol": "-1",
              "IpRanges": null,
              "Ipv6Ranges": null,
              "PrefixListIds": null,
              "ToPort": 80,
              "UserIdGroupPairs":
                [
                  {
                    "Description": null,
                    "GroupId": "sg-def123",
                    "GroupName": null,
                    "PeeringStatus": null,
                    "UserId": "123456789012",
                    "VpcId": null,
                    "VpcPeeringConnectionId": null,
                  },
                ],
            },
          ],
        "IpPermissionsEgress":
          [
            {
              "FromPort": null,
              "IpProtocol": "-1",
              "IpRanges": [{ "CidrIp": "0.0.0.0/0", "Description": null }],
              "Ipv6Ranges": null,
              "PrefixListIds": null,
              "ToPort": null,
              "UserIdGroupPairs": null,
            },
          ],
        "OwnerId": "123456789012",
        "Tags": { "environment": "pci" },
        "VpcId": "vpc-abc111222333",
      }
  - Name: Security Group Does Not Restrict Inter Security Group Traffic
    ExpectedResult: false
    Resource:
      {
        "Description": "example VPC security group",
        "GroupId": "sg-abc123",
        "GroupName": "default",
        "IpPermissions":
          [
            {
              "FromPort": null,
              "IpProtocol": "-1",
              "IpRanges": null,
              "Ipv6Ranges": null,
              "PrefixListIds": null,
              "ToPort": null,
              "UserIdGroupPairs":
                [
                  {
                    "Description": null,
                    "GroupId": "sg-def123",
                    "GroupName": null,
                    "PeeringStatus": null,
                    "UserId": "123456789012",
                    "VpcId": null,
                    "VpcPeeringConnectionId": null,
                  },
                ],
            },
          ],
        "IpPermissionsEgress":
          [
            {
              "FromPort": null,
              "IpProtocol": "-1",
              "IpRanges": [{ "CidrIp": "0.0.0.0/0", "Description": null }],
              "Ipv6Ranges": null,
              "PrefixListIds": null,
              "ToPort": null,
              "UserIdGroupPairs": null,
            },
          ],
        "OwnerId": "123456789012",
        "Tags": { "environment": "pci" },
        "VpcId": "vpc-abc111222333",
      }
  - Name: Security Group Does Not Restrict Internet Traffic
    ExpectedResult: true
    Resource:
      {
        "Description": "example VPC security group",
        "GroupId": "sg-abc123",
        "GroupName": "default",
        "IpPermissions":
          [
            {
              "FromPort": null,
              "IpProtocol": "-1",
              "IpRanges": [{ "CidrIp": "0.0.0.0/0", "Description": null }],
              "Ipv6Ranges": null,
              "PrefixListIds": null,
              "ToPort": null,
              "UserIdGroupPairs": null,
            },
          ],
        "IpPermissionsEgress":
          [
            {
              "FromPort": null,
              "IpProtocol": "-1",
              "IpRanges": [{ "CidrIp": "0.0.0.0/0", "Description": null }],
              "Ipv6Ranges": null,
              "PrefixListIds": null,
              "ToPort": null,
              "UserIdGroupPairs": null,
            },
          ],
        "OwnerId": "123456789012",
        "Tags": { "environment": "pci" },
        "VpcId": "vpc-abc111222333",
      }


# ------ paired body: aws_security_group_restricts_inter_security_group_traffic.py ------

def policy(resource):

    if resource["IpPermissions"] is None:
        return True

    for permission in resource["IpPermissions"]:
        # Only check Security Group -> Security Group permissions
        if not permission["UserIdGroupPairs"]:
            continue
        # Check if the permission is set to "All Ports"
        if permission["FromPort"] is None or permission["ToPort"] is None:
            return False
        # Check if the permission is set to "All TCP" or "All UDP" ports
        if permission["FromPort"] == 0 and permission["ToPort"] == 65535:
            return False

    return True