I was also having this same problem. I developed a synchronous solution thanks to the research done by @tpeczek in the following SO article: Unable to authenticate to ASP.NET Web Api service with HttpClient
My solution uses a WebClient, which as you correctly noted passes the credentials without issue. The reason HttpClient doesn't work is because of Windows security disabling the ability to create new threads under an impersonated account (see SO article above.) HttpClient creates new threads via the Task Factory thus causing the error. WebClient on the other hand, runs synchronously on the same thread thereby bypassing the rule and forwarding its credentials.
Although the code works, the downside is that it will not work async.
var wi = (System.Security.Principal.WindowsIdentity)HttpContext.Current.User.Identity;
var wic = wi.Impersonate();
try
{
var data = JsonConvert.SerializeObject(new
{
Property1 = 1,
Property2 = "blah"
});
using (var client = new WebClient { UseDefaultCredentials = true })
{
client.Headers.Add(HttpRequestHeader.ContentType, "application/json; charset=utf-8");
client.UploadData("http://url/api/controller", "POST", Encoding.UTF8.GetBytes(data));
}
}
catch (Exception exc)
{
// handle exception
}
finally
{
wic.Undo();
}
Note: Requires NuGet package: Newtonsoft.Json, which is the same JSON serializer WebAPI uses.