~ Certitude Security Advisory - CSA-2021-002 ~  
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PRODUCT : Windows Containers  
VENDOR : Microsoft  
AFFECTED VERSION : Windows 10, Windows Server  
IDENTIFIERS : CVE-2021-1645  
PATCH VERSION : KB4598229, KB4598230, KB4598242, KB4598243  
FOUND BY : Marc Nimmerrichter, Certitude Lab  
Windows containers is a feature that extends the container concept well-known  
from Linux environments to Windows. Just like containers on Linux, Windows  
containers utilize a shared kernel but container processes are somewhat  
isolated from one another.  
The Windows Data Protection API (DP API) allows applications to encrypt  
arbitrary data. An application does not have to manage keys, but instead, any  
data can be passed to the API, which then returns an encrypted blob.  
Similarly, an application can pass a previously encrypted blob to DP API to  
retrieve the plain text. The cryptographic key used for these encryption  
operations is either tied to the user context or is unique to a machine.  
There was a design issue with DP API in containers which resulted in DP API  
using the same key in all containers. Additionally, these keys were public in  
base-image layers published by Microsoft.  
Organizations using DP API inside containers should apply patches to Windows  
and use the latest base images. However, the fix causes a design change, which  
might render the use of DP API difficult for many use-cases.  
Vulnerability Overview  
The vulnerability described applies to both, user- and machine-key DP API  
encryption within Windows Docker containers. In our description we will use  
machine key encryption, but the same issue exists if data is encrypted with  
the user-key.  
Normally, a machine key is tied to a (virtual-)machine. Therefore, a machine  
is not able to decrypt data encrypted by an application on another machine.  
However, due to a design issue, DP API machine keys used in containers came  
from the container images. Since Windows docker images are based on the same  
base images, the DP API keys of containers were identical. As the base image  
is public, the DP API keys were public too!  
Therefore, DP API operations performed by any Windows container application  
were ineffective, as the encryption key that was used is public. Organizations  
that used DP API in Windows Docker containers and relied on it to store  
encrypted data in a potentially insecure location, should consider this data  
as compromised.  
First, start a docker container called Alice on VM1:  
\$ docker run --name Alice -it  
Then, encrypt a file in the Alice container using the powershell script  
C:\>powershell.exe -File vault.ps1 -StoreSecret "This is my secret text"  
C:\>type secret.txt  
Start a docker container Bob on VM2:  
\$ docker run --name Bob -it  
The following command shows that the file encrypted by Alice on VM1 can be  
decrypted in the Bob container on VM2:  
C:\>powershell.exe -File vault.ps1 secret.txt  
This is my secret text  
The vault.ps1 PowerShell script from used in  
this PoC:  
``` {.powershell```}  
[string] $StoreSecret,  
[string] $filename )  
[void] [Reflection.Assembly]::LoadWithPartialName("System.Security")  
$scope = [System.Security.Cryptography.DataProtectionScope]::CurrentUser  
if ($StoreSecret -eq "") {  
$data = Get-Content $filename  
$ciphertext = [System.Convert]::FromBase64String($data)  
$plaintext = [System.Security.Cryptography.ProtectedData]::Unprotect(  
$ciphertext, $null, $scope )  
} else {  
$plaintext = [System.Text.UTF8Encoding]::UTF8.GetBytes($StoreSecret)  
$ciphertext = [System.Security.Cryptography.ProtectedData]::Protect(  
$plaintext, $null, $scope )  
[System.Convert]::ToBase64String($ciphertext) > $filename  
Microsoft fixed this vulnerability with a patch for Windows Server and Windows  
10 operating systems and in their docker base-images. Users should apply both,  
OS updates and base-image updates, to address this issue. Please also refer to  
However, the patch comes with a caveat: As the issue is a design problem, it  
could not be fixed in a straightforward way. Windows containers now generate a  
DP API key when the container is first started. This also means that all  
containers use different keys. There is currently no supported way to share  
keys between containers or transfer a key from one container to another. This  
is impractical, because containers are often relatively short-lived. Moreover,  
when a container is scaled up, new containers will not be able to work with  
previously encrypted blobs. This reduces the potential use-cases of DP API  
with containers.  
(c) 2021 Certitude Consulting GmbH