How to Fix Azure App Service Empty JSON in Entra ID SSO

How Do We Solve Azure App Service Empty Responses for Entra ID Files?

While working on a recent enterprise modernization project for a SaaS FinTech platform, we were tasked with upgrading their legacy authentication flow to a modern Single Sign-On (SSO) architecture. The application was built on .NET 10.0 using Blazor and hosted on Azure App Service. To achieve seamless user identity management, we integrated Entra External ID (EEID).

Part of the EEID integration required us to complete the publisher verification process. This process is standard practice; it mandates hosting a specific configuration file named microsoft-identity-association.json inside a publicly accessible /.well-known/ directory at the root of the domain. In our development environment, serving this file through the Blazor app using the new MapStaticAssets feature worked perfectly. However, the moment we deployed to our production and staging slots on Azure App Service, we encountered a bizarre issue.

Fetching the file returned a 200 OK status code, but the response body was completely empty—the Content-Length was precisely 0. This blocked our publisher verification, effectively halting our production release for the SSO integration. We realized that this infrastructure-level quirk was intercepting our application logic. We documented this challenge because we want other technical decision-makers and engineering teams to avoid wasting hours debugging similar phantom deployment issues.

Why is the Publisher Verification JSON Critical for Entra External ID?

In the context of Entra External ID (formerly Azure AD B2C/External Identities), publisher verification helps end users and organizational administrators know that the application they are authenticating against is legitimate. It builds trust by tying the app registration to a verified Microsoft Partner Network (MPN) account.

To prove domain ownership, Microsoft requires the application to host a JSON payload containing the application ID and the tenant ID at a specific path: https://yourdomain.com/.well-known/microsoft-identity-association.json. The authentication provider initiates an automated HTTP GET request to this endpoint during the verification setup. If the JSON payload is unreadable, missing or in this case, completely empty, verification fails.

Why Did the Well-Known JSON File Return a 200 OK with 0 Bytes?

The symptoms were highly specific and somewhat contradictory. When checking our Azure App Service logs and tracing the HTTP requests, we observed the following behavior:

• The request for /.well-known/microsoft-identity-association.json did not return a 404 (Not Found) or a 403 (Forbidden). It returned a 200 OK.
• The Content-Length header was set to 0.
• Other static JSON files served from different directories loaded perfectly fine.
• Application Insights showed no exceptions or Kestrel-level pipeline errors.

Because it worked locally, we knew the .NET 10 application itself was theoretically capable of serving the file. At first, we suspected the Web Application Firewall (WAF) might be stripping the payload due to a false-positive security rule. However, explicitly bypassing the WAF yielded the exact same empty response. To test if it was a file deployment issue, we used FTPS to manually upload the JSON file directly into the physical directory of the Azure App Service. Even then, an empty response persisted. This proved the issue was environmental—the Azure App Service infrastructure was intercepting the request before our Blazor static file middleware could fulfill it.

What Diagnostic Steps Did We Take to Fix the Empty JSON Response?

When you encounter silent failures in managed cloud environments, you have to systematically rule out various layers of the hosting architecture. We considered these solutions during our debugging phase:

Did Web Application Firewall (WAF) Block the Request?

Our initial hypothesis was WAF interference. WAFs often inspect requests to hidden directories (those starting with a dot, like /.well-known/). We temporarily added a bypass rule for this specific URI path. When the response remained empty, we confirmed the WAF was simply forwarding the traffic and the drop was happening closer to the application host.

Was IIS or Static File Middleware Dropping the Request?

Azure App Service (specifically the Windows variant) sits behind IIS. By default, IIS has strict rules about serving files from hidden directories or serving specific extensions without proper MIME types. We injected a custom web.config to explicitly allow the .json MIME type and permit access to the .well-known segment. While this sometimes resolves 404.8 (Hidden Segment) errors, it did not solve our 200 OK / 0 bytes issue. The request was already being approved by IIS, but the payload was being swallowed.

Was App Service Authentication (Easy Auth) Interfering?

