Posté par: igor51 le mars 26, 2017, 09:01:24
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Introduction
Throughout the past few months, FireEye Labs has observed an
increased use of Windows Management Instrumentation (WMI) queries for
environment detection and evasion of dynamic analysis and
virtualization engines. WMI provides high-level interaction with
Windows objects using C/C++, VBScript, JScript, C#, and more in the
form of WMI Query Language (WQL). Last year, FireEye published a
paper detailing an in-depth analysis of WMI infrastructure and
potential abuse of WMI services by malware writers.
In this post we will present an analysis of some samples found in
the wild in 2016. For the purposes of this blog post, we will focus on
evasion only, ignoring other malicious aspects of the samples.
Evasion after Anti-Virus Detection
Anti-virus can be detected by a WMI query as they are registered in
AntiVirusProduct class under
root\SecurityCenter2 (root\SecurityCenter before Vista)
namespace. We analyzed a sample that checked the operating system from
Win32_OperatingSystem class under
root\cimv2 namespace first and if the OS version was
above 6 (Windows Vista and above), then anti-virus check was
performed. Figure 1 shows the VBScript code of anti-virus check.
Figure 1: Anti-virus product checks in VBScript
code using WMI query
Anti-virus and other user information is sent to the server for
fetching the right payload or performing evasion, as shown in Figure 2.
Figure 2: Anti-virus and other info being sent
to the server and actions against response
Virtualization Detection and Evasions
One of the samples was found to monitor many security products using
different techniques, but most popular virtualization software (such
as VMware and VirtualBox) were being detected using WMI queries. It
retrieves BIOS information from
Win32_BIOS class under
root\cimv2 namespace. Specific fields/columns can also be
retrieved similar to an SQL query. The following queries were found in
this sample binary (Figure 3).
Figure 3: Virtualization software checks by the
malware using WMI queries
The query yields the following result when executed in PowerShell in
Bochs Emulator, as shown in Figure 4.
Figure 4: Query result in PowerShell in Bochs Emulator
Figure 5 shows the full scale environment detection being performed
by this sample. Other services may be checked by enumerating the
running processes or using Windows Registry.
Figure 5: Security products being monitored by
the malware
Another sample used
Win32_ComputerSystem class for virtual machines
detection, as show in Figure 6.
Figure 6: ComputerSystem WMI query found in the sample
The result of the query (Figure 9) has ’Model’ field (Figure 7),
which holds the virtual machine information in case of VMware,
VirtualBox and Virtual Machine.
Figure 7: Model column retrieval
When any of the three strings matched with ‘Model’ field
output, virtualization gets detected by matching the stored value with
the one created in the process, as evident in Figure 8.
Figure 8: VirtualBox, VMware and Virtual Machine checks
When the aforementioned query was executed in PowerShell in VMware
workstation 12.0, it gave the result illustrated in Figure 9.
Figure 9: Query result in PowerShell 2.0
Another sample has used Win32_DiskDrive to detect Virtual Box
(Figure 10), Virtual Hard disk (Figure 11) and VMware (Figure 12).
When any of the virtual machines get detected, the process terminates
itself, evading the behavioral analysis.
Figure 10: VirtualBox detection
Figure 11: Virtual Hard disk detection
Figure 12: VMware detection
Process Detection
We analyzed a sample that not only checked a specific process from
Win32_Process class under namespace
root\cimv2, but also killed it. Immunity debugger, a
well-known debugger, is terminated and its folder is deleted after
changing permissions using Windows Script Host shell, as evident in
the code in Figure 13.
Figure 13: Immunity debugger being terminated
and folder being deleted
Moreover, anti-virus vendor Kingsoft Corporation’s processes are
also forced to stop execution, meaning its anti-virus processes are
being killed. The code is shown in Figure 14. Usually samples use
CreateToolHelp32Snapshot, Process32First and Process32Next APIs for
finding a process, but here it is evident that
one WMI query comes in handy to replace tens of lines of code.
Figure 14: Code designed to kill processes
associated with Kingsoft (an anti-virus product company)
Windows Services Detection
Another sample, an MS Office key generator, checked Windows Office
Software Protection Service through WMI queries (Figure 15). This
service enables software vendors to enforce
licensing on the client machines. If this service is not running,
it is started as shown in Figure 16. Once the Office Software
Protection Service object is retrieved, it is then used to install MS
Office product key.
Figure 15: OfficeSoftwareProtectionService check
using WMI query
Figure 16: Code to start/restart OfficeSoftwareProtectionService
Summary
During analysis and research, it has been observed that WMI queries,
shown in Figure 17, can be used for environment detection and (with
more details) for evasion as well. There may be more queries that are
not listed here, but we suggest security researchers should at least
monitor these queries for evaluating the samples.
Figure 17: Possible WMI queries for environment detection/evasion
Conclusion
Malware writers are always in search of new ways to evade analysis
frameworks and sandboxes in order to make the payload execution
successful in their targeted environments and platforms. WMI provides
a simple way of environment detection that can be used to evade
sandboxes and dynamic analysis tools, which seems to be underestimated
by reverse engineers and others in the security community. Mitigation
Steps should be taken to monitor WMI queries that could lead to
possible evasion.