A Standard-SKU Application Gateway load-balances attacks as happily as legitimate traffic

Why it happens

The web application firewall on Application Gateway is not a feature you switch on later in a settings blade; it is a SKU tier chosen at creation, and Standard_v2 is the cheaper, more obvious pick. Teams choosing a SKU are usually thinking about load balancing and TLS termination, not attack inspection, so Standard wins and the decision is never revisited.

Sometimes it is a deliberate architecture call (inspection handled elsewhere, or a cost decision made with eyes open). But often nobody decided anything: the gateway works, traffic flows, and the absence of a WAF produces no error, no warning, and no line item to question. To be clear about severity: this is a defense-in-depth gap, not an open door by itself; an attacker still needs an exploitable backend. The gateway just does nothing to stop them looking for one.

What it costs / blast radius

Every request an attacker sends — SQL injection probes, XSS payloads, path traversal, known-CVE exploit attempts — is TLS-terminated, health-checked, and delivered to your backend with the same care as a paying customer's. Your applications' own input handling is the only defense layer left. (Azure behavior; authored assessment.)

The fix has a real, known price: at East US list price the WAF_v2 fixed charge is about $0.443/hour versus $0.246/hour for Standard_v2, roughly $144/month more before capacity units, which also bill higher ($0.0144 versus $0.008 per capacity-unit hour). (List prices; your region and contract may differ.) That is the honest trade: a modest, predictable spend for managed OWASP rule filtering at the edge. The upgrade is an in-place SKU change on v2, not a rebuild.

See it

resource appgw 'Microsoft.Network/applicationGateways@2024-05-01' = {
  name: 'corp-appgw'
  location: location
  properties: {
    sku: {
      name: 'Standard_v2'
      tier: 'Standard_v2'   // no WAF: nothing inspects request payloads
    }
    // ...listeners, backend pools, routing rules
  }
}

resource wafPolicy 'Microsoft.Network/ApplicationGatewayWebApplicationFirewallPolicies@2024-05-01' = {
  name: 'corp-appgw-waf'
  location: location
  properties: {
    policySettings: {
      state: 'Enabled'
      mode: 'Prevention'    // see AZF-0091: Detection mode blocks nothing
    }
    managedRules: {
      managedRuleSets: [
        {
          ruleSetType: 'OWASP'
          ruleSetVersion: '3.2'
        }
      ]
    }
  }
}

resource appgw 'Microsoft.Network/applicationGateways@2024-05-01' = {
  name: 'corp-appgw'
  location: location
  properties: {
    sku: {
      name: 'WAF_v2'
      tier: 'WAF_v2'
    }
    firewallPolicy: {
      id: wafPolicy.id
    }
    // ...listeners, backend pools, routing rules unchanged
  }
}