Multi-Cloud Networking Risks: Inconsistent Security Between AWS, Azure, GCP

The Illusion of Connectivity

Connecting workloads across Amazon Web Services (AWS), Microsoft Azure, and Google Cloud Platform (GCP) is often presented as a straightforward technical challenge. Indeed, connectivity options exist for all three major cloud providers. However, the reality is considerably more complex. Each platform's underlying networking model operates differently, meaning a design pattern that functions seamlessly within one cloud does not translate directly to others without deliberate adaptation and careful consideration.

The primary risk in multi-cloud networking is not the failure of any single platform's connectivity option. Instead, it lies in the inconsistency of how security and routing are applied across these distinct environments. Each cloud provider has genuinely different native models, leading to gaps at the seams between them. A security review focused on a single platform will invariably miss these inter-cloud vulnerabilities because it only examines one piece of the overall picture.

Divergent Networking Architectures

AWS Virtual Private Clouds (VPCs), Azure Virtual Networks (VNets), and GCP Virtual Private Cloud (VPC) networks are not merely differently named versions of the same concept. They represent fundamentally distinct approaches to network segmentation, resource isolation, and traffic management. Understanding these differences is paramount for building a secure and reliable multi-cloud infrastructure.

AWS VPCs:

• Subnets: VPCs are divided into subnets, which are segments of an IP address range. Subnets can be public or private, dictating whether instances within them can be directly accessed from the internet.
• Route Tables: Each subnet is associated with a route table that controls traffic flow. These tables define rules for directing network traffic to specific destinations.
• Security Groups & Network ACLs: Security Groups act as stateful firewalls at the instance level, controlling inbound and outbound traffic. Network Access Control Lists (NACLs) provide stateless firewalling at the subnet level.
• Gateways: Connectivity to the internet, on-premises networks, or other VPCs relies on various gateways, such as Internet Gateways, NAT Gateways, Virtual Private Gateways, and Transit Gateways.

Azure Virtual Networks (VNets):

• Subnets: Similar to AWS, Azure VNets are divided into subnets. Resources within a VNet are automatically assigned private IP addresses from the VNet address space.
• Network Security Groups (NSGs): NSGs function as stateful firewalls for VNets, controlling traffic to and from Azure resources. They are associated with network interfaces or subnets.
• User Defined Routes (UDRs): UDRs allow administrators to override Azure's default system routes, enabling traffic inspection appliances or custom routing configurations.
• Gateways: Azure offers various gateway types, including VPN Gateways for connecting to on-premises networks and Virtual Network Gateways for inter-VNet connectivity.

GCP VPC Networks:

• Global by Default: Unlike AWS and Azure, GCP VPC networks are global resources. Subnets within a GCP VPC are regional, but the network itself spans all GCP regions.
• Firewall Rules: GCP uses firewall rules that are applied globally to VPC networks. These rules are stateless and can be targeted by network tags or service accounts.
• Routes: GCP automatically creates routes for subnets and can manage custom static and dynamic routes.
• Network Connectivity: GCP provides options like Cloud VPN, Cloud Interconnect, and VPC Network Peering for connecting VPC networks and on-premises environments.

The Insecurity of Seams

The critical danger zone in multi-cloud networking emerges precisely because these distinct models must interoperate. When traffic moves from an AWS VPC to an Azure VNet, or from Azure to a GCP VPC, the security controls and routing decisions are layered and translated.

Consider a scenario where an application runs across instances in AWS and Azure. AWS security groups might permit traffic from a specific IP range, while Azure NSGs permit traffic from a different set. If the connection method between the two clouds (e.g., a VPN tunnel managed by a third-party appliance) allows traffic from a broader range, a security gap is created. The AWS security group might be correctly configured for its environment, and the Azure NSG correctly configured for its own, but the interplay between them, facilitated by the transport mechanism, could be less secure than either individual configuration implies.

This inconsistency is difficult to detect through standard, single-cloud security audits. A security team might meticulously review the firewall rules, NACLs, and security groups within AWS and find them compliant. They could perform a similar audit on Azure and find it equally secure. However, the combined posture, particularly at the connection points, might be inadvertently weakened. The problem is not that the tools are missing, but that the application of policies across disparate systems introduces complexity and potential misconfigurations that are hard to spot.