Policy-based Authorization in ASP.NET Core

In this tutorial. we are going to look at the Policy-based Authorization in ASP.NET Core. We learned Claim based authorization in the previous tutorial. We can create simple policies using that approach and it is sufficient to handle most of the situations. But if we run into complex authorization requirements, then we need to build a custom policy-based authorization.

Authorization Policy

Authorization Policies are the backbone of the ASP.NET Core Authorization Framework. Even when you use claim-based or role-based authorization, you are actually using Policy-based Authorization.

A Policy defines a collection of requirements, that the user must satisfy in order to access a resource. The user must satisfy all the requirements.

We Add the policy using the AddAuthorization method in the ConfigureServices of the startup class.

The policies can be simple as calling RequireClaim on policy builder. You can also create complex policies by creating requirements and handlers.

Simple Authorization Policies

The following examples show the Simple Authorization Policies using claims & Roles.

Policy with Single Claim

The following policy creates an AdminOnly Policy. The user must have the Admin claim. The value of the Claim does not matter here.

Policy with Single Claim & Value

The following is the ITOnly Policy, which requires the user the have a claim Permission with the value IT. The Permission claim with any other value is not allowed.

Policy with Multiple Claims

You can chain multiple Claims together as shown below. The SuperIT policy requires user the have Permission claim with Value IT and separate IT claim. The user must satisfy both conditions.

Policy only for Authorized Users

This policy is similar to adding Authorize attribute.

Policy using a Role

Policy using a User Name

Applying the Authorization Policy

We apply the policy using the Authorize attribute.

The following code applies the AdminOnly policy to the SomeSecureAction

Custom Policy using a Func

The Policies, we created above are simple and straightforward. But if you have little complex authorization requirements, then you can create a custom function and use the method in RequireAssertion

The custom lambda function must return a true if the policy is satisfied.

Custom Policy using requirement & Handlers

We can create a custom Policy by using the Authorization Requirement & Authorization Handler.

Authorization Requirement

The Authorization Requirement defines the collection of conditions that the policy must evaluate. For the Policy to be successful, it must satisfy all the requirements. It is similar to AND Condition. If one of the requirements fails. then the policy fails.

The following diagram describes the structure of a Policy using Requirement. The Policy contains three requirements.

For The policy to be successful all three requirements must return success. It is like a AND Condition. If one of them fails, then the Policy fails.

Authorization Policy Using Requirement and Handler in ASP.NET Core

Authorization Handler

The authorization handler contains the logic to checks to see if the requirement is Valid. An Authorization Requirement can contain more than one Authorization Handler.

An Authorization Handler can return one of the three values

  1. Fail
  2. Succeded
  3. Do Nothing.

If one of the handlers return fail, then the requirement fails. It Fails irrespective of the results of other handlers.

If None of the requirement handlers fails, but at least one of the requirement handler returns Success then the requirement is Success.

If none of the requirement handlers return a value, then the requirement fails.

Note that all the handlers are invoked, irrespective of the outcome.

Now, let us understand it with an example

Example of Requirement & Requirement handler

Consider the following example.

To create a new product in a portal the user must satisfy the following conditions.

  1. The user is an employee of the Organization. Carry an Employee claim.
  2. The user is a VIP Customer. Carry an VIP Claim.
  3. But, the user is not allowed to post if his account is disabled due to bad reviews. The disabled account carry Disabled claim.

The above conditions represent a policy. We call it as canManageProduct policy

Each of the above statements is our Handlers. Hence we have three handlers

  1. IsEmployeeHandler
  2. IsVIPCustomerHandler
  3. IsAccountNotDisabledHandler

We can, in fact, create a single handler and test all of the above. That will serve our purpose for this tutorial. But in a real-world application, you need to look at reusing the handlers in another requirement. Hence it makes sense to split it up into smaller pieces.

In the above, if IsEmployeeHandler OR IsVIPCustomerHandler returns true, then the user can edit the Product. Hence these two falls under a single requirement.

But the IsAccountNotDisabledHandler is different. If the Account is disabled, then the user must not be allowed to create the Product. Hence it must return Fail

  • IsAllowedToManageProductRequirement
    • IsEmployeeHandler returns sucess else nothing
    • IsVIPCustomerHandler returns sucess else nothing
    • IsAccountNotDisabledHandler returns fail else nothing

Another way to build the requirement is by creating the two requirements.

  • IsAllowedToManageProductRequirement
    • IsEmployeeHandler returns sucess else nothing
    • IsVIPCustomerHandler returns sucess else nothing
  • IsAccountEnabledRequirement
    • IsAccountNotDisabledHandler returns true else nothing

In the second method, we can make use of IsAccountEnabledRequirement in another Policy.

Now, let us implement it in an example project. We will use the project that we created in Adding & Managing Claims in ASP.NET Core Identity. The Project has a CRUD option for the product table and also option to add /remove claims of the user

Authorization Requirement

Create a folder Authorization under the solution root.

Under Authorization folder create the class IsAllowedToEditProductRequirement

A Requirement class must implement IAuthorizationRequirement from the Microsoft.AspNetCore.Authorization namespace. IAuthorizationRequirement does not contain any methods or properties. Hence we do not need to implement anything.

If the Requirement needs any input, then you can add a public constructor with the input.

Similarly, create the IsAccountEnabledRequirement class.

Creating Authorization Handlers

The handlers must inherit from the AuthorizationHandler. It has one method HandleRequirementAsync, which we must implement in our handler class.

The following code shows the signature of the HandleRequirementAsync. We get the context and the requirement that is being checked as the parameter. The context class has User (ClaimPrincipal) object. We use that to check whether the user has any claim.

If the user passes the requirement, then the handler must call the context.Succeed method before returning.

If the user fails the requirement, then the handler must call the context.Fail() before returning.

If the user neither fails nor passes, then the handler must just return.

The following listing shows the codes of our Handler classes.

All the handlers are similar in nature. They access the User object from the context class and check if the user has a certain claim.




Registering the Handlers

The next step is to register the handlers in DI. We use the AddSingleton to register them. But if the handlers inject any of the other services, which has a different lifetime scope, then you may have to use it.

Create the Policy based on Requirement

Finally, We create the canManageProduct Policy using the requirement.

Applying the Policy

Apply the policy on any controller using the Authorize attribute as shown below.

Thats it.

Testing the Application

Now, run the app. Register and log in.

Try Creating a new Product, you will see the Access denied page.

Add the Employee Claim, logout and login. Now you will be able to create a new product

Add the disabled Claim, logout, and log in. Now, you will see the Access denied page while creating the new product..

Policy based Authorization example in ASP.NET Core


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