ASP.NET Core Cookie not set with CORS - asp.net-core

Question

I try to set a auth cookie. The ui and the data server are under different subdomains. So I need to activate CORS.
services.AddCors(options => options.AddPolicy("SubdomainDefault", builder => builder
.WithOrigins("https://ui.domain.de")
.AllowCredentials()
.AllowAnyHeader()
.Build()
));
On the same subdomain the cookie is set, but with different subdomains it is visible in the headers, but not set.
Request URL: https://server2.domain.de/...
Request Method: POST
Status Code: 200 OK
Remote Address: X.X.X.X:443
Referrer Policy: strict-origin-when-cross-origin
Response Header
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: https://ui.domain.de
Content-Length: 1017
Content-Type: application/json; charset=utf-8
Date: Fri, 29 Oct 2021 10:27:11 GMT
Server: Microsoft-IIS/8.5
Set-Cookie: auth=XXX; domain=.domain.de; path=/; secure; samesite=strict; httponly
Vary: Origin
X-Powered-By: ARR/3.0
Request Headers
Accept: application/json, text/plain, */*
Accept-Encoding: gzip, deflate, br
Accept-Language: de-DE,de;q=0.9,en;q=0.8,en-US;q=0.7
Cache-Control: no-cache
Connection: keep-alive
Content-Length: 0
Host: server2.domain.de
Origin: https://ui.domain.de
Pragma: no-cache
Referer: https://ui.domain.de/
sec-ch-ua: "Google Chrome";v="95", "Chromium";v="95", ";Not A Brand";v="99"
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Windows"
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-site
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.54 Safari/537.36
Has anyone a idea?

Answer 1

Assuming your second request is actually a XMLHttpRequest you need to set the withCredentials flag, otherwise the cookies are not sent.
Here's an excerpt from https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#requests_with_credentials:
const invocation = new XMLHttpRequest();
const url = 'https://bar.other/resources/credentialed-content/';
function callOtherDomain() {
if (invocation) {
invocation.open('GET', url, true);
invocation.withCredentials = true;
invocation.onreadystatechange = handler;
invocation.send();
}
}