Problem

When we use app.OAuthBearerAuthenticationExtensions, the next chain is called:

public static class OAuthBearerAuthenticationExtensions
  {
    public static IAppBuilder UseOAuthBearerAuthentication(this IAppBuilder app, OAuthBearerAuthenticationOptions options)
    {
      if (app == null)
        throw new ArgumentNullException(nameof (app));
      app.Use((object) typeof (OAuthBearerAuthenticationMiddleware), (object) app, (object) options);
      app.UseStageMarker(PipelineStage.Authenticate);
      return app;
    }
  }

Then an object of type OAuthAuthorizationServerMiddleware uses internal class OAuthAuthorizationServerHandler where JsonTextWriter is used:

using (var jsonTextWriter = new JsonTextWriter((TextWriter) new StreamWriter((Stream) memory)))
{
    jsonTextWriter.WriteStartObject();
    jsonTextWriter.WritePropertyName("access_token");
    jsonTextWriter.WriteValue(accessToken);
    jsonTextWriter.WritePropertyName("token_type");
    jsonTextWriter.WriteValue("bearer");
    // and so on
    this.Response.ContentLength = new long?((long) body.Length);
    await this.Response.WriteAsync(body, this.Request.CallCancelled);
}

There are two limitations here:
*) JsonTextWriter is a pure class that cannot be configured, it just writes string as StringBuilder, so Json.Settings = new MySettings() cannot be applied. Also JsontTextWriter does not support complex objects. Arrays can be only written as jsonTextWriter.WriteStartArray() and jsonTextWriter.WriteEndArray(), but this is ignored in the OAuthAuthorizationServerHandler.
*) Some classes are internal and cannot be overwritten or inherited.

It seems that Microsoft developers did not foresee this problem and just limited custom properties to IDictionary<string, string>.

Solution 1

Instead of app.UseOAuthBearerAuthentication(...) apply your own code

app.Use<MyOAuthBearerAuthenticationMiddleware>(options);
app.UseStageMarker(PipelineStage.Authenticate);

You can derive a class from OAuthBearerAuthenticationMiddleware and use for your own purposes.

Solution 2

Override the token Endpoint response. This is a tricky thing.

1) Create a custom middleware that will wrap other calls and override Body response stream.

class AuthenticationPermissionsMiddleware : OwinMiddleware
{
    public AuthenticationPermissionsMiddleware(OwinMiddleware next) 
        : base(next)
    {            
    }

    public override async Task Invoke(IOwinContext context)
    {
        if (!context.Request.Path.Equals("/Token")
        {
            await Next.Invoke(context);
            return;
        }

        using (var tokenBodyStream = new MemoryStream())
        {
            // save initial OWIN stream
            var initialOwinBodyStream = context.Response.Body;

            // create new memory stream
            context.Response.Body = tokenBodyStream;

            // other middlewares will will update our tokenBodyStream
            await Next.Invoke(context);

            var tokenResponseBody = GetBodyFromStream(context.Response);
            var obj = JsonConvert.DeserializeObject(tokenResponseBody);
            var jObject = JObject.FromObject(obj);

            // add your custom array or any other object
            var scopes = new Scope[];
            jObject.Add("scopes", JToken.FromObject(scopes));
            var bytes = Encoding.UTF8.GetBytes(jObject.ToString());
            context.Response.Body.Seek(0, SeekOrigin.Begin);
            await tokenBodyStream.WriteAsync(bytes, 0, bytes.Length);

            context.Response.ContentLength = data.LongLength;
            tokenBodyStream.Seek(0, SeekOrigin.Begin);

            // get back result to the OWIN stream
            await context.Response.Body.CopyToAsync(initialOwinBodyStream);
            }
        }
    }

    private string GetBodyFromStream(IOwinResponse response)
    {
        using (var memoryStream = new MemoryStream())
        {
            response.Body.Seek(0, SeekOrigin.Begin);
            response.Body.CopyTo(memoryStream);
            memoryStream.Seek(0, SeekOrigin.Begin);
            using (var reader = new StreamReader(memoryStream))
            {
                return reader.ReadToEnd();
            }
        }
    }
}

2) Use the new middleware before UseOAuthBearerTokens in the authentication startup method.

app.Use<AuthenticationPermissionsMiddleware>();
app.UseOAuthBearerTokens(options);