Application Gateway Security Goes Beyond WAF

Azure Application Gateway is a Layer 7 load balancer with built-in Web Application Firewall (WAF) capabilities. While many teams enable WAF and consider themselves protected, a truly hardened Application Gateway deployment requires attention to TLS configuration, backend security, network isolation, and comprehensive monitoring. This guide covers every hardening step.

Threat Landscape and Attack Surface

Hardening Azure Application Gateway requires understanding the threat landscape specific to this service. Azure services are attractive targets because they often store, process, or transmit sensitive data and provide control-plane access to cloud infrastructure. Attackers probe for misconfigured services using automated scanners that continuously sweep Azure IP ranges for exposed endpoints, weak authentication, and default configurations.

The attack surface for Azure Application Gateway includes several dimensions. The network perimeter determines who can reach the service endpoints. The identity and access layer controls what authenticated principals can do. The data plane governs how data is protected at rest and in transit. The management plane controls who can modify the service configuration itself. A comprehensive hardening strategy addresses all four dimensions because a weakness in any single layer can be exploited to bypass the controls in other layers.

Microsoft’s shared responsibility model means that while Azure secures the physical infrastructure, network fabric, and hypervisor, you are responsible for configuring the service securely. Default configurations prioritize ease of setup over security. Every Azure service ships with settings that must be tightened for production use, and this guide walks through the critical configurations that should be changed from their defaults.

The MITRE ATT&CK framework for cloud environments provides a structured taxonomy of attack techniques that adversaries use against Azure services. Common techniques relevant to Azure Application Gateway include initial access through exposed credentials or misconfigured endpoints, lateral movement through overly permissive RBAC assignments, and data exfiltration through unmonitored data plane operations. Each hardening control in this guide maps to one or more of these attack techniques.

Compliance and Regulatory Context

Security hardening is not just a technical exercise. It is a compliance requirement for virtually every regulatory framework that applies to cloud workloads. SOC 2 Type II requires evidence of security controls for cloud services. PCI DSS mandates network segmentation and encryption for payment data. HIPAA requires access controls and audit logging for health information. ISO 27001 demands a systematic approach to information security management. FedRAMP requires specific configurations for government workloads.

Azure Policy and Microsoft Defender for Cloud provide built-in compliance assessments against these frameworks. After applying the hardening configurations in this guide, run a compliance scan to verify your security posture against your applicable regulatory standards. Address any remaining findings to achieve and maintain compliance. Export compliance reports on a scheduled basis to satisfy audit requirements and demonstrate continuous adherence.

The Microsoft cloud security benchmark provides a comprehensive set of security controls mapped to common regulatory frameworks. Use this benchmark as a checklist to verify that your hardening effort covers all required areas. Each control includes Azure-specific implementation guidance and links to the relevant Azure service documentation.

Step 1: Enable WAF v2 in Prevention Mode

Detection mode logs threats but does not block them. Production deployments must use Prevention mode with the latest Core Rule Set (CRS).

# Create WAF policy with Prevention mode
az network application-gateway waf-policy create \
  --name waf-policy-prod --resource-group rg-network

az network application-gateway waf-policy policy-setting update \
  --policy-name waf-policy-prod --resource-group rg-network \
  --state Enabled --mode Prevention --max-request-body-size-in-kb 128

# Add CRS 3.2 rule set
az network application-gateway waf-policy managed-rule rule-set add \
  --policy-name waf-policy-prod --resource-group rg-network \
  --type OWASP --version 3.2

# Add Bot Manager rules
az network application-gateway waf-policy managed-rule rule-set add \
  --policy-name waf-policy-prod --resource-group rg-network \
  --type Microsoft_BotManagerRuleSet --version 1.0

Step 2: Enforce TLS 1.2 with Strong Cipher Suites

# Apply predefined SSL policy that enforces TLS 1.2+ and strong ciphers
az network application-gateway ssl-policy set \
  --gateway-name appgw-prod --resource-group rg-network \
  --policy-type Predefined --policy-name AppGwSslPolicy20220101

The AppGwSslPolicy20220101 policy enforces TLS 1.2 minimum and removes weak cipher suites like TLS_RSA_WITH_AES_128_CBC_SHA. You can also create a custom policy to specify exact cipher suites.

