Azure Logic Apps now supports writing custom connectors, which are just custom REST APIs for which you can customize the experience so that they feel like the built-in Logic Apps connectors. I wanted to give try my hand at writing one, so decided on a simple use case: Writing a connector that provides a way to retrieve a secret stored in Azure Key Vault.

Key Vault Connector

This part 1 in a series of articles on my experience writing this.

Update 2017-10-17: A few details on the post have been updated due to me confusing summary and x-ms-summary.

Connector Implementation

In this post, I’d like to talk a bit about the connector implementation. I decided to implement this as an ASP.NET Core 2.0 WebAPI application that uses the Microsoft.Azure.KeyVault library to query the secret information.

For my initial test, all I implemented was two simple operations:

  • List available secrets in the selected Key Vault store
  • Get the value of a specified secret (for the latest version, or for a specific one)

For the most part, implementation is fairly straightforward, but there are a couple of interesting things that are worth discussing.

OpenAPI Extensions

The connector uses the Swashbuckle library to generate the OpenAPI (Swagger) description for the API. A Logic Apps custom connector has an extended OpenAPI definition that includes things such as:

  • Operation descriptions
  • Additional metadata for object members and operations (such as descriptions)
  • Dynamic parameters and schemas

You can either write the connector OpenAPI description by hand and upload it, or start with a standard definition and then customize it in the Azure Portal. I quickly found out that every time you point Logic Apps to your updated OpenAPI definition after changes will overwrite all your customizations, so you’d need to do them all over again.

This makes sense, since the idea is that you’d normally upload an OpenAPI definition that already contains the necessary extensions, but if not, it quickly gets really annoying.

What I did to simplify this process somewhat was writing a couple of Swashbuckle extensions to produce at least a working definition right from the start.

For every operation in your connector, you should at least have a description and an x-ms-summary field. You’ll also want to correctly name your operation as you want it to appear in the Logic Apps designer by setting the operationId.

I ended up writing my operations like this:

[HttpGet()]
[SwaggerOperation(OperationId = "ListSecrets")]
[Summary("Lists all secrets")]
[Description("Lists the secrets stored in Key Vault")]
public  async Task<IEnumerable<Secret>> Get(String vaultName)

Notice how I use the DescriptionAttribute to specify the longer operation description, and the SummaryAttribute to add the value for summary. Both are processed by custom IOperationFilter extensions for Swashbuckle. Here’s the code for handling the summary:

[AttributeUsage(AttributeTargets.Method | AttributeTargets.Parameter)]
public class SummaryAttribute : Attribute {
  public String Summary { get; private set; }

  public SummaryAttribute(String summary) {
    this.Summary = summary;
  }
}

public class SummaryFilter : IOperationFilter {
  public void Apply(Operation operation, OperationFilterContext context) {
    var apiDesc = context.ApiDescription;
    var summary = apiDesc.ActionAttributes()
                         .OfType<SummaryAttribute>()
                         .FirstOrDefault();
    if ( summary != null ) {
      operation.Summary = summary.Summary;
    }
  }
}

The code for handling the description is very similar to this. All that remained was registering these extensions during startup, like this:

services.AddSwaggerGen(c => {
  c.SwaggerDoc("v1", new Info { Title = "KeyVaultController", Version = "v1" });
  c.OperationFilter<SummaryFilter>();
  c.OperationFilter<DescriptionFilter>();
});

I’ve been looking at a similar mechanism for adding support for x-ms-dynamic-values, but there are two reasons I haven’t done so yet:

  • It’s too complex for expressing in attributes, so need a better way of adding these descriptions.
  • Despite spending hours trying everything I could think of, I was completely unable to get dynamic values in parameters to work. I’m sure I was doing something wrong, but I can’t seem to figure out what.

Once I can get these to work, I’ll figure out a nice way to represent this in code.

Authentication

The second interesting part in the code is the authentication part. Making a connector that internally handled authentication to Key Vault using a Service Principal would have been trivial, but very unflexible. What if I wanted to support arbitrary Key Vaults?

So what I decided to do was force the consumer of the connector authenticate via Azure Active Directory with a user that had permissions to access Key Vault, and then do an On-behalf-of flow to get a token to Key Vault. I wasn’t entirely sure how this would work when I started, but turned out to be very possible, and even documented (though the docs could be clearer).

Authentication basically works like this:

           +---------------+          +---------------+          +-----------------+
 User      |               | Provide  |               | Provide  |                 |
 Identity  |   Logic App   | Token    |   Connector   | Token    |    Key Vault    |
+--------> |    Client     +--------> |   WebApi      +--------> |                 |
           |               |          |               |          |                 |
           +-------+-------+          +-------+-------+          +-----------------+
                   |                          |
                   |                          | Obtain
                   |                          | on-behalf-of
                   |                          | token
                   |                          |
                   |                          v
                   | Auth with        +---------------+
                   | client app       |               |
                   +----------------> |   Azure AD    |
                                      |               |
                                      +---------------+

When using Azure AD authentication, Logic Apps requires that we register 2 applications in AAD: One is a client web application, and the other is the app representing the connector itself.

