How does Azure App Service takeover work?

Q: How does Azure App Service takeover work?

Azure App Service web app names (app-name.azurewebsites.net) are shared in a global public namespace. If app-name is deleted, an attacker creates a new web app named app-name and adds your custom domain.

CRITICAL SEVERITYCLOUDCVSS 9.1Immediate (<90s)ID: microsoft-azure

Microsoft Azure App Service Dangling DNS & Takeover Fingerprint

An Azure App Service plan or slot was deleted while the DNS CNAME record remains active. Any Azure customer can create an App Service with the unallocated subdomain prefix and claim ownership.

DNS CNAME Delegation Rules

Target CNAME Delegation

*.azurewebsites.net / *.cloudapp.net / *.trafficmanager.net

DNS Resolution Indicator

CNAME points to *.azurewebsites.net. Azure edge returns HTTP 404 with 'Web Site not found'.

Expected HTTP Status Code

404 Not Found

Authoritative Response Token

Vulnerable Body Indicator String

“404 Web Site not found”

CLI Verification Command

curl -sI http://subdomain.example.com | grep '404 Web Site not found'

When SubdomainWatch scanner identifies this response on an unresolved CNAME target, an automated alert is triggered within 800ms.

Remediation Guide & Defensive Action Plan

Immediate Action Required: Delete the dangling CNAME record from your authoritative DNS zone or re-bind the custom domain to an active Azure App Service plan.

Defensive Verification Checklist:

Audit authoritative DNS zone records for any CNAME pointing to *.azurewebsites.net / *.cloudapp.net / *.trafficmanager.net.
Verify whether the target resource or bucket still exists under your organization's cloud tenant.
If decommissioned, delete the DNS record immediately to close the public exposure window.
Deploy a pre-merge IaC security gate (such as SubdomainWatch Takeover Guard) to intercept dangling records before pull requests merge.

Terraform Remediation Example

# Delete dangling Azure CNAME or recreate azurerm_app_service_custom_hostname_binding

Open Dedicated Microsoft Azure App Service Terraform Solution Guide →

ATTACK SURFACE DETECTION COMPARISON

Why Static DNS Scanners Miss Microsoft Azure App Service Takeovers

Passive DNS repositories like SecurityTrails and heavy scanners like Detectify record domain records retrospectively or trigger alert fatigue. SubdomainWatch conducts active recursive CNAME testing with zero false positives.

SubdomainWatch vs SecurityTrails →SubdomainWatch vs Detectify →All 5 Competitor Benchmarks →

Microsoft Azure App Service Takeover FAQs