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A subtle design limitation in Microsoft Azure’s Private Endpoint implementation is creating an unexpected denial-of-service (DoS) risk for cloud environments that rely heavily on Private Link for secure connectivity.
Azure’s DNS behavior with Private Endpoints and Private DNS zones across multiple virtual networks (VNETs) can break name resolution, causing outages even when the service and its public endpoint are still functioning.
The flaw “… could expose Azure resources to denial of service (DoS) attacks,” said researchers at Palo Alto Networks’ Unit 42.
Inside the Azure Private Endpoint DNS Issue
At the heart of this issue is how Azure prioritizes DNS resolution once a Private DNS zone is linked to a virtual network.
Private Endpoints are designed to keep traffic off the public internet by assigning a private IP to an Azure service and routing access through Azure’s backbone.
To make that work seamlessly, Azure relies on Private DNS zones — such as privatelink[.]blob[.]core[.]windows[.]net — to translate a service’s hostname into the correct private address.
In a standard deployment, that DNS zone includes the required records (typically A records) so workloads inside the connected network resolve the service name to the private endpoint IP instead of the public endpoint.
The trouble begins when the same Private DNS zone is extended across multiple VNETs, especially in hub-and-spoke or segmented environments where not every network has identical Private Endpoint coverage.
The failure pattern generally looks like this:
- A Private Endpoint is created in VNET2, and Azure generates or uses a Private DNS zone tied to that endpoint.
- The Private DNS zone is then linked to VNET1 to enable name resolution across networks.
- But VNET1 does not have a corresponding DNS A record for the target storage account (or other service).
Once that link is in place, Azure DNS in VNET1 will prefer the Private DNS zone for resolving that hostname.
If the record isn’t there, the lookup can fail with an NXDOMAIN-style response, meaning the name can’t be resolved at all.
The result is a denial-of-service (DoS) condition where workloads in VNET1 suddenly lose access — even though the underlying Azure resource is still healthy, the public endpoint hasn’t changed, and no firewall rules were touched.
That’s what makes this issue especially disruptive: the outage is caused entirely by DNS behavior, not by an attacker taking down the resource or modifying production settings on the service itself.
A simple change to VNET links or zone relationships can break connectivity across environments in seconds.
The researchers found this weakness typically shows up in three real-world scenarios:
- Accidental internal misconfiguration: Teams expand Private Endpoint coverage over time and unintentionally introduce DNS conflicts when linking zones across multiple networks.
- Third-party deployments: Security vendors or managed services may deploy Private Endpoints for scanning, monitoring, or integration purposes, unintentionally altering DNS behavior and disrupting production traffic flows.
- Malicious insider or compromised admin activity: An attacker with enough Azure permissions could weaponize Private Endpoint and DNS zone linking to intentionally block access — creating a DoS event without needing to flood traffic or exploit the target resource directly.
The impact can cascade beyond a single outage because Azure Storage underpins many services, including Azure Functions, CI/CD pipelines, and applications that depend on blobs for configuration and state.
If DNS failures block storage access, outages can cascade — functions fail, deployments stall, and systems relying on Key Vault secrets or container artifacts can break.
Mitigating Azure Private Endpoint DNS Failures
Organizations don’t have to accept this Private Link DNS risk as unavoidable.
With the right mix of DNS governance, access controls, and monitoring, teams can reduce the chance of resolution failures turning into outages.
- Enable “fallback to internet” (NxDomainRedirect) on private DNS zone VNET links to prevent NXDOMAIN failures from breaking resolution, while weighing the security tradeoff of allowing public endpoint fallback.
- Maintain complete and accurate private DNS A records for all Private Link–enabled resources across linked networks to avoid resolution gaps that can cause outages.
- Centralize and standardize private endpoint name resolution using Azure Private Resolver or custom DNS servers with conditional forwarding for privatelink.* zones in multi-VNET and hybrid environments.
- Restrict who can create private endpoints or modify private DNS zones and VNET links using least privilege RBAC, and add change control for DNS and Private Link configuration updates.
- Reduce blast radius by limiting private DNS zone linking to only the VNETs that require it and separating DNS zones by environment or workload where appropriate.
- Continuously audit and monitor private endpoint and private DNS changes using Azure Policy, resource graph queries, and alerting for risky link changes or NXDOMAIN spikes.
Collectively, these controls help organizations keep Private Link deployments secure while reducing the risk of DNS-driven outages.
Private Link Can Trigger DoS-Like Failures
This issue is a reminder that cloud security controls can introduce availability risks when they aren’t paired with disciplined DNS design and governance.
Azure Private Link reduces public exposure, but its “all-or-nothing” DNS behavior means a missing record or broad zone link can trigger DoS-like outages.
That balance between stronger isolation and operational resilience is why organizations are turning to zero-trust solutions that protect access without relying solely on network-level assumptions.
