Can you download an Azure Blob using Virtual Machine Identity Access Token through PowerShell without using Access Keys - azure

Question

I'm setting up a VM to do some bootstrapping on creation.
Part of this is to download a blob from an azure storage account to the VM.
These are all in the same subscription, resource group, etc.
I can do it this way fine:
function Get-BlobUsingVMIdentity
{
param(
[Parameter(Mandatory = $true)] $containerName,
[Parameter(Mandatory = $true)] $blobName,
[Parameter(Mandatory = $true)] $outputFolder
)
write-host "Defining package information"
mkdir $outputFolder -force
write-host "Getting Instance meta data"
$instanceInfo = Invoke-WebRequest -UseBasicParsing -Uri 'http://169.254.169.254/metadata/instance/?api-version=2018-02-01' `
-Headers #{Metadata="true"} `
| select -expand content `
| convertfrom-json `
| select -expand compute
$storageAccountName = "$($instanceInfo.resourceGroupName.replace('-rg',''))sa" # This is custom since we know our naming schema
$resourceGroupName = $($instanceInfo.resourceGroupName)
$subscriptionId = $($instanceInfo.subscriptionId)
write-host "Got storageAccountName [$storageAccountName], resourceGroupName [$resourceGroupName], subscriptionId [$subscriptionId]"
write-host "Getting VM Instance Access Token"
$response = Invoke-WebRequest -UseBasicParsing -Uri 'http://169.254.169.254/metadata/identity/oauth2/token?api-version=2018-02-01&resource=https%3A%2F%2Fmanagement.azure.com%2F' `
-Headers #{Metadata = "true" }
$content = $response.Content | ConvertFrom-Json
$access_token = $content.access_token
write-host "Getting SAS Token From Storage Account"
$params = #{canonicalizedResource = "/blob/$($storageAccountName)/$($containerName)"; signedResource = "c"; signedPermission = "rcw"; signedProtocol = "https"; signedExpiry = "2031-09-23T00:00:00Z" }
$jsonParams = $params | ConvertTo-Json
$sasResponse = Invoke-WebRequest -UseBasicParsing -Uri "https://management.azure.com/subscriptions/$($subscriptionId)/resourceGroups/$($resourceGroupName)/providers/Microsoft.Storage/storageAccounts/$($storageAccountName)/listServiceSas/?api-version=2017-06-01" `
-Method POST `
-Body $jsonParams `
-Headers #{Authorization="Bearer $access_token"}
$sasContent = $sasResponse.Content | ConvertFrom-Json
$sasCred = $sasContent.serviceSasToken
write-host "Manually download blob"
$params = #{signedResource = "c"; signedPermission = "rcw"; signedProtocol = "https"; signedExpiry = "2031-09-23T00:00:00Z" }
$jsonParams = $params | ConvertTo-Json
$sasResponse = Invoke-WebRequest -UseBasicParsing -Uri "https://$($storageAccountName).blob.core.windows.net/$($containerName)/$($blobName)?api-version=2017-06-01" `
-Method POST `
-Body $jsonParams `
-Headers #{Authorization="Bearer $access_token"}
$sasContent = $sasResponse.Content | ConvertFrom-Json
$sasCred = $sasContent.serviceSasToken
write-host "Setting up storage context"
$ctx = New-AzStorageContext -StorageAccountName $storageAccountName -SasToken $sasCred
write-host "Downloading package"
Get-AzStorageBlobContent `
-Blob $blobName `
-Container $containerName `
-Destination $outputFolder `
-Context $ctx `
-Force
}
This works fine, except I have to grant full/write access to the identity in order for it to use the access key.
Is there a similar approach that would allow read only access to the blob?
My goals are:
1. No credentials stored anywhere
2. Download blob to VM from azure storage
3. No statically defined variables (ex: subscriptionid)
4. Read only access to the blob/storage account.
Appreciate any help!