# Custom SSL policy for strict compliance
az network application-gateway ssl-policy set \
  --gateway-name appgw-prod --resource-group rg-network \
  --policy-type Custom --min-protocol-version TLSv1_2 \
  --cipher-suites TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256

Step 3: Enable End-to-End TLS

By default, Application Gateway terminates TLS and sends unencrypted traffic to backends. Enable end-to-end TLS to encrypt the backend connection as well.

# Upload backend trusted root certificate
az network application-gateway root-cert create \
  --gateway-name appgw-prod --resource-group rg-network \
  --name backend-root-cert --cert-file backend-ca.cer

# Configure backend HTTPS
az network application-gateway http-settings update \
  --gateway-name appgw-prod --resource-group rg-network \
  --name backend-https-settings --protocol Https --port 443 \
  --root-certs backend-root-cert

Step 4: Configure NSG on Application Gateway Subnet

# Create NSG for the Application Gateway subnet
az network nsg create --name nsg-appgw --resource-group rg-network

# Allow inbound from Internet on ports 80/443
az network nsg rule create --nsg-name nsg-appgw --resource-group rg-network \
  --name AllowHTTPS --priority 100 --direction Inbound \
  --source-address-prefixes Internet --destination-port-ranges 80 443 \
  --protocol Tcp --access Allow

# Allow Gateway Manager for health probes (required)
az network nsg rule create --nsg-name nsg-appgw --resource-group rg-network \
  --name AllowGatewayManager --priority 110 --direction Inbound \
  --source-address-prefixes GatewayManager --destination-port-ranges 65200-65535 \
  --protocol Tcp --access Allow

# Associate NSG with subnet
az network vnet subnet update --resource-group rg-network \
  --vnet-name vnet-prod --name snet-appgw --network-security-group nsg-appgw

Ports 65200-65535 are required for the Gateway Manager service tag. Blocking these ports will cause health probe failures and the gateway will become unhealthy.

Step 5: Enable DDoS Protection

# Create DDoS protection plan
az network ddos-protection create --name ddos-plan --resource-group rg-network

# Associate with VNet
az network vnet update --name vnet-prod --resource-group rg-network \
  --ddos-protection-plan ddos-plan --ddos-protection true

Identity and Access Management Deep Dive

Identity is the primary security perimeter in cloud environments. For Azure Application Gateway, implement a robust identity and access management strategy that follows the principle of least privilege.

Managed Identities: Use system-assigned or user-assigned managed identities for service-to-service authentication. Managed identities eliminate the need for stored credentials (connection strings, API keys, or service principal secrets) that can be leaked, stolen, or forgotten in configuration files. Azure automatically rotates the underlying certificates, removing the operational burden of credential rotation.

Custom RBAC Roles: When built-in roles grant more permissions than required, create custom roles that include only the specific actions needed. For example, if a monitoring service only needs to read metrics and logs from Azure Application Gateway, create a custom role with only the Microsoft.Insights/metrics/read and Microsoft.Insights/logs/read actions rather than assigning the broader Reader or Contributor roles.

Conditional Access: For human administrators accessing Azure Application Gateway through the portal or CLI, enforce Conditional Access policies that require multi-factor authentication, compliant devices, and approved locations. Set session lifetime limits so that administrative sessions expire after a reasonable period, forcing re-authentication.

Just-In-Time Access: Use Azure AD Privileged Identity Management (PIM) to provide time-limited, approval-required elevation for administrative actions. Instead of permanently assigning Contributor or Owner roles, require administrators to activate their role assignment for a specific duration with a business justification. This reduces the window of exposure if an administrator’s account is compromised.

