Custom authorization policies with IAuthorizationRequirementData

Article
06/16/2023

Consider the following sample that implements a custom MinimumAgeAuthorizationHandler:

using AuthRequirementsData.Authorization;
using Microsoft.AspNetCore.Authorization;

var builder = WebApplication.CreateBuilder();

builder.Services.AddAuthentication().AddJwtBearer();
builder.Services.AddAuthorization();
builder.Services.AddControllers();
builder.Services.AddSingleton<IAuthorizationHandler, MinimumAgeAuthorizationHandler>();

var app = builder.Build();

app.MapControllers();

app.Run();

The MinimumAgeAuthorizationHandler class:

using Microsoft.AspNetCore.Authorization;
using System.Globalization;
using System.Security.Claims;

namespace AuthRequirementsData.Authorization;

class MinimumAgeAuthorizationHandler : AuthorizationHandler<MinimumAgeAuthorizeAttribute>
{
    private readonly ILogger<MinimumAgeAuthorizationHandler> _logger;

    public MinimumAgeAuthorizationHandler(ILogger<MinimumAgeAuthorizationHandler> logger)
    {
        _logger = logger;
    }

    // Check whether a given MinimumAgeRequirement is satisfied or not for a particular
    // context.
    protected override Task HandleRequirementAsync(AuthorizationHandlerContext context,
                                               MinimumAgeAuthorizeAttribute requirement)
    {
        // Log as a warning so that it's very clear in sample output which authorization
        // policies(and requirements/handlers) are in use.
        _logger.LogWarning("Evaluating authorization requirement for age >= {age}",
                                                                    requirement.Age);

        // Check the user's age.
        var dateOfBirthClaim = context.User.FindFirst(c => c.Type == 
                                                                 ClaimTypes.DateOfBirth);
        if (dateOfBirthClaim != null)
        {
            // If the user has a date of birth claim, check their age.
            var dateOfBirth = Convert.ToDateTime(dateOfBirthClaim.Value,
                                                           CultureInfo.InvariantCulture);
            var age = DateTime.Now.Year - dateOfBirth.Year;
            if (dateOfBirth > DateTime.Now.AddYears(-age))
            {
                // Adjust age if the user hasn't had a birthday yet this year.
                age--;
            }

            // If the user meets the age criterion, mark the authorization requirement
            // succeeded.
            if (age >= requirement.Age)
            {
                _logger.LogInformation(
                    "Minimum age authorization requirement {age} satisfied", 
                      requirement.Age);
                context.Succeed(requirement);
            }
            else
            {
                _logger.LogInformation("Current user's DateOfBirth claim ({dateOfBirth})"
                   + " does not satisfy the minimum age authorization requirement {age}",
                    dateOfBirthClaim.Value,
                    requirement.Age);
            }
        }
        else
        {
            _logger.LogInformation("No DateOfBirth claim present");
        }

        return Task.CompletedTask;
    }
}

The custom MinimumAgePolicyProvider:

using Microsoft.AspNetCore.Authentication.JwtBearer;
using Microsoft.AspNetCore.Authorization;
using Microsoft.Extensions.Options;

namespace AuthRequirementsData.Authorization;

class MinimumAgePolicyProvider : IAuthorizationPolicyProvider
{
    const string POLICY_PREFIX = "MinimumAge";
    public DefaultAuthorizationPolicyProvider FallbackPolicyProvider { get; }
    public MinimumAgePolicyProvider(IOptions<AuthorizationOptions> options)
    {
        FallbackPolicyProvider = new DefaultAuthorizationPolicyProvider(options);
    }
    public Task<AuthorizationPolicy> GetDefaultPolicyAsync() => 
                            FallbackPolicyProvider.GetDefaultPolicyAsync();
    public Task<AuthorizationPolicy?> GetFallbackPolicyAsync() =>
                            FallbackPolicyProvider.GetFallbackPolicyAsync();

