Summary

In this article, I would share a .NET 6.0 Web API sample code that supports Basic Authentication. Here is the sample code

TOC

• Concept
  • Configuration
  • Custom attribute
  • Middleware
  • Allowanonymous attribute
• Sample request

Concept

Configuration

The Web API validates an incoming request header based on the username and password configured in appsettings.json or Configuration at Azure App Service. The app exctracts the value through IConfiguration class.

appsettings.json

"BasicAuth": {
   "UserName": "",
   "Password": ""
}

Custom attribute

A custom attribute BasicAuth is on the controller and allow only requests which attach the username and password on the request header.

WeatherforecastController

[HttpGet("RequireAuth")]
[AllowAnonymous]
[AuthorizeBasicAuth]
public ActionResult<Weatherforecast> GetBasicAuth()
{
    ...

The custom attribute AuthorizeBasicAuthAttribute class inherits AuthorizeAttribute and IAuthorizationFilter. At OnAuthorization method, it sets UnauthorizedResult() to the context result if the basic authentication validation HttpContext.Items["BasicAuth"] is not true.

AuthorizeBasicAuthAttribute

public void OnAuthorization(AuthorizationFilterContext context)
{
    if (context.HttpContext.Items["BasicAuth"] is not true)
    {
        context.Result = new UnauthorizedResult();
        return;
    }
}

Middleware

In the custom middleware BasicAuthMiddleware, it validates the username and password attached to the request header and add true to HttpContext.Items['BasicAuth'] so it can pass the validation result to the later step of the authorization filter.

BasicAuthMiddleware

Encoding encoding = Encoding.GetEncoding("iso-8859-1");
string usernameAndPassword = encoding.GetString(Convert.FromBase64String(authHeaderVal.Parameter));
string username = usernameAndPassword.Split(new char[] { ':' })[0];
string password = usernameAndPassword.Split(new char[] { ':' })[1];
if (username == this.configuration.GetValue<string>("BasicAuth:UserName") && password == this.configuration.GetValue<string>("BasicAuth:Password"))
{
    httpContext.Items["BasicAuth"] = true;
}

The validation process is during the custom middleware BasicAuthMiddleware because it is difficult for the custom attribute filter to extract configuration values from IConfiguration class. According to the Microsoft documentation Dependency Injection, a filter attribute cannot have dependency injection constructors.

Filters that are implemented as attributes and added directly to controller classes or action methods cannot have constructor dependencies provided by dependency injection (DI). Constructor dependencies cannot be provided by DI because attributes must have their constructor parameters supplied where they're applied.

Allowanonymous attribute

I intentionally add [AllowAnonymous] to the controller because .NET built-in authentication is conducted in the middleware but the request header is supposed to have only a basic authentication information.

• Without [AllowAnonymous] attribute in the controller, the controller does not give the request a permission to access.
• Without app.UseAuthentication, the authorization process does not work. (401 Unauthorized)
• Without app.UseAuthorization, the middleware that supports authorization cannot deal with the authorization metadata (500 internal server error.)

Program.cs

app.UseAuthentication();
app.UseAuthorization();
app.UseMiddleware<BasicAuthMiddleware>();

Sample request

Powershell

$basicAuthUsername = "{The user name configured in the Web API}"
$basicAuthSecret = "{The password name configured in the Web API}"
$bytes = [System.Text.Encoding]::ASCII.GetBytes($basicAuthUsername + ':' + $basicAuthSecret)
$authHeader = [Convert]::ToBase64String($bytes)
$Uri = "{App URL}/Weatherforecast/RequireAuth"
$headers = @{
  "Authorization" = 'Basic ' + $authHeader
}
Invoke-RestMethod -Uri $Uri -Method Get -Headers $headers

bash

$basicAuthUsername = "{The user name configured in the Web API}"
$basicAuthSecret = "{The password name configured in the Web API}"
curl {App URL}/Weatherforecast/RequireAuth \
  -H "accept: application/json" \
  -H "Authorization:Basic $(echo -n $basicAuthUsername:$basicAuthSecret | openssl base64)"