Create 2025-02-25-aspnetcore-authorizationservice-to-validate-policies.md

Author: robertmuehsig

_posts/2025-02-25-aspnetcore-authorizationservice-to-validate-policies.md

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+---
+layout: post
+title: "Handling Multiple ASP.NET Core Policies with Manual Authorization Checks"
+description: "When implementing authorization in ASP.NET Core, Microsoft provides several ways to check multiple policies. While using **multiple `[Authorize(Policy = "...")]` attributes** works in some or most cases, there are scenarios where a more flexible approach is needed."
+date: 2025-02-25 00:30
+author: Robert Muehsig
+tags: [ASP.NET Core]
+language: en
+---
+
+{% include JB/setup %}
+
+# ASP.NET Core Policies
+
+[ASP.NET Core policies](https://learn.microsoft.com/en-us/aspnet/core/security/authorization/policies) provide a structured and reusable way to enforce authorization rules across your application.
+The built-in features are very flexible, but we had trouble with one scenario - but depending how you write your "Requirements" this might even be possible with the built-in features. 
+Our approach was to use the authorization service to check certain policies manually - which works quite good!
+
+# The Challenge: Combining Policies with OR Logic
+
+In one of our API use cases, we needed to allow access **either** for certain clients (e.g., specific admininstrative applications) **or** for certain users in the database. The two approaches differ:
+
+- **Client Authorization**: This is relatively straightforward and can be handled using the built-in `RequireClaim` approach.
+- **User Authorization**: This required checking database permissions, meaning a **custom authorization requirement** was necessary.
+
+Since both authorization paths are valid, they need to be evaluated using **OR logic**: if **either** condition is met, access should be granted.
+
+# Solution: Using the Authorization Service for Manual Policy Checks
+Instead of relying solely on `[Authorize]` attributes, we can leverage the **`IAuthorizationService`** to manually check policies in our code.
+
+## Step 1: Define the Authorization Policies
+In `Program.cs`, we define multiple policies:
+
+```csharp
+builder.Services.AddAuthorization(options =>
+{
+    options.AddPolicy(KnownApiPolicies.AdminApiPolicyForServiceAccount, policy =>
+    {
+        policy.RequireClaim("scope", "admin-client");
+    });
+    options.AddPolicy(KnownApiPolicies.AdminApiPolicyForLoggedInUserAdministrator, policy =>
+    {
+        policy.Requirements.Add(new DbRoleRequirement(Custom.UserAdminInDatabase));
+    });
+});
+```
+
+## Step 2: Manually Validate User Authorization
+Using `IAuthorizationService`, we can manually check if the user meets **either** of the defined policies.
+
+```csharp
+private async Task<AuthorizationResultType> ValidateUserAuthorization()
+{
+    var user = User;
+
+    var serviceAccountAuth = await _authorizationService.AuthorizeAsync(user, KnownApiPolicies.AdminApiPolicyForServiceAccount);
+    if (serviceAccountAuth.Succeeded)
+    {
+        return AuthorizationResultType.ServiceAccount;
+    }
+
+    var userAuth = await _authorizationService.AuthorizeAsync(user, KnownApiPolicies.AdminApiPolicyForLoggedInUserAdministrator);
+    if (userAuth.Succeeded)
+    {
+        return AuthorizationResultType.LoggedInUser;
+    }
+
+    return AuthorizationResultType.None;
+}
+```
+
+## Step 3: Apply the Authorization Logic in the Controller
+
+```csharp
+[HttpGet]
+public async Task<ActionResult<...>> GetAsync()
+{
+    var authResult = await ValidateUserAuthorization();
+    
+    if (authResult == AuthorizationResultType.None)
+    {
+        return Forbid(); // Return 403 if unauthorized
+    }
+    
+    using var contextScope = authResult switch
+    {
+        AuthorizationResultType.ServiceAccount => // ... do something with the result,
+        AuthorizationResultType.LoggedInUser => // ...,
+        _ => throw new UnauthorizedAccessException()
+    };
+    
+    return Ok(_userService.GetUsers(...));
+}
+```
+
+# Recap
+
+We use the `IAuthorizationService.AuthorizeAsync` method to check multiple policies and depending on the outcome, we can handle it depending on our needs.
+This approach retains the same overall structure as the "default" policy-based authorization in ASP.NET Core but provides more flexibility by allowing policies to be evaluated dynamically via the service.
+
+Keep in mind (as mentioned at the beginning): This is just one way of handling authorization. As far as we know, it works well without drawbacks while offering the flexibility we need.