Dissecting One of APT29’s Fileless WMI and PowerShell Backdoors (POSHSPY)

Mandiant has observed APT29 using a stealthy backdoor that we call
POSHSPY. POSHSPY leverages two of the tools the group frequently uses:
PowerShell and Windows Management Instrumentation (WMI). In the
investigations Mandiant has conducted, it appeared that APT29 deployed
POSHSPY as a secondary backdoor for use if they lost access to their
primary backdoors.

POSHSPY makes the most of using built-in Windows features –
so-called “living off the land” – to make an especially stealthy
backdoor. POSHSPY’s use of WMI to both store and persist the backdoor
code makes it nearly invisible to anyone not familiar with the
intricacies of WMI. Its use of a PowerShell payload means that only
legitimate system processes are utilized and that the malicious code
execution can only be identified through enhanced
logging
or in memory. The backdoor’s infrequent beaconing, traffic
obfuscation, extensive encryption and use of geographically local,
legitimate websites for command and control (C2) make identification
of its network traffic difficult. Every aspect of POSHSPY is efficient
and covert.

Mandiant initially identified an early variant of the POSHSPY
backdoor deployed as PowerShell scripts during an incident response
engagement in 2015. Later in that same engagement, the attacker
updated the deployment of the backdoor to use WMI for storage and
persistence. Mandiant has since identified POSHSPY in several other
environments compromised by APT29 over the past two years.

We first discussed APT29’s use of this backdoor as part of our “No
Easy Breach” talk. For additional details on how we first identified
this backdoor, and the epic investigation it was part of, see the slides
and presentation.

Windows Management Instrumentation

WMI is an administrative framework that is built into every version
of Windows since 2000. WMI provides many administrative capabilities
on local and remote systems, including querying system information,
starting and stopping processes, and setting conditional triggers. WMI
can be accessed using a variety of tools, including the Windows WMI
Command-line (wmic.exe), or through APIs accessible to programming and
scripting languages such as PowerShell. Windows system WMI data is
stored in the WMI common information model (CIM) repository, which
consists of several files in the System32\wbem\Repository directory.

WMI classes are the primary structure within WMI. WMI classes can
contain methods (code) and properties (data). Users with sufficient
system-level privileges can define custom classes or extend the
functionality of the many default classes.

WMI permanent event subscriptions can be used to trigger actions
when specified conditions are met. Attackers often use this
functionality to persist the execution of backdoors at system start
up. Subscriptions consist of three core WMI classes: a Filter, a
Consumer, and a FilterToConsumerBinding. WMI Consumers specify an
action to be performed, including executing a command, running a
script, adding an entry to a log, or sending an email. WMI Filters
define conditions that will trigger a Consumer, including system
startup, the execution of a program, the passing of a specified time
and many others. A FilterToConsumerBinding associates Consumers to
Filters. Creating a WMI permanent event subscription requires
administrative privileges on a system.

We have observed APT29 use WMI to persist a backdoor and also store
the PowerShell backdoor code. To store the code, APT29 created a new
WMI class and added a text property to it in order to store a string
value. APT29 wrote the encrypted and base64-encoded PowerShell
backdoor code into that property.

APT29 then created a WMI event subscription in order to execute the
backdoor. The subscription was configured to run a PowerShell command
that read, decrypted, and executed the backdoor code directly from the
new WMI property. This allowed them to install a persistent backdoor
without leaving any artifacts on the system’s hard drive, outside of
the WMI repository. This “fileless” backdoor methodology made the
identification of the backdoor much more difficult using standard host
analysis techniques.

POSHSPY WMI Component

The WMI component of the POSHSPY backdoor leverages a Filter to
execute the PowerShell component of the backdoor on a regular basis.
In one instance, APT29 created a Filter named BfeOnServiceStartTypeChange (Figure 1), which they
configured to execute every Monday, Tuesday, Thursday, Friday, and
Saturday at 11:33 am local time. 

