Jquery – How do I resolve a CORS issue in an Ajax POST to ASP.NET Core 6.0 Web API?

I have an ASP.NET website (not a web app) that makes an Ajax POST call using JQuery 2.1.4 to an ASP.NET Core 6.0 Web API.

When I make the call to the method I am getting a CORS error. Yet, when I make a GET Ajax request to a different method in the same controller, the request is successful.

My Ajax POST:

function insertLNItemStorageRequirement() {
  var tempLNStorage = {
    TItem: storageTempReq.t_item,
    TSrq1: storageTempReq.t_srq1,
    TSrq2: storageTempReq.t_srq2,
    TSq27: storageTempReq.t_sq27,
    TStmp: storageTempReq.t_stmp,
    TRcdVers: 0,
    TRefcntd: 0,
    TRefcntu: 0,
  };

  $.ajax({
    type: "POST",
    url: commonAPIURL + "api/LN/InsertItemStorageRequirement",
    data: JSON.stringify(tempLNStorage),
    contentType: "application/json; charset=utf-8",
    //dataType: "json",
    xhrFields: { withCredentials: true },
    success: function (response) {},
    failure: function (response) {
      alert(response.responseText);
    },
    error: function (response) {
      alert(response.responseText);
    },
  });
}

Here is the ASP.NET Core Web API method being called:

[HttpPost("InsertItemStorageRequirement")]
[Produces(typeof(IActionResult))]
public IActionResult InsertItemStorageRequirement([FromBody] Ttccgs016424 itemStorageReq)
{
    Ttccgs016424 newItemStorageReq = _LN.Ttccgs016424s.FirstOrDefault(s => s.TItem == itemStorageReq.TItem);

    if (newItemStorageReq == null)
    {
        newItemStorageReq = new Ttccgs016424()
        {
            TItem = itemStorageReq.TItem,
            TSrq1 = itemStorageReq.TSrq1,
            TSrq2 = itemStorageReq.TSrq2,
            TSq27 = itemStorageReq.TSq27,
            TStmp = itemStorageReq.TStmp,
            TRcdVers = itemStorageReq.TRcdVers,
            TRefcntd = itemStorageReq.TRefcntd,
            TRefcntu = itemStorageReq.TRefcntu
        };

        try
        {
            _LN.Ttccgs016424s.Add(newItemStorageReq);
            _LN.SaveChanges();
        }
        catch (Exception ex)
        {
            return BadRequest(ex.Message);  
        }

        return StatusCode(StatusCodes.Status201Created);
    }
    else
    {
        return StatusCode(StatusCodes.Status412PreconditionFailed, "Item Storage Requirement already exists.");
    }
}

Here is my CORS configuration in my startup.cs for the API (note that my origin is part of the origins array):

services.AddCors(setup => {
    setup.DefaultPolicyName = "open";
    setup.AddDefaultPolicy(p => {
        p.AllowAnyHeader();
        p.WithMethods("OPTIONS", "GET", "POST", "PUT", "DELETE");
        p.WithOrigins(origins);
        p.AllowCredentials();
     });
});

Request headers sent:

OPTIONS /api/LN/InsertItemStorageRequirement HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate, br, zstd
Accept-Language: en-US,en;q=0.9
Access-Control-Request-Headers: content-type
Access-Control-Request-Method: POST
Connection: keep-alive
Host: localhost:31227
Origin: http://localhost:14612
Referer: http://localhost:14612/
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-site
User-Agent: Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Mobile Safari/537.36 Edg/128.0.0.0

Response:

HTTP/1.1 401 Unauthorized
Cache-Control: private
Content-Type: text/html; charset=utf-8
Server: Microsoft-IIS/10.0
WWW-Authenticate: Negotiate
WWW-Authenticate: NTLM
X-Powered-By: ASP.NET
Date: Wed, 11 Sep 2024 23:43:25 GMT
Content-Length: 5995

I’ve tried changing the AJAX call to use credentials: 'include'.

And tried changed the

data: '{itemStorageReq: "' + JSON.stringify(tempLNStorage) + '"}'

Neither have resulted in a successful POST to the API.

GET requests to a different method in the same controller of the API are successful.

I suspect that this isn’t truly a CORS issue, but rather a data mismatch between what the Ajax is sending and what the API is expecting.

Any suggestions on how to troubleshoot this issue?

Update
IIS has the CORs module installed:

Update 2 9/23/24

Moved my app.UseCors() to between app.UseRouting() and app.UseEndpoints() before calling authenticate and authorize in the configure() of startup.cs.

app.UseRouting();

app.UseCors("open");

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

app.UseEndpoints(endpoints =>
{
        endpoints.MapControllers();
});

Also, I am using endpoint routing, which according to MS, means that the CORS Module won’t automatically respond to OPTIONS requests: https://learn.microsoft.com/en-us/aspnet/core/security/cors?view=aspnetcore-6.0#httpoptions-attribute-for-preflight-requests.

In the same link MS recommends writing code in your endpoint to authorize the OPTIONS. So something like this?

[HttpOptions("InsertItemStorageRequirement")]
public IActionResult PreFlightRoute() {
       return NoContent();
}