    public Task<AuthorizationPolicy?> GetPolicyAsync(string policyName)
    {
        if (policyName.StartsWith(POLICY_PREFIX, StringComparison.OrdinalIgnoreCase) &&
            int.TryParse(policyName.Substring(POLICY_PREFIX.Length), out var age))
        {
            var policy = new AuthorizationPolicyBuilder(
                                                JwtBearerDefaults.AuthenticationScheme);
            policy.AddRequirements(new MinimumAgeRequirement(age));
            return Task.FromResult<AuthorizationPolicy?>(policy.Build());
        }

        return Task.FromResult<AuthorizationPolicy?>(null);
    }
}

ASP.NET Core only uses one authorization policy provider. If the custom implementation doesn't handle all policies, including default policies, etc., it should fall back to an alternate provider. In the preceding sample, a default authorization policy provider is:

Constructed with options from the dependency injection container.
Used if this custom provider isn't able to handle a given policy name.

If a custom policy provider is able to handle all expected policy names, setting the fallback policy with GetFallbackPolicyAsync() isn't required.

class MinimumAgePolicyProvider : IAuthorizationPolicyProvider
{
    const string POLICY_PREFIX = "MinimumAge";
    public DefaultAuthorizationPolicyProvider FallbackPolicyProvider { get; }
    public MinimumAgePolicyProvider(IOptions<AuthorizationOptions> options)
    {
        FallbackPolicyProvider = new DefaultAuthorizationPolicyProvider(options);
    }
    public Task<AuthorizationPolicy> GetDefaultPolicyAsync() => 
                            FallbackPolicyProvider.GetDefaultPolicyAsync();
    public Task<AuthorizationPolicy?> GetFallbackPolicyAsync() =>
                            FallbackPolicyProvider.GetFallbackPolicyAsync();

Policies are looked up by string name, therefore parameters, for example, age, are embedded in the policy names. This is abstracted away from developers by the more strongly-typed attributes derived from AuthorizeAttribute. For example, the [MinimumAgeAuthorize()] attribute in this sample looks up policies by string name.

public Task<AuthorizationPolicy?> GetPolicyAsync(string policyName)
{
    if (policyName.StartsWith(POLICY_PREFIX, StringComparison.OrdinalIgnoreCase) &&
        int.TryParse(policyName.Substring(POLICY_PREFIX.Length), out var age))
    {
        var policy = new AuthorizationPolicyBuilder(
                                            JwtBearerDefaults.AuthenticationScheme);
        policy.AddRequirements(new MinimumAgeRequirement(age));
        return Task.FromResult<AuthorizationPolicy?>(policy.Build());
    }

    return Task.FromResult<AuthorizationPolicy?>(null);
}

The MinimumAgeAuthorizeAttribute uses the IAuthorizationRequirementData interface that allows the attribute definition to specify the requirements associated with the authorization policy:

using Microsoft.AspNetCore.Authorization;

namespace AuthRequirementsData.Authorization;

class MinimumAgeAuthorizeAttribute : AuthorizeAttribute, IAuthorizationRequirement,
                                     IAuthorizationRequirementData
{
    public MinimumAgeAuthorizeAttribute(int age) => Age = age;
    public int Age { get; }

    public IEnumerable<IAuthorizationRequirement> GetRequirements()
    {
        yield return this;
    }
}

The GreetingsController displays the user's name when they satisfy the minimum age policy:

using AuthRequirementsData.Authorization;
using Microsoft.AspNetCore.Mvc;

namespace AuthRequirementsData.Controllers;

[ApiController]
[Route("api/[controller]")]
public class GreetingsController : Controller
{
    [MinimumAgeAuthorize(16)]
    [HttpGet("hello")]
    public string Hello() => $"Hello {(HttpContext.User.Identity?.Name ?? "world")}!";
}

The complete sample can be found in the AuthRequirementsData folder of the AspNetCore.Docs.Samples repository.

The sample can be tested with dotnet user-jwts and curl:

dotnet user-jwts create --claim http://schemas.xmlsoap.org/ws/2005/05/identity/claims/dateofbirth=1989-01-01
curl -i -H "Authorization: Bearer <token from dotnet user-jwts>" http://localhost:<port>/api/greetings/hello

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Custom authorization policies with IAuthorizationRequirementData

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