Blazor Server SignalR circuit authorization caching with revoked credentials

Addressing Stale Authorization in SignalR Circuits

To force the re-evaluation of authorization policies on every SignalR hub invocation without tearing down the circuit, you must move the authorization check from the component level (which is cached) to the hub pipeline. The recommended pattern is implementing a custom IHubFilter.

Implementation Strategy

A custom IHubFilter allows you to intercept every hub method call. By injecting IAuthorizationService and UserManager, you can verify the user's current status against the database before the hub method executes.

Performance Implications: Calling UserManager.GetUserAsync on every invocation introduces a database round-trip. At scale, this can create a significant bottleneck. To mitigate this, implement a short-lived cache (e.g., 5–30 seconds) using IMemoryCache to store the user's authorization state, balancing security freshness with database load.

Revocation and Signaling

Since the framework does not natively push revocation events, you can implement a manual signaling mechanism. By maintaining a mapping of User IDs to Connection IDs, the server can push a specific message to the client to trigger a redirect.

Recommended Steps

1. Create a Hub Filter: Implement IHubFilter to perform the check.

   public class AuthorizationRevalidationFilter : IHubFilter
   {
       private readonly IAuthorizationService _authService;
       private readonly UserManager<IdentityUser> _userManager;
   
       public AuthorizationRevalidationFilter(IAuthorizationService authService, UserManager<IdentityUser> userManager)
       {
           _authService = authService;
           _userManager = userManager;
       }
   
       public async ValueTask<HubInvocationContext> InvokeAsync(HubInvocationContext invocationContext, Func<HubInvocationContext, ValueTask<HubInvocationContext>> next)
       {
           var user = await _userManager.GetUserAsync(invocationContext.Context.User);
           if (user == null)
           {
               throw new HubException("User no longer exists or is disabled.");
           }
   
           var result = await _authService.AuthorizeAsync(invocationContext.Context.User, null, "YourRequiredPolicy");
           if (!result.Succeeded)
           {
               throw new HubException("Authorization revoked.");
           }
   
           return await next(invocationContext);
       }
   }

2. Register the Filter:

   services.AddSignalR(options =>
   {
       options.AddFilter<AuthorizationRevalidationFilter>();
   });

3. Push Revocation: When a user is locked out or roles are changed, use IHubContext to notify the client:

   await _hubContext.Clients.User(userId).SendAsync("ForceLogout");

4. Client-Side Response: Add a listener in the Blazor component or JavaScript to handle the ForceLogout event by redirecting the user to the login page.

Assumptions and Constraints

• CircuitHandler Limitation: A CircuitHandler is unsuitable for per-invocation checks because its methods (like OnConnectionUpAsync) only trigger during circuit lifecycle changes, not during individual hub calls.
• Identity Store: This approach assumes the use of ASP.NET Core Identity. If using a custom JWT or external provider, the UserManager logic must be replaced with the corresponding identity provider's validation logic.
• Version Assumption: This implementation targets .NET 6+ where IHubFilter is available.

Diagnostic Requirement

To refine this recommendation: Are you using a distributed cache (like Redis) for session state, or is the application running on a single server? This determines if IMemoryCache is sufficient or if a distributed cache is required for the re-validation window.