Setting up authentication

First thing I needed to do to support this was implement OAuth-based authentication in the WebApi itself.

I did this by implementing JSON Web Token (JWT) Bearer authentication, in ConfigureServices():

services.AddAuthentication(o => {
  o.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
  o.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
}).AddJwtBearer(options => {
  options.Authority = Configuration["Auth:Authority"];
  options.Audience = Configuration["Auth:Audience"];
  options.SaveToken = true;
  // hack: don't validate issuers for now to support multi-tenant
  options.TokenValidationParameters.ValidateIssuer = false;
});

We first add authentication setting the default authentication and challenge scheme to JwtBearer. I first missed this step when implementing the solution and took me a lot of time to figure out this was why authentication was not working!

Then we add the authentication mechanism, setting the Authority and Audience, like this:

  • Authority: I set this to https://login.microsoftonline.com/common, since I wanted to register the connector as a multi-tenant application (I have not tested this yet, though).
  • Audience: This is just the Application Id of the application as registered in Azure AD. Again, since I was registering this as a multi-tenant application, it needs to be something like https://[tenantname].onmicrosoft.com/[guid]. Note that if the Azure AD had a custom domain name, this would be slightly different.

A couple more things are interesting in this registration:

  • Setting SaveToken to true is important, as we’ll need the raw bearer token later on for the on-behalf-of flow.
  • Setting ValidateIssuer to false. This is definitely a bad idea, as we won’t be validate the issuer signature on the received tokens, but it was the simplest way for now to make it work. The reason for that is that since the authority is set to the common endpoint, rather than a specific Azure Active Directory tenant, we don’t have the necessary keys to validate the issuer signature. Normally, you’d implement this validation manually against a list of known issuers, but that was not really something I needed for now. Also, to some extent I don’t care, since only the issuer tenant will be able to complete the on-behalf-of flow, anyway.

Including authentication information in OpenAPI

In order to be able to configure the authentication settings in the custom Logic App connector, we need that the OpenAPI description includes the supported authentication mechanism. So we want Swashbuckle to include this information in our service description.

To do this, I added this when configuring the Swagger generation in ConfigureServices():

services.AddSwaggerGen(c => {
  // ...
  c.AddSecurityDefinition("AAD", new OAuth2Scheme {
    AuthorizationUrl = "https://login.windows.net/common/oauth2/authorize",
    TokenUrl = "https://login.windows.net/common/oauth2/token",
    Flow = "accessCode",
    Scopes = new Dictionary<String, String>()
  });
  // ...
});

Obtaining the Key Vault token

The final piece of the authentication puzzle was obtaining the delegated token to Key Vault in the service operations. To do this, I use the overload of the KeyVaultClient class that takes an authentication callback, which I implemented like this:

protected async Task<String> Authenticate(String tenantId, String resource, String scope) {
  Logger.LogTrace("Authenticate({0}, {1}, {2})", tenantId, resource, scope);
  String clientId = Configuration["Auth:ClientId"];
  String clientSecret = Configuration["Auth:ClientSecret"];

  var appCredentials = new ClientCredential(clientId, clientSecret);

  var assertion = await GetUserAssertion();

  var context = new AuthenticationContext(tenantId);
  var token = await context.AcquireTokenAsync(resource, appCredentials, assertion);
  return token.AccessToken;
}

protected async Task<UserAssertion> GetUserAssertion() {
  var auth = await this.HttpContext.AuthenticateAsync();
  var token = auth.Properties.GetTokenValue("access_token");
  return new UserAssertion(token);
}

This is straightforward code using the Azure Active Directory Authentication library. When the Authenticate callback fires, we get the AAD tenant, the resource to authenticate against and the scope.

The code does the following things:

  • Obtain the client credentials. This is just the Application Id and Client Secret that authenticates our application to Azure Active Directory.
  • Obtain the user assertion. We do this by extracting the saved JWT bearer token from the authentication context (this is why enabling SaveToken is necessary)
  • Obtain the on-behalf-of token from AAD.

One improvement here would be to use a Key Vault based certificate for authenticating against Azure AD rather than using a client secret, but I didn’t want to add this complexity for now. Managed Service Identity would be even better, but this is an unsupported scenario at the moment.

Final thoughts

In a followup post I will discuss how to register the necessary applications in Azure Active Directory as well as how to register the custom connector.

The code described in this point can be found on GitHub.


Tomas Restrepo

Software developer located in Colombia. Sr. PFE at Microsoft.