Service Principal Hygiene: If managed identities cannot be used (for example, for external services or CI/CD pipelines), use certificate-based authentication for service principals rather than client secrets. Certificates are harder to accidentally expose than text secrets, and Azure Key Vault can automate their rotation. Set short expiration periods for any client secrets and monitor for secrets that are approaching expiration.

Step 6: Configure Custom Health Probes

# Create HTTPS health probe
az network application-gateway probe create \
  --gateway-name appgw-prod --resource-group rg-network \
  --name health-probe --protocol Https \
  --host-name-from-http-settings true \
  --path /health --interval 30 --threshold 3 --timeout 30 \
  --match-status-codes 200-399

Custom health probes detect backend failures faster than default probes. Use a dedicated /health endpoint that checks database connectivity and dependencies.

Step 7: Add Custom WAF Rules

# Block requests from specific countries (geo-filtering)
az network application-gateway waf-policy custom-rule create \
  --policy-name waf-policy-prod --resource-group rg-network \
  --name BlockCountries --priority 10 --action Block --rule-type MatchRule

# Block requests with SQL injection patterns in query strings
az network application-gateway waf-policy custom-rule create \
  --policy-name waf-policy-prod --resource-group rg-network \
  --name BlockSQLi --priority 20 --action Block --rule-type MatchRule

Step 8: Enable Diagnostic Logging

az monitor diagnostic-settings create \
  --name appgw-diag \
  --resource "/subscriptions/{sub}/resourceGroups/rg-network/providers/Microsoft.Network/applicationGateways/appgw-prod" \
  --workspace law-prod-id \
  --logs '[
    {"category":"ApplicationGatewayAccessLog","enabled":true},
    {"category":"ApplicationGatewayFirewallLog","enabled":true},
    {"category":"ApplicationGatewayPerformanceLog","enabled":true}
  ]'

Security Monitoring and Threat Detection

Hardening configurations are only effective if you can detect when they are bypassed, misconfigured, or degraded. Implement comprehensive security monitoring for Azure Application Gateway that covers authentication events, authorization decisions, configuration changes, and data access patterns.

Enable Microsoft Defender for Cloud and activate the relevant protection plan for this service type. Defender provides threat detection powered by Microsoft’s global threat intelligence, behavioral analytics that identify suspicious patterns, and just-in-time alerts when potential security incidents are detected. Review and triage Defender alerts daily, and integrate them into your security incident response workflow.

Configure Microsoft Sentinel to ingest logs from Azure Application Gateway and apply analytics rules that detect attack indicators. Common detection scenarios include brute force authentication attempts, access from unusual geographic locations, privilege escalation through role assignment changes, and data exfiltration through unusual data transfer patterns. Create custom analytics rules for scenarios specific to your environment, such as access outside of maintenance windows or modifications by unauthorized automation accounts.

Implement Azure Policy assignments that continuously monitor your resources for configuration drift from your hardened baseline. Use the audit effect to detect non-compliant resources and the deny effect to prevent the creation of resources that do not meet your security standards. Review policy compliance reports weekly and remediate any drift immediately, as configuration changes that weaken security controls may indicate either accidental misconfiguration or deliberate tampering.

Conduct tabletop exercises that simulate security incidents involving Azure Application Gateway. Walk through scenarios such as compromised credentials, data breach notification, ransomware attack, and insider threat. These exercises test your team’s ability to detect, contain, and recover from security incidents using the hardening controls and monitoring capabilities you have implemented. Document lessons learned and improve your security controls based on the gaps identified during the exercise.

