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Description

GCP Firewall Rules are specific to a VPC Network. Each rule either allows or denies traffic when its conditions are met. Its conditions allow users to specify the type of traffic, such as ports and protocols, and the source or destination of the traffic, including IP addresses, subnets, and instances.

Firewall rules are defined at the VPC network level and are specific to the network in which they are defined. The rules themselves cannot be shared among networks. Firewall rules only support IPv4 traffic. When specifying a source for an ingress rule or a destination for an egress rule by address, an IPv4 address or IPv4 block in CIDR notation can be used. Generic (0.0.0.0/0) incoming traffic from the Internet to a VPC or VM instance using RDP on Port 3389 can be avoided.

Rationale​

GCP Firewall Rules within a VPC Network. These rules apply to outgoing (egress) traffic from instances and incoming (ingress) traffic to instances in the network. Egress and ingress traffic flows are controlled even if the traffic stays within the network (for example, instance-to-instance communication). For an instance to have outgoing Internet access, the network must have a valid Internet gateway route or custom route whose destination IP is specified. This route simply defines the path to the Internet, to avoid the most general (0.0.0.0/0) destination IP Range specified from the Internet through RDP with the default Port 3389. Generic access from the Internet to a specific IP Range should be restricted.

Impact​

All Remote Desktop Protocol (RDP) connections from outside of the network to the concerned VPC(s) will be blocked. There could be a business need where secure shell access is required from outside of the network to access resources associated with the VPC. In that case, specific source IP(s) should be mentioned in firewall rules to white-list access to RDP port for the concerned VPC(s).

Audit​

From Google Cloud Console​

  1. Go to VPC network.
  2. Go to the Firewall Rules.
  3. Ensure Port is not equal to 3389 and Action is not Allow.
  4. Ensure IP Ranges is not equal to 0.0.0.0/0 under Source filters.

From Google Cloud CLI​

        gcloud compute firewall-rules list --format=table'(name,direction,sourceRanges,allowed.ports)'

Ensure that there is no rule matching the below criteria:

• SOURCE_RANGES is 0.0.0.0/0

• AND DIRECTION is INGRESS

• AND IPProtocol is TCP or ALL

• AND PORTS is set to 3389 or range containing 3389 or Null (not set)

Note:

• When ALL TCP ports are allowed in a rule, PORT does not have any value set (NULL)

• When ALL Protocols are allowed in a rule, PORT does not have any value set (NULL)

References​

  1. https://cloud.google.com/vpc/docs/firewalls#blockedtraffic
  2. https://cloud.google.com/blog/products/identity-security/cloud-iap-enables-context-aware-access-to-vms-via-ssh-and-rdp-without-bastion-hosts

Additional Information​

Currently, GCP VPC only supports IPV4; however, Google is already working on adding IPV6 support for VPC. In that case along with source IP range 0.0.0.0, the rule should be checked for IPv6 equivalent ::/0 as well.