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.
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.
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.