How to catch WebAPI Core unauthenticated status

Question

I have the below code in my Web Core API

using Microsoft.AspNetCore.Authentication.JwtBearer;
using Microsoft.Identity.Web;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllers();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();
builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
                .AddMicrosoftIdentityWebApi(builder.Configuration.GetSection("AzureAd"));

builder.Services.AddEndpointsApiExplorer();
var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();
app.UseAuthentication();
app.UseAuthorization();

app.MapControllers();

app.Run();

My Controller has [Authorize] at the top. When the API runs, if sending a successful bearer token and request, I can retrieve data and if incorrect I can see a 401 status in Postman.

The way it currently works means when I attempt to access data I don’t get a friendly message back, just an empty response but with a 401 status. How could I return a friendly message? I tried debugging but when an unauthenticated attempt nothing is being highlighted where I could check the status and return a friendly message.

I am using Azure AD with client credentials for this API to be authenticated.

Accepted Answer

Hi @Mr Edge

How to catch WebAPI Core unauthenticated status

You can try to use the UseStatusCodePages middleware to capture the http response and format the returned error message. Code like this:


app.UseStatusCodePages(async statusCodeContext =>
{
    switch (statusCodeContext.HttpContext.Response.StatusCode)
    {
        case 401:
            statusCodeContext.HttpContext.Response.StatusCode = (int)HttpStatusCode.Unauthorized;
            await statusCodeContext.HttpContext.Response.WriteAsJsonAsync(new ErrorMessage { httpStatus = 500, Message = "some message" });

            ////If you want to return a html page, use the following code:
            //statusCodeContext.HttpContext.Response.ContentType = "text/html"; 
            //await statusCodeContext.HttpContext.Response.WriteAsync("
");
            //await statusCodeContext.HttpContext.Response.WriteAsync("401 ERROR From API!


"); 
            //await statusCodeContext.HttpContext.Response.WriteAsync( "Home

");
            //await statusCodeContext.HttpContext.Response.WriteAsync("
"); 
            break;
        case 403:
            statusCodeContext.HttpContext.Response.StatusCode = 400;
            await statusCodeContext.HttpContext.Response.WriteAsJsonAsync(new ErrorMessage { httpStatus = 500, Message = "some message" });
            break;
    }
});

Then, when using Postman to access the endpoint without token, the result as below:

User's image

More detail information about using UseStatusCodePages, see Handle errors in ASP.NET Core

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Best regards,

Dillion

Answer

the 401 error is perfectly clear and can be handled with code. if instead you returned a 200 and message that said not authenticated. the code would have to parse the response looking for the error. this might make sense if api response always includes errors, the calling app needs to process.

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