A critical architectural weakness in Azure’s Private Endpoint deployments could allow both accidental and intentional denial of service (DoS) attacks against cloud resources.
The vulnerability stems from how Azure’s Private DNS zone resolution interacts with hybrid networking configurations, potentially affecting over 5% of Azure storage accounts and multiple critical services.
The Core Vulnerability
The issue arises from Azure Private Link’s binary design philosophy. When a Private Endpoint is created for an Azure resource such as a storage account, Key Vault, or CosmosDB instance, a corresponding Private DNS zone (e.g., privatelink.blob.core.windows.net) is automatically linked to the virtual network.
Azure’s DNS resolution engine prioritises this Private DNS zone, forcing all matching name resolutions through it rather than public DNS infrastructure.
The problem manifests when a Private Endpoint exists in one virtual network (VNET2) but other virtual networks (VNET1) require access to the same resource via its public endpoint.
If the Private DNS zone is linked or shared across virtual networks, Azure’s resolution logic forces VNET1 traffic through the Private DNS zone.
Since no DNS A record exists for the storage account in that zone, name resolution fails entirely effectively creating a denial of service condition for previously functioning workloads.
Attack Vectors and Scenarios
Three deployment scenarios enable this vulnerability:
Accidental (Internal): Network administrators deploy Private Endpoints for improved security, inadvertently linking Private DNS zones across multiple virtual networks through shared infrastructure.
Accidental (Vendor): Third-party vendors deploy Private Endpoints as part of security solutions or monitoring tools, cascading DNS resolution issues throughout the environment.
Malicious (Attacker): Threat actors with Azure environment access intentionally deploy Private Endpoints to disrupt service availability, particularly targeting high-value resources like Key Vaults or Function Apps.
This vulnerability creates cascading failures across Azure services. Denying service to storage accounts disrupts Azure Functions and subsequent application deployments.
DoS targeting Key Vaults prevents dependent processes from accessing encryption keys and secrets.
CosmosDB, Azure Container Registry, Function Apps, and OpenAI accounts all face similar exposure, as reported by paloaltonetworks.
Research indicates at least one susceptible resource exists across these service types in most Azure environments, representing widespread organizational risk.
Detection and Mitigation Strategy
Azure Resource Graph Queries: Security teams can deploy predefined Resource Graph Explorer queries to identify vulnerable configurations:
- Virtual networks linked to Private DNS zones without corresponding resources
- Storage accounts with public endpoint access but no Private Endpoint connectivity
- Resources allowing public network access with network ACL restrictions
Microsoft acknowledges this issue as a known limitation. Two partial solutions exist: enabling fallback-to-internet DNS resolution when creating virtual network links (though this contradicts Private Link’s security objective), or manually adding DNS A records for affected resources in Private DNS zones (operationally intensive for production environments).
Organizations should conduct comprehensive Azure environment scanning using Resource Graph queries to map Private Link configurations.
Implement fallback DNS resolution where appropriate, combine it with manual DNS record management for critical resources, and monitor network logs for failed DNS resolutions indicating potential attacks.
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