What happened to my Thread.CurrentPrincipal

This is not the IPrincipal you are looking for...

David Pine

4 minute read

Overview

Like the title claims, if you’re using ASP.NET Core and expecting the Thread.CurrentPrincipal or ClaimsPrincipal.Current to be populated you’d be wrong. This is not the IPrincipal you’re looking for. In this post we’ll discuss what happened and what you need to do now. Additionally we’ll cover all the peripheral benefits as a result of this change.

History

If you have ever done any .NET Framework development, you’ve probably seen the Thread class. You’re probably familiar with the Thread.CurrentPrincipal member. This member of the Thread class is defined as follows:

public static IPrincipal CurrentPrincipal
{
    [System.Security.SecuritySafeCritical]  // auto-generated
    get
    {
        lock (CurrentThread)
        {
            var principal = (IPrincipal)CallContext.Principal;
            if (principal == null)
            {
                principal = GetDomain().GetThreadPrincipal();
                CallContext.Principal = principal;
            }
            return principal;
        }
    }

    [System.Security.SecuritySafeCritical]  // auto-generated
    [SecurityPermission(SecurityAction.Demand, Flags = SecurityPermissionFlag.ControlPrincipal)]
    set
    {
        CallContext.Principal = value;
    }
}

Gets or sets the thread’s current principal (for role-based security).

Does anyone see the issue with this? Can you say, “publically static mutable state, oh my”?! You should be alarmed. This property was never a good idea and today it simply doesn’t belong. ASP.NET Core is not responsible for assigning this. You might not agree with that decision, but it is final.

Before diving into this, pop over to Microsoft.AspNetCore.Security – Issue 332 for more of the back story.

Thread.CurrentPrincipal Today in ASP.NET Core

As part of the ASP.NET Core framework, the following middleware packages are provided:

If you opt-in to using this middleware and you provide a login page (or expose an external provider) you’d end up creating an instance of a ClaimsPrincipal that represents an authenticated user. Subsequent requests to the web server would be handed the cookie that holds the user’s claims. However the Thread.CurrentPrincipal would not actually reflect the ClaimsPrincipal object that was created as the result of the login. In fact, it would simply be an instance of the GenericPrincipal implementation. Likewise, walking up to the ClaimsPrincipal.Current property and asking it for the current claims principal in context wouldn’t give you what you might expect either. Additionally, the ClaimsPrincipal.Current internally relies on the Thread.CurrentPrincipal for its value.

public static ClaimsPrincipal Current
{
    get
    {
         return ClaimsPrincipal.s_principalSelector() ??
                ClaimsPrincipal.SelectClaimsPrincipal();
    }
}

private static ClaimsPrincipal SelectClaimsPrincipal()
{
    return Thread.CurrentPrincipal as ClaimsPrincipal ?? 
           new ClaimsPrincipal(Thread.CurrentPrincipal);
}

You might be asking yourself, “how do I access this value then?”. If you’re in the context of a controller then you already have access to it via the .User property. Otherwise, the answer is “dependency injection”. Wherever you’re in need of the identity for accessing claims, use .cstor injection and give yourself the IPrincipal you need. If you’re in the context of an action you might be tempted to use the [FromServices] attribute to inject the IPrincipal instance, but remember that this actually comes from the current user - which is already accessible via the controller’s .User property.

Dependency Injection

ASP.NET Core made dependency injection (DI) a first class citizen, and it is so easy to use. The Startup.cs by convention has a ConfigureServices method where you’ll register the dependencies. In the case of ASP.NET Core you’ll need to do the following:

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
    services.AddTransient<IPrincipal>(
        provider => provider.GetService<IHttpContextAccessor>().HttpContext.User);

    // ...
}

You might have noticed that we explicitly register the IHttpContextAccessor and corresponding implementation. This is necessary as that service is not registered for us. With that in place we can now specify that any class asking for an IPrincipal will be given the IHttpContextAccessor.HttpContext.User as the implementation. This is exactly what we needed. The exact same pattern is true for class libraries that want to leverage identity-based claim values from the ClaimsPrincipal implementation of the IPrincipal.

Example

using System.Security.Claims;
using System.Security.Principal;

public class SampleService : ISampleService
{
    private readonly ClaimsPrincipal _principal;

    public SampleService(IPrincipal principal)
    {
        _principal = principal as ClaimsPrincipal;
    }

    public Task ConsumeAsync()
    {
        var dobClaim = _principal?.FindFirst(ClaimTypes.DateOfBirth);
        // Take some action ...
    }
}

From the example above we can see how simple it is to use dependency injection. Simply define the IPrincipal as a .cstor parameter, store it in a field and consume it as needed.

But Why?

There are a lot of people who feel as though DI is overrated and that it is much easier to ask the Thread.CurrentPrincipal for its value. While that is easy, it is also risky…think about it. Since it is mutable anyone (even 3rd party libraries) can set it. Additionally, you avoid concerns about trying to synchronize static state between the Thread and ClaimsPrincipal classes. Scott Hanselman blogged about a similar issue nearly thirteen years ago!

One of the first benefits that comes to mind from this is the fact that with DI the code is unit-testable. It is extremely easy to mock out an IPrincipal to use for unit testing. Additionally, this alleviates all of the concerns about synchronization and reliability. The IPrincipal you’re given is the principal you’d expect and it is fully populated accordingly.

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