Answer 1

According to my understanding, you want to use Azure VM MSI to access Azure storage. If so, please refer to the following steps:
Enable a system-assigned managed identity on a VM
Connect-AzAccount
$vm = Get-AzVM -ResourceGroupName myResourceGroup -Name myVM
Update-AzVM -ResourceGroupName myResourceGroup -VM $vm -AssignIdentity:$SystemAssigned
Grant your VM access to an Azure Storage container
Connect-AzAccount
$spID = (Get-AzVM -ResourceGroupName myRG -Name myVM).identity.principalid
New-AzRoleAssignment -ObjectId $spID -RoleDefinitionName "Storage Blob Data Reader" -Scope "/subscriptions/<mySubscriptionID>/resourceGroups/<myResourceGroup>/providers/Microsoft.Storage/storageAccounts/<myStorageAcct>/blobServices/default/containers/<container-name>"
access blob
# get AD access token
$response = Invoke-WebRequest -Uri 'http://169.254.169.254/metadata/identity/oauth2/token?api-version=2018-02-01&resource=https%3A%2F%2Fmanagement.azure.com%2F' `
-Headers #{Metadata="true"}
$content =$response.Content | ConvertFrom-Json
$access_token = $content.access_token
# call Azure blob rest api
$url="https://<myaccount>.blob.core.windows.net/<mycontainer>/<myblob>"
$RequestHeader = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
$RequestHeader.Add("Authorization", "Bearer $access_token")
$RequestHeader.Add("x-ms-version", "2019-02-02")
$result = Invoke-WebRequest -Uri $url -Headers $RequestHeader
$result.content
Update
According to my test, when we get token to access Azure blob, we need to change resouce as https://storage.azure.com/
# get AD access token
$response = Invoke-WebRequest -Uri 'http://169.254.169.254/metadata/identity/oauth2/token?api-version=2018-02-01&resource=https://storage.azure.com/' `
-Headers #{Metadata="true"}
$content =$response.Content | ConvertFrom-Json
$access_token = $content.access_token
# call Azure blob rest api
$url="https://<myaccount>.blob.core.windows.net/<mycontainer>/<myblob>"
$RequestHeader = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
$RequestHeader.Add("Authorization", "Bearer $access_token")
$RequestHeader.Add("x-ms-version", "2019-02-02")
$result = Invoke-WebRequest -Uri $url -Headers $RequestHeader
$result.content

Answer 2

Update
Found a much easier way to do it!
The key was to use:
Connect-AzAccount -identity
This auto logs in a the identity and allows you to interact through AZ module instead of trying to hack API calls together.
You only need [Storage Blob Data Reader] assigned to the container to pull this off.
Working example looks like this:
function Get-BlobUsingVMIdentity
{
param(
[Parameter(Mandatory = $true)] $containerName,
[Parameter(Mandatory = $true)] $blobName,
[Parameter(Mandatory = $true)] $outputFolder
)
write-host "Get instance info"
$instanceInfo = Invoke-WebRequest -UseBasicParsing -Uri 'http://169.254.169.254/metadata/instance/?api-version=2018-02-01' `
-Headers #{Metadata="true"} `
| select -expand content `
| convertfrom-json `
| select -expand compute
$storageAccountName = "$($instanceInfo.resourceGroupName.replace('-rg',''))sa" # This is custom since we know our naming schema
write-host "Clear existing identies to keep cache fresh"
Clear-AzContext -force
write-host "Authenticate using identity"
$account = Connect-AzAccount -identity
if(-not $account.Context.Subscription.Id)
{
write-error "Failed to authenticate with identity. Ensure VM has identity enabled and is assigned the correct IAM roles"
return
}
write-host "Get storage context"
$ctx = New-AZStorageContext -StorageAccountName $storageAccountName
write-host "Getting blob"
Get-AzStorageBlobContent `
-Blob $blobName `
-Container $containerName `
-Destination $outputFolder `
-Context $ctx `
-Force
}
Get-BlobUsingVMIdentity `
-containerName "deploy" `
-blobName "deploy.zip" `
-outputFolder "c:\deploy\"