Microsoft Expands Secure Hostname Model to Functions and Logic Apps

Microsoft has completed the rollout of its Secure Unique Default Hostnames capability across all Azure App Service workloads, with general availability now extended to Azure Functions and Logic Apps (Standard). This expansion brings critical DNS security protections previously available only for Web Apps to serverless and workflow automation services, addressing long-standing subdomain takeover vulnerabilities.

Historically, Azure resources used predictable default hostnames following the <SiteName>.azurewebsites.net pattern. While simple, this approach created security gaps when customers deleted resources without cleaning up DNS records. Malicious actors could later recreate resources with the same name and hijack traffic intended for the original application, leading to potential data breaches or phishing attacks.

The new security model assigns cryptographically unique hostnames using the format <SiteName>-<Hash>.<Region>.azurewebsites.net. This region-scoped randomization ensures:

• No two resources can ever share identical default hostnames
• Automatic protection against dangling DNS entry exploits
• Consistent security baselines across all App Service workloads

For infrastructure teams, Azure CLI now supports explicit hostname scope configuration through the --domain-name-scope parameter during resource creation. This allows precise control over naming reuse boundaries at four levels: NoReuse, ResourceGroupReuse, SubscriptionReuse, or TenantReuse. Automation scripts creating Function Apps or Web Apps should incorporate this parameter immediately to enforce secure defaults.

While existing applications remain unaffected, Microsoft strongly recommends adopting this model for all new deployments due to:

1. Elimination of subdomain takeover attack vectors
2. Reduced operational overhead for DNS hygiene management
3. Future-proof alignment with Azure's evolving security architecture

For organizations evaluating cloud providers, this update highlights Azure's continued investment in foundational security automation. Unlike competitors who often treat such protections as premium add-ons, Microsoft builds these safeguards directly into core platform services at no additional cost. Teams migrating to Azure should factor this proactive security-by-default approach into platform evaluations.

Implementation requires updating deployment pipelines and CLI scripts to include the scope parameter. Documentation should be revised to reflect the new naming conventions. For technical details, refer to Microsoft's original announcement of the hostname security model and GA expansion details.