How to get secrets from HashiCorp Vault into .NET configuration with C#

Time to read: 9 minutes

November 28, 2022

Written by

Volkan Paksoy

Contributor

Reviewed by

Niels Swimberghe

Twilion

How to get secrets from HashiCorp Vault into .NET configuration with C#

Configuration management has always been a challenge for developers. It gets especially tricky when it comes to storing sensitive configuration values such as API keys, tokens, certificates, passwords etc. In this article, you will learn how to use HashiCorp Vault with C# .NET to manage your application’s secrets.

Prerequisites

You'll need the following things in this tutorial:

A free Twilio account
A Twilio Phone Number with SMS capabilities.
An OS that supports .NET (Windows/macOS/Linux)
.NET 6.0 SDK (newer and older versions may work too)
A code editor or IDE (Recommended: Visual Studio Code with the C# plugin, Visual Studio, or JetBrains Rider)
Docker Engine (You can install the engine using Docker Desktop (Windows, macOS, and Linux), Colima (macOS and Linux), or manually on any OS.)

Problem Statement

Let’s start by implementing a simple application to demonstrate the issue. It’s a simple .NET console application that sends a single SMS via Twilio.

Run the following commands in a terminal to create the project and add the Twilio NuGet package:

				mkdir VaultDemo 
cd VaultDemo
dotnet new console
dotnet add package Twilio

		

Open the project with your IDE and replace the contents of Program.cs with the code below:

				using Twilio;
using Twilio.Rest.Api.V2010.Account;
using Twilio.Types;

var accountSid = "{ YOUR TWILIO ACCOUNT SID }";
var authToken = "{ YOUR TWILIO AUTH TOKEN }";
var senderPhoneNumber = "{ SENDER PHONE NUMBER }";
var recipientPhoneNumber = "{ RECIPIENT PHONE NUMBER }";

TwilioClient.Init(accountSid, authToken);

MessageResource.Create(
    body: "Nothing fancy, just a simple SMS.",
    from: new PhoneNumber(senderPhoneNumber),
    to: new PhoneNumber(recipientPhoneNumber)
);

		

Replace the placeholders with actual values. To obtain your Twilio Account SID, AuthToken, and Twilio Phone Number, log in to the Twilio Console and copy the values shown in the account info section:

The account information section on Twilio Console which shows the Account SID, Auth Token (masked) and Twilio phone number

For the recipient phone number, use your actual phone number so that you can receive the SMS and confirm the application is working.

Run the application by running the following command in the terminal:

dotnet run

You now have a working, standalone application without any dependencies on external configuration. Hardcoding secrets this way might only work for throw-away code. Even then, it’s ill-advised, and you should never use this approach. If you forget to delete the code, you will expose your secrets in cleartext.

Most applications use a version control system (such as GitHub, GitLab etc). In enterprise, it’s safe to say all code is pushed to a source code repository. If you hardcode your secrets and push your secrets to the version control system, anybody who has access to the code will have access to your secrets too. If you’re working on a public open-source project, you have now exposed your secrets to the entire world.

If you are using Git as your version control system, even if you delete the sensitive data immediately, it will still exist in your git history.

As a general rule of thumb, putting secrets in your source code is considered to be a terrible practice.

Environment Variables

A better approach is using environment variables. This way, you can completely separate your code from your config. If you subscribe to the Twelve-Factor-App philosophy, you can see they describe this approach as “Store config in the environment”. You can read more about it in their Config section.

Now, update your application and replace the hardcoded values with the following lines:

				var accountSid = Environment.GetEnvironmentVariable("TWILIO_ACCOUNT_SID");
var authToken = Environment.GetEnvironmentVariable("TWILIO_AUTH_TOKEN");
var senderPhoneNumber = Environment.GetEnvironmentVariable("SENDER_PHONE_NUMBER");
var recipientPhoneNumber = Environment.GetEnvironmentVariable("RECIPIENT_PHONE_NUMBER");

		

For macOS and Linux, set the environment variable like this:

export {KEY}={VALUE}

If you're using PowerShell on Windows or another OS, use this command:

$Env:{KEY} = "{VALUE}"

If you're using CMD on Windows, use this command:

set "{KEY}={VALUE}"

Replace {KEY} with the name of the configuration/secret key (such as TWILIO_ACCOUNT_SID). Replace {VALUE} with the value of the configuration/secret (the values you hardcoded in the previous example).

Repeat the above for all 4 configuration values (TWILIO_ACCOUNT_SID, TWILIO_AUTH_TOKEN, TWILIO_PHONE_NUMBER and RECIPIENT_PHONE_NUMBER).