Figure 1: “BfeOnServiceStartTypeChange” WMI
Query Language (WQL) filter condition

The BfeOnServiceStartTypeChange Filter was
bound to the CommandLineEventConsumer WindowsParentalControlsMigration. The WindowsParentalControlsMigration consumer was
configured to silently execute a base64-encoded PowerShell command.
Upon execution, this command extracted, decrypted, and executed the
PowerShell backdoor payload stored in the HiveUploadTask text property of the RacTask class. The PowerShell command contained
the payload storage location and encryption keys. Figure 2 displays
the command, called the “CommandLineTemplate”, executed by the WindowsParentalControlsMigration consumer.

Figure 2: WindowsParentalControlsMigration CommandLineTemplate

Figure 3 contains the decoded PowerShell command from the “CommandLineTemplate.”

Figure 3: Decoded CommandLineTemplate PowerShell code

POSHSPY PowerShell Component

The full code for a POSHSPY sample is available here.

The POSHSPY backdoor is designed to download and execute additional
PowerShell code and Windows binaries. The backdoor contains several
notable capabilities, including:

1. Downloading and executing PowerShell code as an EncodedCommand

2. Writing executables to a randomly-selected directory under Program Files, and naming the EXE to match the
chosen directory name, or, if that fails, writing the executable to a
system-generated temporary file name, using the EXE extension

3. Modifying the Standard Information timestamps (created, modified,
accessed) of every downloaded executable to match a randomly selected
file from the System32 directory that was created prior to 2013

4. Encrypting communications using AES and RSA public key cryptography

5. Deriving C2 URLs from a Domain Generation Algorithm (DGA) using
lists of domain names, subdomains, top-level domains (TLDs), Uniform
Resource Identifiers (URIs), file names, and file extensions

6. Using a custom User Agent string or the system’s User Agent
string derived from urlmon.dll

7. Using either custom cookie names and values or randomly-generated
cookie names and values for each network connection

8. Uploading data in 2048-byte chunks

9. Appending a file signature header to all encrypted data, prior to
upload or download, by randomly selecting from the file types:

  • ICO
  • GIF
  • JPG
  • PNG
  • MP3
  • BMP

The sample
in this example used 11 legitimate domains owned by an organization
located near the victim. When combined with the other options in the
DGA, 550 unique C2 URLs could be generated. Infrequent beaconing, use
of DGA and compromised infrastructure for C2, and appended file
headers used to bypass content inspection made this backdoor difficult
to identify using typical network monitoring techniques.

Conclusion

POSHSPY is an excellent example of the skill and craftiness of
APT29. By “living off the land” they were able to make an extremely
discrete backdoor that they can deploy alongside their more
conventional and noisier backdoor families, in order to help ensure
persistence even after remediation. As stealthy as POSHSPY can be, it
comes to light quickly if you know where to look. Enabling and
monitoring enhanced
PowerShell logging
can capture malicious code as it executes and
legitimate WMI persistence is so rare that malicious persistence
quickly stands out when enumerating it across an environment. This is
one of several sneaky backdoor families that we have identified,
including an off-the-shelf domain
fronting backdoor
and HAMMERTOSS.
When responding to an APT29 breach, it is vital to increase
visibility, fully scope the incident before responding and thoroughly
analyze accessed systems that don’t contain known malware.

Additional Reading

This PowerShell
logging blog post
contains more information on improving
PowerShell visibility in your environment.

This excellent
whitepaper
by William Ballenthin, Matt Graeber and Claudiu
Teodorescu contains additional information on WMI offense, defense and forensics.

This presentation
by Christopher Glyer and Devon Kerr contains additional information on
attacker use of WMI in past Mandiant investigations.

The FireEye FLARE team released a WMI
repository-parsing tool
that allows investigators to extract
embedded data from the WMI repository and identify WMI persistence. 

This is a Security Bloggers Network syndicated blog post authored by Nick Harbour. Read the original post at: Threat Research Blog