Inter-microservice communication validation using IdentityServer4 involves several key steps to ensure that each service can securely identify and authenticate requests from other services in the system. This process typically leverages the OAuth 2.0 client credentials grant, where each microservice acts as an OAuth 2.0 client. Here's a breakdown of the steps and an example implementation in a .NET environment:

Step 1: Setup IdentityServer4 First, configure IdentityServer4 with the necessary clients, scopes, and API resources. Each microservice will be set up as a client with access to specific scopes.

Example Configuration in IdentityServer:

public static IEnumerable<ApiScope> ApiScopes =>
    new List<ApiScope>
    {
        new ApiScope("service1.read", "Read access to Service 1"),
        new ApiScope("service2.write", "Write access to Service 2"),
    };

public static IEnumerable<Client> Clients =>
    new List<Client>
    {
        new Client
        {
            ClientId = "service1",
            AllowedGrantTypes = GrantTypes.ClientCredentials,
            ClientSecrets = { new Secret("secret1".Sha256())},
            AllowedScopes = { "service1.read" }
        },
        new Client
        {
            ClientId = "service2",
            AllowedGrantTypes = GrantTypes.ClientCredentials,
            ClientSecrets = { new Secret("secret2".Sha256())},
            AllowedScopes = { "service2.write" }
        }
    };

Grant Types

IdentityServer4 is highly configurable and supports extensions and customization of these grant types to meet specific security requirements and use cases. Each grant type serves different client scenarios, so choosing the right one is critical for the security and functionality of the application.

Authorization Code: Employed by client applications for access tokens post-user authentication, ideal for server-side apps without exposed source code.

Implicit: Formerly for clients without secure storage for secrets, like native apps; now less advised due to security issues, with a shift towards Authorization Code with PKCE.

Hybrid: Merges Authorization Code and Implicit grants, suitable for clients that manage secrets securely and need browser-based token delivery.

Client Credentials: For client applications accessing their own resources instead of representing a user, commonly used by services or daemons.

Resource Owner Password Credentials (ROPC): Permits direct user credential transmission to the authorization server; less secure, advised only when other methods are impractical.

Device Code: For devices lacking a browser or with input limitations, like smart TVs; requires identity verification via a secondary device.

Refresh Token: Not a standalone grant type but used to renew access tokens via a refresh token, paired with methods like Authorization Code.

Step 2: Obtain Access Tokens Each microservice uses its client credentials to request an access token from IdentityServer4. This token is then used to authenticate requests to other services.

Example Token Request:

var client = new HttpClient();
var disco = await client.GetDiscoveryDocumentAsync("https://your-identityserver-url");
if (disco.IsError) throw new Exception(disco.Error);

var tokenResponse = await client.RequestClientCredentialsTokenAsync(new ClientCredentialsTokenRequest
{
    Address = disco.TokenEndpoint,
    ClientId = "service1",
    ClientSecret = "secret1",
    Scope = "service2.write"
});

if (tokenResponse.IsError) throw new Exception(tokenResponse.Error);

Step 3: Secure API Endpoints Ensure that your microservices validate the access token in each incoming request. This typically involves configuring the JWT Bearer Token middleware in ASP.NET Core applications.

Example API Configuration:

public void ConfigureServices(IServiceCollection services)
{
    services.AddAuthentication("Bearer")
        .AddJwtBearer("Bearer", options =>
        {
            options.Authority = "https://your-identityserver-url";
            options.RequireHttpsMetadata = false;
            options.Audience = "service2";
        });

    services.AddAuthorization(options =>
    {
        options.AddPolicy("ApiScope", policy =>
        {
            policy.RequireAuthenticatedUser();
            policy.RequireClaim("scope", "service2.write");
        });
    });
}

public void Configure(IApplicationBuilder app)
{
    app.UseRouting();
    app.UseAuthentication();
    app.UseAuthorization();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllers();
    });
}

Step 4: Call Secured Endpoints When Service 1 needs to call Service 2, it adds the access token to the Authorization header of the HTTP request.

Example Service Call:

var apiClient = new HttpClient();
apiClient.SetBearerToken(tokenResponse.AccessToken);

var response = await apiClient.PostAsync("https://service2/api/data", new StringContent("..."));
if (!response.IsSuccessStatusCode)
{
    Console.WriteLine($"Failed to call service 2: {response.StatusCode}");
}
else
{
    var content = await response.Content.ReadAsStringAsync();
    Console.WriteLine($"Service 2 response: {content}");
}

Step 5: Continuous Security Assessment Regularly update IdentityServer4 and your microservices to the latest versions. Monitor authentication and authorization failures for signs of security issues. Conduct periodic security reviews and penetration tests to ensure the integrity of your inter-service communication.