Run the application and confirm it’s still working. Now you have given yourself the opportunity of checking in your code to the version control system as there are no sensitive values in it anymore.

User Secrets

You can also improve the local development environment security by using .NET user secrets. This way, the secrets are stored in a JSON configuration file in the user profile directory. Since the secrets are persisted, you don’t have to enter them over and over again, whereas with the environment variable you will lose the values if you close the terminal.

To use user secret in the demo application, run the following command:

dotnet user-secrets init

Now you can add the secrets by running the following commands:

dotnet user-secrets set "KEY" "VALUE"

To be able to read these values back, you will need configuration extension NuGet packages from Microsoft. Run the following commands to add those libraries:

				dotnet add package Microsoft.Extensions.Configuration
dotnet add package Microsoft.Extensions.Configuration.EnvironmentVariables
dotnet add package Microsoft.Extensions.Configuration.UserSecrets

		

You can also use the appSettings.json file, command-line arguments, and more to read the configuration values. You can read more about various configuration options at configuration in .NET.

Update Program.cs as shown below:

				
using Microsoft.Extensions.Configuration;
using Twilio;
using Twilio.Rest.Api.V2010.Account;
using Twilio.Types;

IConfiguration config = new ConfigurationBuilder()
    .AddUserSecrets<Program>(optional: true, reloadOnChange: false)
    .AddEnvironmentVariables()
    .Build();

var accountSid = config["TWILIO_ACCOUNT_SID"];
var authToken = config["TWILIO_AUTH_TOKEN"];
var senderPhoneNumber = config["SENDER_PHONE_NUMBER"];
var recipientPhoneNumber = config["RECIPIENT_PHONE_NUMBER"];

TwilioClient.Init(accountSid, authToken);

MessageResource.Create(
    body: "Nothing fancy, just a simple SMS.",
    from: new PhoneNumber(senderPhoneNumber),
    to: new PhoneNumber(recipientPhoneNumber)
);

		

Run the application, and it should still work.

This example leverages both user secrets and environment variables. You can still add environment variables to overwrite the values in user secrets. The last key loaded wins, and all the other ones are overwritten. So be careful when specifying multiple providers. The benefit of this approach is now you are not directly reading from environment variables, which means you have more control over where you store your configuration.

Now you’re in a more secure position when storing the secrets locally in your development environment. What about deployments, though? When you deploy your application, it will fail because it won’t find the secrets. You can choose to remotely log in to your servers and set the environment variables or user secrets, but it’s impractical and doesn’t scale.

A better approach is to use a central, secure place to store your configuration so that all instances of your application can easily read those values.

Many services provide this functionality, such as Azure Key Vault, AWS Secrets Manager, and HashiCorp Vault. In this article, you will learn how to set up HashiCorp Vault using Docker and configure your application to get your configuration from it.

Set Up HashiCorp Vault

Vault is an open-source project and can be found on HashiCorp’s GitHub repository. Since it’s open-source, you have a few alternatives for using Vault:

Clone/fork the GitHub repository and build it yourself
Download and run one of the installers created for your platform
Run it in a Docker container
Sign up and use their cloud-based solution

The first 3 options are self-hosted and completely free. For the hosted solution, you can sign up for free and evaluate it for 30 days. In this article, you’ll run it in a Docker container.

The focus of the article is not setting up an enterprise-grade Vault cluster, which would be too complicated to cover in a single article. You will see how to run it for your development environment in a single Docker container, but the principle is the same. So if you later purchase a cloud-hosted solution, all you have to do is change the vault address. You can find out more about Vault pricing and packaging here.

Vault is one of the select few official images, and it’s quite popular on Docker Hub:

Vault image properties on Docker Hub showing it"s an official image with more than 100 million downloads and 1K stars

Run the following command to pull the latest Vault image and run Vault in a container:

docker run -d -p 8200:8200 --cap-add=IPC_LOCK --name=dev-vault vault

In the command above -d flag indicates to run it in the background. --cap-add=IPC_LOCK prevents sensitive values from being swapped to disk. As the name of the container implies, this is for development only. Everything runs in memory and all the secrets will disappear if you stop the container.

Now, open a browser and go to http://localhost:8200. You should see a sign-in screen like this:

Vault sign in page showing the authentication method (Token selected) and a input with Token label

To get your token, first, find the container id by running the command below:

docker ps | grep vault

You should see something like this:

Terminal showing the output of docker ps | grep vault command and the container id as the first value (892327726ea3) on the output line.

The first value you see is the container id (in this example it’s 892327726ea3)

Copy that value and run the command below by replacing { YOUR CONTAINER ID } with the value you copied:

docker logs { YOUR CONTAINER ID }

At the end of the logs, you should see your root token:

Container logs showing the root token along with warning about this not being suitable for a production environment.

