Abusing and Detecting Alternative Data Channel Command Execution on Azure Virtual Machines

Currently, command execution on virtual machines (VM) in Azure happens through the cmdlet Invoke-AzVMRunCommand. There are other specific ways, such as using an Azure Runbook if a RunAs account is being used. However, after some experimentation, there is another data channel that can be abused by Azure VMs to allow an attacker to run commands on a machine without the use of Invoke-AzVMRunCommand* by leveraging userData. The asterisk (*) is there because technically Invoke-AzVMRunCommand is needed once to setup this technique. Before getting into the code and examples, a few things must be covered.

The userData field on an Azure VM is used to include setup scripts or other metadata during provisioning. Through the portal, it looks like this:

Figure 1: Modifying the ‘user data’ field on an Azure VM.

While intended to be used for provisioning, it is also possible to modify the contents of this property even after the VM is created. The VM is able to fetch this property through a REST API call.

$userData = Invoke-RestMethod -Headers @{"Metadata"="true"} -Method GET -NoProxy -Uri "http://169.254.169.254/metadata/instance/compute/userData?api-version=2021-01-01&format=text"
[System.Text.Encoding]::UTF8.GetString([Convert]::FromBase64String($userData))
Figure 2: Calling the REST API to gather the ‘userdata’ property contents from within the VM.

The REST API in this case runs on the Azure Instance Metadata Service (IMDS). IMDS is intended to be something query-able from the VM in order to fetch metadata about itself, such as name, region, disk space, etc. and is only able to be reached by the localhost as the security boundary for IMDS is the resource it is bound to, which in this case it is the virtual machine. The userData property can then be retrieved through the VM locally over 169.254.169.254 via IMDS and it can also be edited through the Azure portal and Graph REST API.

Invoke-RestMethod -Method PATCH -Uri https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Compute/virtualMachines/{vmName}?api-version=2021-07-01 -Body $Json -Header $Headers -ContentType 'application/json'

While the local VM can query the IMDS REST API with a GET request, GET is the only approved verb, meaning PUT and PATCH was not possible with the http://169.254.169.254/metadata/instance URI (IMDS), meaning the local VM cannot modify any metadata (including userData) through IMDS. The metadata can only be modified with the Azure REST API. The permission needed to modify this property is Microsoft.Compute/virtualMachines/write which is included in your typical VM management RBAC roles (VM Contributor, Contributor, etc.).

To summarize:

  • VMs can locally retrieve the userData property from the IMDS REST API
  • Users can modify this property through the portal or Azure REST API

The hypothesized technique then looks like this:

The first challenge was then automating the Azure VM to poll the IMDS REST API for the userData field. If commands are constantly sent, then the VM will have to autonomously make the request to IMDS, decode the command, then run the command. Basically, the VM needs an agent. The simplest method I could think of for a basic agent, was to create a PowerShell script that can be uploaded with Invoke-AzVMRunCommand and will do three things:

  1. Create a Scheduled Task that will run the script when an Event occurs. The chosen event was an Azure-specific event ID that happens several times every minute, ensuring the script is constantly executing.
  2. Make the IMDS REST API request to retrieve the uploaded data/command.
  3. Run the command and upload the result back to the userData field

The final challenge was then sending back the output of the command that was run. Since VMs cannot upload data to IMDS, but it is possible to upload over Azure REST, then including the Azure REST AccessToken in the original uploaded data would allow the VM to make authenticated requests to the Azure REST API and thus use the URI https://management.azure.com/subscriptions/ which does support PATCH & PUT.

To summarize the full technique:

  • By using Invoke-AzVMRunCommand, a PowerShell script is uploaded that will act as an “agent”. The script is autonomous and will deploy a Scheduled Task that will execute the rest of the script on an Event.
  • The initial upload to the userData field that contains the arbitrary command to be run will also include the Azure REST API access token. The data that is uploaded to the userData property will then be the arbitrary command to be run and the Azure REST API access token.
  • The VM will call the IMDS REST API to get the contents of the userData property, decode it, run the command, then use the Azure REST API to make a PUT request to upload the results of the command, which is done by using the smuggled access token.
  • The userData property can then be queried again to see the results of the command.

In PowerZure, this can now be accomplished with the two commands Invoke-AzureVMUserDataCommand and Invoke-AzureVMUserDataAgent.

Detection and Threat Hunting Azure Alternate Data Channels

There’s several assumptions made for this attack to be successful.

  1. The account used to upload data has VM write privileges
  2. Invoke-AzVMRunCommand is able to be executed by users without approval
  3. The VM is on and running

If any of these assumptions are not true, then the technique will fail. In addition, there’s several artifacts left behind by this technique.

  • The scripting agent from PowerZure is located in C:\WindowsAzure\SecAgent\AzureInstanceMetadataService.ps1
  • Invoke-AZVmRunCommand leaves behind the command or script that was run in C:\Packages\Plugins\Microsoft.CPlat.Core.RunCommandWindows\1.1.9\Downloads

Within Azure, Invoke-AzVMRunCommand will leave behind a log in the Activity log.

These logs should always trigger alerts and should be reviewed. Finally, since commands are just being executed from within the PS script agent, PowerShell logging will capture all activity. I personally have never seen the ‘userData’ field ever populated in the Azure portal, so check if anything is there and review its purpose.

Acknowledgements

Special thank you to @_wald0, @jsecurity101, and @matterpreter.

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