Check for Unrestricted DNS Access

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Risk level: High (not acceptable risk)

Ensure that Google Cloud VPC network firewall rules do not allow unrestricted access (i.e. 0.0.0.0/0) on TCP and UDP port 53 in order to reduce the attack surface and protect the virtual machine (VM) instances associated with these rules. TCP/UDP port 53 is used by the Domain Name System during DNS resolution (DNS lookup), when the requests are sent from clients to DNS servers or between DNS servers.

Security

Allowing unrestricted DNS access to your Google Cloud virtual machines (VMs) through VPC network firewall rules can increase opportunities for malicious activities such as Denial of Service (DoS) attacks and Distributed Denial of Service (DDoS) attacks. VPC firewall rules should be configured so that access to specific resources is restricted to just those hosts or networks that have a legitimate business requirement for access.


Audit

To determine if your Google Cloud VPC firewall rules allow unrestricted access on TCP and/or UDP port 53, perform the following actions:

Using GCP Console

01 Sign in to Google Cloud Management Console.

02 Select the Google Cloud Platform (GCP) project that you want to examine from the console top navigation bar.

03 Navigate to VPC Network dashboard at https://console.cloud.google.com/networking.

04 In the navigation panel, select Firewall, click inside the Filter table box, set Type to Ingress and Disabled to False, to list all the active ingress rules created for the resources inside the selected project.

05 Check the filtered list for any inbound rules with the Protocols / ports attribute set to tcp:53 or tcp:0-65535, udp:53 or udp:0-65535, Action to Allow, and Filters to IP ranges: 0.0.0.0/0. If one or more rules match the filter criteria, there are VPC network firewall rules that allow unrestricted access on TCP/UDP port 53, therefore the DNS access to the associated Google Cloud VM instances is not secured.

06 Repeat steps no. 2 – 5 for each GCP project deployed in your Google Cloud account.

Using GCP CLI

01 Run projects list command (Windows/macOS/Linux) using custom query filters to list the IDs of the Google Cloud Platform (GCP) projects currently available in your Google Cloud account:

gcloud projects list
	--format="table(projectId)"

02 The command output should return the requested GCP project identifiers (IDs):

PROJECT_ID
cc-project5-stack-123123
cc-web-platform-stack-112233

03 Run compute networks list command (Windows/macOS/Linux) using the ID of the GCP project that you want to examine as identifier parameter and custom query filters to describe the name of each VPC network created within the selected project:

gcloud compute networks list
	--project cc-project5-stack-123123
	--format="table(name)"

04 The command output should return the name(s) of the VPC network(s) created for the specified project:

NAME
cc-web-stack-network

05 Run compute firewall-rules list command (Windows/macOS/Linux) using the name of the VPC network that you want to examine as identifier parameter and custom filtering to list all the firewall rules defined for the selected Virtual Private Cloud (VPC):

gcloud compute firewall-rules list
	--filter network=cc-web-stack-network
	--sort-by priority
	--format=table"(name,disabled,direction,sourceRanges,allowed[].map().firewall_rule().list())"

06 The command output should return the requested information available for the existing VPC firewall rules:

NAME                   DISABLED   DIRECTION   SOURCE_RANGES     ALLOW
cc-web-allow-dns       False      INGRESS     ['0.0.0.0/0']     tcp:53
cc-web-allow-icmp      False      INGRESS     ['0.0.0.0/0']     icmp
cc-web-allow-ssh       False      INGRESS     ['0.0.0.0/0']     tcp:22

Check the compute firewall-rules list command output for any active firewall rules (i.e. DISABLED attribute set to False) with the DIRECTION set to INGRESS, SOURCE_RANGES set to ['0.0.0.0/0'], and ALLOW set to tcp:53 or tcp:0-65535, udp:53 or udp:0-65535. If one or more rules match the search criteria, there are VPC network firewall rules that allow unrestricted access on TCP/UDP port 53, therefore the DNS access to the associated Google Cloud VM instances is not restricted/secured.

07 Repeat step no. 5 and 6 for each VPC network created for the selected GCP project.

08 Repeat steps no. 3 – 7 for each GCP project deployed in your Google Cloud account.

Remediation / Resolution

To update your VPC network firewall rules configuration in order to restrict Domain Name System (DNS) access to trusted entities only (i.e. authorized IP addresses or IP ranges), perform the following actions:

Using GCP Console

01 Sign in to Google Cloud Management Console.

02 Select the Google Cloud Platform (GCP) project that you want to examine from the console top navigation bar.

03 Navigate to VPC Network dashboard at https://console.cloud.google.com/networking.

04 In the navigation panel, select Firewall to access the list of VPC firewall rules created for the resources within the selected project.

05 Click on the name of the rule that allows unrestricted inbound access on TCP/UDP port 53 (see Audit section part I to identify the appropriate firewall rule), then click on the Edit button from the dashboard top menu to access the rule configuration settings.

06 On the selected firewall rule configuration page, perform the following:

  1. Remove the non-compliant 0.0.0.0/0 IP address range from the Source IP ranges configuration box to deny unrestricted (public) inbound access on TCP/UDP port 53.
  2. Type the source IP address(es) or IP address range(s) into the Source IP ranges box to define the source for the incoming traffic on TCP/UDP port 53. The allowed IP address blocks must be specified in CIDR format (e.g. 192.168.1.0/24). The IP range(s) can include addresses inside your VPC network and outside your network. Source IP range(s) can be used to define sources both inside and outside Google Cloud Platform (GCP).
  3. Click Save to apply the configuration changes.

07 If required, repeat step no. 5 and 6 to reconfigure other VPC network firewall rules that allow unrestricted inbound access on TCP/UDP port 53.

08 Repeat steps no. 2 – 7 for each GCP project available within your Google Cloud account.

Using GCP CLI

01 Run compute firewall-rules update command (Windows/macOS/Linux) to reconfigure the VPC firewall rule that allows unrestricted inbound access on TCP port 53 (see Audit section part II to identify the appropriate firewall rule), by replacing the insecure 0.0.0.0/0 IP source range with a trusted, authorized IP address/IP range. The IP range(s) can include addresses available within your VPC network and outside your network. The allowed IP address blocks must be defined in CIDR format. You can specify a single value (e.g. 192.168.1.0/24) or a comma-separated list of multiple values (e.g. 192.168.1.0/32, 192.168.1.0/24):

gcloud compute firewall-rules update cc-web-allow-dns
	--allow tcp:53
	--source-ranges=192.168.1.0/24
	--description="Allows DNS access from authorized IPv4 address range"

02 The command output should return the ID of the reconfigured VPC firewall rule:

Updated [https://www.googleapis.com/compute/v1/projects/cc-project5-stack-123123/global/firewalls/cc-web-allow-dns].

03 If required, repeat step no. 1 and 2 to reconfigure other VPC network firewall rules that allow unrestricted inbound access on TCP/UDP port 53 (DNS).

04 Repeat steps no. 1 – 3 for each GCP project deployed in your Google Cloud account.

References

Publication date Apr 21, 2021

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You are auditing:

Check for Unrestricted DNS Access

Risk level: High