Add content about enabling CORS for SignalR without applying the policy globally.

[ryan-h]

I encountered a situation where the CORS policy used for SignalR could not be enabled as the middleware default for the entire application. Instead I needed to use a specific policy only for the requests to the SignalR hubs.

Maybe it would be helpful to add a subsection about "Cross-origin resource sharing" that describes how to branch the request pipeline in order to apply a CORS policy specifically for SignalR, which would then allow an application to use attributes to enable CORS elsewhere instead of using middleware.

public void Configure(IApplicationBuilder app)
{
    ...

    app.Map("/hubs", config =>
    {
         // Must be called before mapping SignalR hubs
         config.UseCors(builder =>
         {
             builder.WithOrigins("https://myorigin:5000")
                 .WithMethods("Get", "Post")
                 .AllowAnyHeader()
                 .AllowCredentials();
         });

         config.UseSignalR(routes =>
         {
             routes.MapHub<ChatHub>("/chat");
         });
    });
}

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Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: 8af11946-ea05-3be2-6fa1-c4c01aa76d8c
• Version Independent ID: 58e87eb0-240b-ab00-c5f4-32a0ab2e0028
• Content: Security considerations in ASP.NET Core SignalR
• Content Source: aspnetcore/signalr/security.md
• Product: aspnet-core
• Technology: aspnetcore-signalr
• GitHub Login: @bradygaster
• Microsoft Alias: anurse

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Associated WorkItem - 566444

[BrennanConroy]

There are 2 ways to do this assuming a named policy has been added

services.AddCors(o =>
{
    o.AddPolicy("SignalRPolicy", o2 =>
    {
        o2.WithOrigins("https://example.com")
            .AllowAnyHeader()
            .WithMethods("GET", "POST")
            .AllowCredentials();
    });
});

endpoints.MapHub<Chat>("/default").RequireCors("SignalRPolicy");

[EnableCors("SignalRPolicy")]
public class Chat : Hub
{
}

And can cross-ref to https://docs.microsoft.com/en-us/aspnet/core/security/cors?view=aspnetcore-3.1#enable-cors-with-endpoint-routing