Bring back logic to remove unwanted env variables

src/Components/WebAssembly/Server/src/DebugProxyLauncher.cs

@@ -52,6 +52,7 @@ private static async Task<string> LaunchAndGetUrl(IServiceProvider serviceProvid
                 UseShellExecute = false,
                 RedirectStandardOutput = true,
             };
+            RemoveUnwantedEnvironmentVariables(processStartInfo.Environment);
 
             var debugProxyProcess = Process.Start(processStartInfo);
             CompleteTaskWhenServerIsReady(debugProxyProcess, tcs);
@@ -64,6 +65,21 @@ private static async Task<string> LaunchAndGetUrl(IServiceProvider serviceProvid
             return await tcs.Task;
         }
 
+        private static void RemoveUnwantedEnvironmentVariables(IDictionary<string, string> environment)
+        {
+            // Generally we expect to pass through most environment variables, since dotnet might
+            // need them for arbitrary reasons to function correctly. However, we specifically don't
+            // want to pass through any ASP.NET Core hosting related ones, since the child process
+            // shouldn't be trying to use the same port numbers, etc. In particular we need to break
+            // the association with IISExpress and the MS-ASPNETCORE-TOKEN check.
+            // For more context on this, see https://github.com/dotnet/aspnetcore/issues/20308.
+            var keysToRemove = environment.Keys.Where(key => key.StartsWith("ASPNETCORE_")).ToList();
+            foreach (var key in keysToRemove)
+            {
+                environment.Remove(key);
+            }
+        }
+
         private static string LocateDebugProxyExecutable(IWebHostEnvironment environment)
         {
             var assembly = Assembly.Load(environment.ApplicationName);