Wednesday, June 24, 2020

How to get Azure REST APIs access tokens using PowerShell

Sometimes, I had to step out of the comfort of the Azure PowerShell  module and call Azure REST APIs directly. Usually, it is required when there is no cmdlet wrapper for some API, or Az module does not support some underlying API functionality.

As you already know, such calls are regular HTTP requests and can be executed by using cmdlets Invoke-WebRequest or Invoke-RestMethod. The essential part of these HTTP requests is authentication. For this purpose, the HTTP request must contain "Authorization" header that contains access token for API.

Little bit of theory

It is relatively easy to get the token when your code has complete control over credentials. For example, it is interactive PowerShell session where user can provide them, or it is a script that has values of the client id and client secret for service principal. However, often, the scripts have to be executed in the automated, non-interactive environment like CI/CD pipelines where underlying CI/CD product (e.g. Azure DevOps) manages the access credentials and prepares Azure Context for script execution (by implicit Connect-AzAccount cmdlet execution). In such scenarios, it possible to utilize Azure PowerShell module ability to transparently get access token when its cmdlets access control/data planes of the different services. For example, Get-AzKeyVault is control plane call against endpoint https://management.azure.com, while Get-AzKeyVaultSecret is data plane call against endpoint https://{some-vault}.vault.azure.net. The module use MSAL to acquire tokens from Azure AD, cache and renew them. A one-liner will return the list of the tokens in the current  Azure PowerShell session:

(Get-AzContext).TokenCache.ReadItems()

Practice

Now, let see how we can use this ability of the Azure PowerShell module for our purpose - call one of Azure APIs. Let say, we need to perform direct API call against our Key Vault. To ensure that token cache has access token for desired API (Key Vault), we will perform a simple secret KV read using cmdlet from Az.KeyVault module:

Get-AzKeyVaultSecret -VaultName $kvName -Name $secretName

Now, token cache has access token for data plane of our Key Vault (assuming current context identity has read access to this KV secrets and Get-AzKeyVaultSecret succeeded; the actual secrets doesn't have to exists).

We can get this token from cache

$tokenCache = (Get-AzContext).TokenCache.ReadItems()
$cacheItem = $tokenCache | Where-Object { $_.Resource -eq 'https://vault.azure.net' }
$kvAccessToken = $cacheItem.AccessToken

and use it to call desired API

$token = ConvertTo-SecureString -String $kvAccessToken -AsPlainText -Force
...
Invoke-RestMethod -Method Post -Uri $URI -ContentType "application/json" `
    -Authentication Bearer -Token $token -Body $body

Side note. In the interactive session, where the user potentially is a member of the multiple Azure AD tenants or Azure PowerShell context contains multiple session for different users, additional filtering of the token cache based on TenantId and DisplayableId (user logon name) will be required.

In my next post, I will show how I used this access token acquisition technique to solve a real life "non-standard" task.

Update (November 26, 2020)

Since the release of Az module version 5.x, cmdlet Get-AzContext doesn't populate TokenCache property anymore. New cmdlet Get-AzAccessToken, avalable starting Az v 5.1.0, can be used now to acquire access tokens:

$kvAccessToken = (Get-AzAccessToken -ResourceUrl 'https://vault.azure.net').Token

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