Step 9: Use Rewrites to Add Security Headers

# Add security headers via rewrite rules
az network application-gateway rewrite-rule set create \
  --gateway-name appgw-prod --resource-group rg-network \
  --name security-headers

az network application-gateway rewrite-rule create \
  --gateway-name appgw-prod --resource-group rg-network \
  --rule-set-name security-headers --name add-hsts \
  --response-headers Strict-Transport-Security="max-age=31536000; includeSubDomains"

az network application-gateway rewrite-rule create \
  --gateway-name appgw-prod --resource-group rg-network \
  --rule-set-name security-headers --name add-csp \
  --response-headers Content-Security-Policy="default-src 'self'"

Step 10: Monitor and Alert

• Alert on UnhealthyHostCount > 0 — backend servers are failing health probes
• Alert on WAF blocked requests spike — potential attack in progress
• Alert on FailedRequests > threshold — application errors
• Forward WAF logs to Microsoft Sentinel for SIEM correlation
• Review Top 10 blocked rules weekly to tune WAF exclusions

Defense in Depth Strategy

No single security control is sufficient. Apply a defense-in-depth strategy that layers multiple controls so that the failure of any single layer does not expose the service to attack. For Azure Application Gateway, this means combining network isolation, identity verification, encryption, monitoring, and incident response capabilities.

At the network layer, restrict access to only the networks that legitimately need to reach the service. Use Private Endpoints to eliminate public internet exposure entirely. Where public access is required, use IP allowlists, service tags, and Web Application Firewall (WAF) rules to limit the attack surface. Configure network security groups (NSGs) with deny-by-default rules and explicit allow rules only for required traffic flows.

At the identity layer, enforce least-privilege access using Azure RBAC with custom roles when built-in roles are too broad. Use Managed Identities for service-to-service authentication to eliminate stored credentials. Enable Conditional Access policies to require multi-factor authentication and compliant devices for administrative access.

At the data layer, enable encryption at rest using customer-managed keys (CMK) in Azure Key Vault when the default Microsoft-managed keys do not meet your compliance requirements. Enforce TLS 1.2 or higher for data in transit. Enable purge protection on any service that supports soft delete to prevent malicious or accidental data destruction.

At the monitoring layer, enable diagnostic logging and route logs to a centralized Log Analytics workspace. Configure Microsoft Sentinel analytics rules to detect suspicious access patterns, privilege escalation attempts, and data exfiltration indicators. Set up automated response playbooks that can isolate compromised resources without human intervention during off-hours.

Continuous Security Assessment

Security hardening is not a one-time activity. Azure services evolve continuously, introducing new features, deprecating old configurations, and changing default behaviors. Schedule quarterly security reviews to reassess your hardening posture against the latest Microsoft security baselines.

Use Microsoft Defender for Cloud’s Secure Score as a quantitative measure of your security posture. Track your score over time and investigate any score decreases, which may indicate configuration drift or new recommendations from updated security baselines. Set a target Secure Score and hold teams accountable for maintaining it.

Subscribe to Azure update announcements and security advisories to stay informed about changes that affect your security controls. When Microsoft introduces a new security feature or changes a default behavior, assess the impact on your environment and update your hardening configuration accordingly. Automate this assessment where possible using Azure Policy to continuously evaluate your resources against your security standards.

Conduct periodic penetration testing against your Azure environment. Azure’s penetration testing rules of engagement allow testing without prior notification to Microsoft for most services. Engage a qualified security testing firm to assess your Azure Application Gateway deployment using the same techniques that real attackers would employ. The findings from these tests often reveal gaps that automated compliance scans miss.

Hardening Checklist

1. WAF v2 in Prevention mode with CRS 3.2 and Bot Manager
2. TLS 1.2+ with strong cipher suites only
3. End-to-end TLS encryption to backends
4. NSG on Application Gateway subnet (allow GatewayManager)
5. DDoS Protection Standard on VNet
6. Custom HTTPS health probes
7. Custom WAF rules for geo-filtering and specific threats
8. Diagnostic logs to Log Analytics
9. Security headers via rewrite rules (HSTS, CSP)
10. Alerts on unhealthy hosts, blocked requests, and failures

For more details, refer to the official documentation: What is Azure Application Gateway?.