Copy your root token and use it in the sign-in screen.

Vault server starts in a sealed state, meaning it doesn’t know how to decrypt the data. Unsealing is the process of obtaining the plaintext root key necessary to read the decryption key to decrypt the data, allowing access to the Vault. You can read more about sealing and unsealing here.

You should see the default secrets engines:

Vault UI showing the default secrets engines: cubbyhole and secret

Cubbyhole and key/value secret engines are enabled by default (and they cannot be disabled). The cubbyhole secrets engine is used to store arbitrary secrets. Paths are scoped per token, and no token can access another token's cubbyhole. In this article, you will use Key/Value secret engine, which is a generic secret engine.

Click secret to view the existing secrets (which are none at the moment). You should see a screen like this:

Secret engine showing no secrets and a Create secret button

Click the Create secret button.

Set the path to your secret as twilioapp. Add your config values as you did in the previous examples. Your screen should look like this:

Vault secret showing the path (twilioapp) and all the config values entered (Values masked)

Click the Save button. Now you should see the values saved as Version 1 of your configuration.

Now that your secrets are Vault, it’s time to modify the application to read these values.

Using Vault C# Client

To access Vault with C#, you are going to use a library called VaultSharp. Run the following command to add the NuGet package to your project:

dotnet add package VaultSharp

Set the user secret VAULT_ADDR to http://127.0.0.1:8200.

Set the VAULT_TOKEN user secret to your root token.

In production, it’s more likely your operations team will provide you with a role that has access to the secrets your application needs, so you won’t have to use tokens this way. This is just for demonstration purposes.

Update Program.cs as shown below:

				using Microsoft.Extensions.Configuration;
using Twilio;
using Twilio.Rest.Api.V2010.Account;
using Twilio.Types;
using VaultSharp;
using VaultSharp.V1.AuthMethods;
using VaultSharp.V1.AuthMethods.Token;
using VaultSharp.V1.Commons;

IConfigurationBuilder configBuilder = new ConfigurationBuilder()
    .AddUserSecrets<Program>(optional: true, reloadOnChange: false)
    .AddEnvironmentVariables();

IConfiguration config = configBuilder.Build();

IAuthMethodInfo authMethod = new TokenAuthMethodInfo(config["VAULT_TOKEN"]);
var vaultClientSettings = new VaultClientSettings(config["VAULT_ADDR"], authMethod);
IVaultClient vaultClient = new VaultClient(vaultClientSettings);

Secret<SecretData> kv2Secret = await vaultClient.V1.Secrets.KeyValue.V2
    .ReadSecretAsync(path: "twilioapp", mountPoint: "secret");

configBuilder.AddInMemoryCollection(kv2Secret.Data.Data.ToDictionary(kv => kv.Key, kv => kv.Value.ToString()));
config = configBuilder.Build();

var accountSid = config["TWILIO_ACCOUNT_SID"];
var authToken = config["TWILIO_AUTH_TOKEN"];
var senderPhoneNumber = config["SENDER_PHONE_NUMBER"];
var recipientPhoneNumber = config["RECIPIENT_PHONE_NUMBER"];

TwilioClient.Init(accountSid, authToken);

MessageResource.Create(
    body: "Nothing fancy, just a simple SMS.",
    from: new PhoneNumber(senderPhoneNumber),
    to: new PhoneNumber(recipientPhoneNumber)
);

		

Run the application again, and you should now be able to get the secrets from your Vault instance.

The implementation above first gets the user secrets to be able to access Vault. Then, reads the secrets from Vault and adds them back to the .NET configuration so that all configuration values can be managed in one place. To make it more reusable, you can refactor it to use an extension method.

If you want to implement a configuration provider to retrieve information from Vault, I recommend reading this article from HashiCorp.

Vault supports various authentication methods such as app role, AWS auth, Azure auth, etc. Since my focus is on the programming side, I used the basic token authentication method. You can find out more about the other auth methods on the library’s GitHub repo.

Now stop the container by running:

docker stop { YOUR CONTAINER ID }

Run the application, and it will get an error as the Vault is not running anymore. Start the same container again by running:

docker start { YOUR CONTAINER ID }

Run your application again, and this time it will fail due to the following error: Unhandled exception. VaultSharp.Core.VaultApiException: {"errors":["permission denied"]}

If you check the logs of the container again, you will see the root token is different. Copy that one and sign in to your Vault via UI again, and you will see that your previous secrets are gone now. This is, as discussed before, because we didn’t provide a persistence engine, and everything was kept in memory. In the next section, you will learn how to fix this issue.

