ASP.NET Core SignalR CORS issue: "Request header field X-Requested-With is not allowed by Access-Control-Allow-Headers in preflight response"

I'm facing an inconsistent CORS issue with SignalR in my ASP.NET Core and Angular application. On my machine, the connection works fine in Chrome but fails with a CORS error in Edge. However, for some other people, the error occurs in both Chrome and Edge, which makes the issue difficult to reproduce consistently.

Access to XMLHttpRequest at 'http://localhost:21023/signalr/negotiate?enc_auth_token=wNYmO41%2F48SHNstaLVXxHCCre29BZQl1NhC6NM3R3rzpXtPQxVzH6jEzA%2FQhXFN5tu6Fk7pO53uppm1mVXMZgxbyRVz26dnepi%2FFyB6axBY%2B6gq1GL%2BuRQgoiFUCjRN2p8w6LevViwKlHyWZZJZO1DGVSjAi1m2U%2Bog9pkHw9%2FTWnx%2BxQedi3enbqm%2FZYOXT%2FBycU4LZ4F6zlZtrxzHwExtSfELJf%2F0pBk4IITKNjDQHBqaqbtRSdePntPkQpHXvZNrJtiNNLL85J%2F82FKzVtA8QFbHGXtNOZ0fxoo2a1PXJ9tDbrwc9J%2BI7IuFSrHNNT3DC28TLcM7kBDr5mCl95lNKMDiM9hflCF7A6D%2BTSCOGKZJGV6uiFiyN1qDqLG95D4BgZchn6jC%2FSTq493ETqjromBYUfLMabD0HGtdrmMAtmjbyHHswplwmBUPw1heNZCPd%2BmVVqg7jpSio9GWs6B0MZWfI3ZTW6Ox%2BGZMPTiv5RdVDpvoNOLyQAarB0Y8sa6zhTa2mQSXFpwgLm1gfUEIY5DDrgAesHYt8DRXab%2B6Jitdbn6nKmVd0aayomDTm0TJmEG0upxcuSg0aIoYnDqauY33PPhb4L4lmOWUOu5ojBHRHAT%2BQynlt9sFSjqXcBoKmUzB2Hg6pEYRHM3Ah3XUOHDtwwxjITCL%2FKV3HFf8hFBgp%2F9rPtYVn5eziCkIoDBfyahUqpUQKQ%2FULKZvHhKIsaoVNSC0DKSKF5teOv1MIp60Te20EDcwJs6YurEXzkWFhFAAVH6%2Fb1bjDY0MQfhOTNeu9JFvHecXdkbqACrzi04714nKTZcu%2F1Zroiikf9HKJ4Ew7QmqrtNgrfLNyThON6hgpJWuz60TzSzaki7dwfYT72u7DjPel0%2B7Qsaz4VcKtVSPHZEoQVWzomMk6QlgiYkM08MUaVEdFNLBENrChUaKgvfG8G%2BhWggKGMGInp%2BuIp4OwbstyyOapAGGDhA%3D%3D' from origin 'http://localhost:4200' has been blocked by CORS policy: Request header field x-requested-with is not allowed by Access-Control-Allow-Headers in preflight response.

I have configured CORS in Startup.cs and tried several different approaches, but it still doesn’t work. I suspect the issue might be related to the middleware order or a missing Authorization header in the CORS configuration.

Backend (ASP.NET Core)

Here is the CORS and SignalR configuration in Startup.cs:

// ConfigureServices method
services.AddCors(
    options => options.AddPolicy(
        _defaultCorsPolicyName,
        builder => builder
            .WithOrigins(
                // App:CorsOrigins in appsettings.json can contain more than one address separated by comma.
                _appConfiguration["App:CorsOrigins"]
                    .Split(",", StringSplitOptions.RemoveEmptyEntries)
                    .Select(o => o.RemovePostFix("/"))
                    .ToArray()
            )
            .AllowAnyHeader()
            .AllowAnyMethod()
            .AllowCredentials()
            .SetIsOriginAllowed(s => true)
    )
);

services.AddSignalR(options =>
{
    options.EnableDetailedErrors = true;
});

// Configure method
app.UseAbp(options => { options.UseAbpRequestLocalization = false; }); // Initializes ABP framework.

app.UseCors(_defaultCorsPolicyName); // Enable CORS!

app.UseStaticFiles();

app.UseAuthentication();

app.UseAbpRequestLocalization();

app.UseSignalR(endpoints =>
{
    endpoints.MapHub<AbpCommonHub>("/signalr", options => {
        options.Transports = HttpTransportType.LongPolling | HttpTransportType.WebSockets;
    });
    endpoints.MapHub<TimekeepingHub>("/signalr-timekeepingHub", options => {
        options.Transports = HttpTransportType.LongPolling | HttpTransportType.WebSockets;
    });
});

Frontend (Angular)
Here is how I connect to the SignalR hub from Angular:

setupMyTimekeepingHub(callback : Function): void {
    const self = this;
    abp.signalr.startConnection(abp.appPath + 'signalr-timekeepingHub', function (connection) {
        console.log(connection);
        self.timekeepingHub = connection; // Save a reference to the hub

        connection.on('connectedSuccess', function (prevSyncDays: string[]) {
            self.listProcessingDays = prevSyncDays;
            self.timekeepingProcess.next({event:"connectedSuccess",data: self.listProcessingDays});
        });

        // Other event handlers...
    }).then(function (connection) {
        abp.log.debug('Connected to timekeepingHub server!');
        if(typeof callback === "function"){
            callback();
        }
    }).catch((err) => {
        console.log(err);
        self.timekeepingProcess.next({event:"errorOccured",data: null});
    });
}

initSignalR(callback): void {
    const encryptedAuthToken = new UtilsService().getCookieValue(AppConsts.authorization.encrptedAuthTokenName);

    abp.signalr = {
        autoConnect: true,
        connect: undefined,
        hubs: undefined,
        qs: AppConsts.authorization.encrptedAuthTokenName + '=' + encodeURIComponent(encryptedAuthToken),
        remoteServiceBaseUrl: AppConsts.remoteServiceBaseUrl,
        startConnection: undefined,
        url: '/signalr'
    };

    jQuery.getScript(AppConsts.appBaseUrl + '/assets/abp/abp.signalr-client.js', () => {
        this.setupMyTimekeepingHub(callback);
    });
}

I have tried the following:

* Changing the middleware order (placing CORS before Authentication)
* Adding a custom middleware to handle preflight OPTIONS requests
* Adding the Authorization header to Access-Control-Allow-Headers
* Removing extra whitespaces in the URL

But it still doesn’t work. Has anyone experienced this issue before? What should I do to resolve this CORS error?

Additional information:

* ASP.NET Core version: 2.2
* SignalR version: 1.1.0
* Angular version: 8.2.14
* ABP Framework version: 4.3.0