Azure App Service has a built-in feature called Easy Auth that manages identity integrations at the platform level. It actively intercepts requests to specific authentication-related paths (like /.auth/). We discovered that certain Azure platform features, including managed certificates and identity extensions, reserve the /.well-known/ namespace (e.g., for ACME challenge validation). In some configurations, if the platform expects to handle a `.well-known` route but doesn’t find its specific internal payload, it short-circuits the pipeline with an empty 200 OK rather than passing the request down to the Kestrel server hosting the .NET application.

Could We Map a Direct Endpoint in Blazor?

Instead of fighting the underlying static file handlers and platform-level intercepts for physical files, we decided to bypass file serving entirely. If we map an explicit API route in our .NET 10 application, we force Kestrel to handle the HTTP request directly as dynamic content rather than a static asset.

How Did We Properly Serve the Microsoft Identity Association JSON in Blazor?

The most resilient fix was to stop relying on MapStaticAssets for this particular file. By removing the physical JSON file from the wwwroot/.well-known directory and injecting it directly into the application routing pipeline, we bypassed the Azure infrastructure’s static file interception completely.

Here is the implementation we deployed to our Program.cs file:

var builder = WebApplication.CreateBuilder(args);
// Standard Blazor service registrations
builder.Services.AddRazorComponents()
    .AddInteractiveServerComponents();
var app = builder.Build();
// Other middleware...
app.UseHttpsRedirection();
app.MapStaticAssets(); // Keep this for normal CSS/JS/images
app.UseAntiforgery();
// Explicit route to serve the Entra ID verification file dynamically
app.MapGet("/.well-known/microsoft-identity-association.json", () =>
{
    var identityConfig = new
    {
        associatedApplications = new[]
        {
            new { applicationId = builder.Configuration["EntraID:ApplicationId"] }
        }
    };
    return Results.Json(identityConfig);
});
app.MapRazorComponents<App>()
    .AddInteractiveServerRenderMode();
app.Run();

Validation Steps and Considerations:

• Content-Type Security: Results.Json() automatically sets the Content-Type: application/json header, which Microsoft’s verification service expects.
• Dynamic Configuration: Instead of hardcoding the Tenant and Application ID in a static file, this approach pulls the values securely from the application configuration (e.g., Azure Key Vault or Environment Variables).
• Bypassing Intercepts: Because this is now a mapped endpoint in the routing table, ASP.NET Core processes it before the static file handler tries to look for a physical file. The Azure platform recognizes it as dynamic application traffic, preventing the empty response bug.

What Can Engineering Teams Learn From App Service Interceptions?

When you hire software developer teams to build cloud-native applications, they need to look beyond the application code and understand the infrastructure hosting it. This incident provided several actionable insights:

• Parity Between Environments is Never Absolute: Development environments running standalone Kestrel will not mirror the exact behavior of Azure App Service sitting behind IIS or YARP reverse proxies. Always test platform-specific paths in a staging slot that mirrors production infrastructure.
• Beware of the .well-known Directory: The /.well-known/ directory is a standardized URI space used by Let’s Encrypt, Apple Universal Links and Microsoft Identity. Cloud providers often map internal extensions to this path, which can silently override your static files.
• Route Over Static Assets for Critical Configurations: For essential verification files, serving them dynamically via API routes rather than static files prevents MIME type issues, hidden segment blocks and infrastructure interceptions.
• Use 200 OK With Caution: A 200 OK does not guarantee success. Automated monitoring should explicitly check the response payload or Content-Length when validating critical platform integrations.
• Evaluate Platform Features: When you hire dotnet developers for enterprise modernization, ensure they audit native cloud features like Azure Easy Auth to prevent conflicts with custom authentication logic.

How Can We Prevent Similar Azure App Service Deployment Issues?

Diagnosing silent failures in production requires a deep understanding of how application code interacts with the underlying cloud infrastructure. In this scenario, moving away from static file handlers and leveraging .NET 10 routing provided a clean, dynamic and resilient solution for serving the Entra External ID verification file. Organizations scaling their architecture need engineering partners who can navigate these infrastructure idiosyncrasies efficiently.

If you are looking to scale your engineering capabilities or need to hire developers with deep expertise in Azure and .NET architectures, contact us to discuss how our dedicated remote teams can accelerate your enterprise modernization.

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