Persisting Secrets

When you run HashiCorp Vault in dev mode, everything is stored in-memory, and the web UI is enabled automatically. To persist secrets, you need to run Vault in server mode.

Vault has an extendable model and supports various storage providers for persisting secrets. You can use the file system, an RDBMS such as MySQL or MSSQL, a NoSQL database such as CouchDB or Amazon DynamoDB, or even a cloud-based storage service such as Google Cloud Storage or Amazon S3. You can find the full list of supported providers here. All the data will be encrypted at rest (as well as in transit), so even if a 3rd party gains access to the stored secrets, they wouldn't be able to read them. In this article, you will use the local file system to persist your secrets.

If you created a container in the previous section, stop and delete it by running the following commands:

				docker stop { YOUR CONTAINER ID }
docker rm { YOUR CONTAINER ID }

Then, create a folder to store the Vault configuration. It can be placed anywhere you like on your filesystem. The following example uses /dev/vault/config under your user's home directory.

mkdir -p $HOME/dev/vault/config

Create a file named config.hcl under that folder and update its contents as shown below:

				ui = true
disable_mlock = true

storage "file" {
  path = "/vault/file"
}

listener "tcp" {
  address = "0.0.0.0:8200"
  tls_disable = "true"
}

api_addr = "http://127.0.0.1:8200"

		

Then, run the following command to create the Vault instance:

docker run -d -p 8200:8200 --volume $HOME/dev/vault:/vault vault server

Open a browser tab and go to http://127.0.0.1:8200.

Keep in mind that this article is not meant to teach you how to install a production-grade Vault server, as it requires high-security, high-availability, backups, monitoring, etc. This is still meant to be used for local development.

This time, you will see a more involved setup since you are now running Vault in server mode:

Image showing the initial setup page for server mode asking for key shares and key threshold

Enter 1 in both key shares and key threshold fields.

You should see a successful initialization screen:

Image showing initial root token and key 1 (both masked). Also a Continue to unseal button on the bottom left and a Download keys button on the bottom right.

Click the Download keys button and get a copy of your keys. Then click the Continue to Unseal button to proceed.

You will then be prompted your Unseal key:

Image showing the Unseal Vault prompt asking for Unseal key portion

Open the JSON file you just downloaded which looks like this:

Image showing the contents of vault keys JSON file

Copy the value of the keys property (this is an array but since you requested 1 key share, it only has 1 element.

Click the Unseal button.

Now you should be redirected to the sign-in page. Copy your root token from the JSON and use it to log in.

Sign in to vault page showing Method Token selected and Token value entered in the field. Sign in button to be clicked next to log in.

You must have noticed it looks different from the dev mode. There is no secret engine called secret. To fix this, click Enable new engine.

Select KV and click Next.

Vault configuration page showing KV engine selected and the Next button to be clicked to proceed.

Enter "secret" in the Path field just to match the previous example and click Enable Engine.

Vault configuration page asking for the path of the KV secrets engine showing a path input and an Enable Engine button at the bottom

Now click the Create secret button as you did in the previous section and create secrets.

Update your new Vault token by running the following command:

dotnet user-secrets set VAULT_TOKEN { YOUR NEW ROOT KEY }

Replace { YOUR NEW ROOT KEY } with the value you copied from the file you downloaded.

Run your application again and you should receive an SMS as you did before.

Here's the difference though:

Open a terminal and find the container id by running

docker ps | grep vault

Now run the following command:

docker restart { YOUR CONTAINER ID }

Replace { YOUR CONTAINER ID } with the id you noted from the previous command.

Check the container logs by running

docker logs { YOUR CONTAINER ID }

If you recall, the last time you did this you saw an error. This time you should see something like this:

Terminal window showing the Vault has started successfully in server mode

So the Vault server is still running.

Go back to the Web UI, and it will ask you for the unseal key again. After you enter the unseal key and the root token, you can see the secrets you entered have persisted successfully.

Conclusion

In this article, I wanted to demonstrate the necessity and benefit of having a centralized, secure system to manage application secrets and configuration. You started from the very primitive hard-coding secrets to code approach to using an external service which is highly reputable and used by many top-level companies all around the world. Granted, your Vault installation is for development only, but the underlying principles still apply to production. I hope you found this article helpful. If you'd like to keep learning about .NET configuration and containerizing applications, I recommend taking a look at these articles:

Volkan Paksoy is a software developer with more than 15 years of experience, focusing mainly on C# and AWS. He’s a home lab and self-hosting fan who loves to spend his personal time developing hobby projects with Raspberry Pi, Arduino, LEGO and everything in between. You can follow his personal blogs on software development at devpower.co.uk and cloudinternals.net.

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