A SignalR or Web PubSub service with public network access enabled takes realtime connections from anywhere

Why it happens

Realtime services get treated as plumbing. The app server holds the connection string, clients negotiate tokens through it, everything works on day one, and the networking tab never gets a second look. Because SignalR and Web PubSub sit behind the application rather than in front of it, they rarely appear on the same review checklist as databases and storage, even though the same access key that lets your server negotiate clients also lets anyone holding it connect directly.

The configuration surface adds its own quirk: when public access is Enabled, the service's network ACLs still apply, but the default ACL allows the traffic, so an untouched resource is open by default. Only publicNetworkAccess: 'Disabled' shuts the public path unconditionally, regardless of what the ACLs say, and nothing in Azure nudges you toward flipping it.

What it costs / blast radius

The blast radius is the live message plane. A leaked access key, and connection strings for these services travel through app settings, CI variables, and server configs like any other, lets an outsider mint client tokens, join hubs and groups, listen to whatever your application broadcasts (notifications, dashboards, chat, presence, live business data), and push messages that your clients will trust as coming from you. Serverless upstream patterns extend that reach into whatever handles the inbound events. The direct cost is minor; the harm is eavesdropping on and injecting into your realtime traffic. (Authored assessment of Azure's behavior, not a measured statistic.)

See it

resource signalr 'Microsoft.SignalRService/signalR@2023-02-01' = {
  name: 'corp-signalr'
  location: location
  sku: { name: 'Standard_S1', capacity: 1 }
  properties: {
    publicNetworkAccess: 'Enabled'   // the default even if you never set it
  }
}

// Identical property on Microsoft.SignalRService/webPubSub resources.
resource signalr 'Microsoft.SignalRService/signalR@2023-02-01' = {
  name: 'corp-signalr'
  location: location
  sku: { name: 'Standard_S1', capacity: 1 }
  properties: {
    publicNetworkAccess: 'Disabled'  // overrides all network ACLs; Private Link only
  }
}