Original Post from FireEye
Author: Sumith Maniath
Malware authors attempt to evade detection by executing their
payload without having to write the executable file on the disk. One
of the most commonly seen techniques of this “fileless”
execution is code injection. Rather than executing the malware
directly, attackers inject the malware code into the memory of another
process that is already running.
Due to its presence on all Windows 7 and later machines and the
sheer number of supported features, PowerShell has been a favorite
tool of attackers for some time. FireEye has published multiple
reports where PowerShell
was used during initial malware delivery or during
post-exploitation activities. Attackers have abused PowerShell to
easily interact with other Windows components to perform their
activities with stealth and speed.
This blog post explores a recent phishing campaign observed in
February 2019, where an attacker targeted multiple customers and
successfully executed their payload without having to write the
executable dropper or the payload to the disk. The campaign
involved the use of VBScript, PowerShell and the .NET framework to
perform a code injection attack using a process hollowing technique.
The attacker abused the functionality of loading .NET assembly
directly into memory of PowerShell to execute malicious code without
creating any PE files on the disk.
The user is prompted to open a document stored on Google Drive. The
name of the file, shown in Figure 1, suggests that the actor was
targeting members of the airline industry that use a particular
aircraft model. We have observed an increasing number of attackers
relying on cloud-based file storage services that bypass firewall
restrictions to host their payload.
Figure 1: Malicious script hosted on
As seen in Figure 2, attempting to open the script raises an alert
from Internet Explorer saying that the publisher could not be
verified. In our experience, many users will choose to ignore the
warning and open the document.
Figure 2: Alert raised by Internet Explorer
Upon execution, after multiple levels of obfuscation, a PowerShell
script is executed that loads a .NET assembly from a remote URL,
functions of which are then used to inject the final payload (NETWIRE
Trojan) into a benign Microsoft executable using process hollowing.
This can potentially bypass application whitelisting since all
processes spawned during the attack are legitimate Microsoft executables.
The initial document contains VBScript code. When the user opens it,
Wscript is spawned by iexplore to execute this file. The script uses
multiple layers of obfuscation to bypass static scanners, and
ultimately runs a PowerShell script for executing the binary payload.
Obfuscation techniques used during different levels of script
execution are shown in Figure 3 and Figure 4.
Figure 3: Type 1 obfuscation technique,
which uses log functions to resolve a wide character
Figure 4: Type 2 obfuscation technique,
which uses split and replace operations
This script then downloads and executes another encoded .vbs script
from a paste.ee URL, as seen in Figure 5. Paste.ee is a less regulated
alternative to Pastebin and we have seen multiple attacks using this
service to host the payload. Since the website uses TLS, most firewall
solutions cannot detect the malicious content being downloaded over
Figure 5: Downloading the second-stage
script and creating a scheduled task
The script achieves persistence by copying itself to Appdata/Roaming
and using schtasks.exe to create a scheduled task that runs the
VBScript every 15 minutes.
After further de-obfuscation of the downloaded second-stage
VBScript, we obtain the PowerShell script that is executed through a
shell object, as shown in Figure 6.
Figure 6: De-obfuscated PowerShell script
The PowerShell script downloads two Base64-encoded payloads from
paste.ee that contain binary executable files. The strings are stored
as PowerShell script variables and no files are created on disk.
Microsoft has provided multiple ways of interacting with the .NET
framework in PowerShell to enhance it through custom-developed
features. These .NET integrations with PowerShell are particularly
attractive to attackers due to the limited visibility that traditional
security monitoring tools have around the runtime behaviors of .NET
processes. For this reason, exploit frameworks such as CobaltStrike
and Metasploit have options to generate their implants in .NET
Here, the attackers have used the Load method from the
System.Reflection.Assembly .NET Framework class. After the
assembly is loaded as an instance of
System.Reflection.Assembly, the members can be accessed through
that object similarly to C#, as shown in Figure 7.
Figure 7: Formatted PowerShell code
The code identifies the installed version of .NET and uses it later
to dynamically resolve the path to the .NET installation folder. The
decoded dropper assembly is passed as an argument to the Load
method. The resulting class instance is stored as a variable.
The objects of the dropper are accessed through this variable and
method R is invoked. Method R of the .NET dropper is
responsible for executing the final payload.
The following are the parameters for method R:
- Path to InstallUtil.exe
(or other .NET framework tools)
- Decoded NETWIRE trojan
When we observed the list of processes spawned during the attack
(Figure 8), we did not see the payload spawned as a separate process.
Figure 8: Processes spawned during attack
We observed that the InstallUtil.exe process was being created in
suspended mode. Once it started execution, we compared its memory
artifacts to a benign execution of InstallUtil.exe and concluded that
the malicious payload is being injected into the memory of the newly
spawned InstallUtil.exe process. We also observed that no arguments
are passed to InstallUtil, which would cause an error under normal
execution since InstallUtil always expects at least one argument.
From a detection evasion perspective, the attacker has chosen an
interesting approach. Even if the PowerShell process creation is
detected, InstallUtil.exe is executed from its original path.
Furthermore, InstallUtil.exe is a benign file often used by internal
automations. To an unsuspecting system administrator, this might not
When we disassembled the .NET code and removed the obfuscation to
understand how code injection was performed, we were able to identify
Windows win32 API calls associated with process hollowing (Figure 9).
Figure 9: Windows APIs used in .NET
dropper for process hollowing
After reversing and modifying the code of the C# dropper to invoke
R from main, we were able to confirm that when the method
R is invoked, InstallUtil.exe is spawned in suspended mode. The
memory blocks of the suspended process are unmapped and rewritten with
the sections of the payload program passed as an argument to method
R. The thread is allowed to continue after changes have been
made to the entry point. When the process hollowing is complete, the
parent PowerShell process is terminated.
High-Level Analysis of the Payload
The final payload was identified by FireEye Intelligence as a
NETWIRE backdoor. The backdoor receives commands from a command and
control (C2) server, performs reconnaissance that includes the
collection of user data, and returns the information to the C2 server.
Capabilities of the NETWIRE backdoor include key logging, reverse
shell, and password theft. The backdoor uses a custom encryption
algorithm to encrypt data and then writes it to a file created in the
The malware also contains a custom obfuscation algorithm to hide
registry keys, APIs, DLL names, and other strings from static
analysis. Figure 10 provides the decompiled version of the custom
decoding algorithm used on these strings.
Figure 10: Decompiled string decoding algorithm
From reversing and analyzing the behavior of the malware, we were
able to identify the following capabilities:
- Record mouse and keyboard
- Capture session logon details
- Capture system
- Take screenshots
- Monitor CPU usage
- Create fake HTTP proxy
From the list of decoded strings, we were able to identify other
features of this sample:
Stealing data from an email
“GoogleChromeUser DataDefaultLogin Data”
“ChromiumUser DataDefaultLogin Data”
“ComodoDragonUser DataDefaultLogin Data”
“Opera SoftwareOpera StableLogin Data”
“select * from moz_login”
Stealing login details from browsers
A complete report on the NETWIRE backdoor family is available to
customers who subscribe to the FireEye Intelligence portal.
Indicators of Compromise
Initial attack vector .vbs file
C2 domains of NETWIRE Trojan
FireEye detection names for the indicators in the attack:
Malware authors continue to use different “fileless”
process execution techniques to reduce the number of indicators on an
endpoint. The lack of visibility into .NET process execution combined
with the flexibility of PowerShell makes this technique all the more effective.
FireEye Endpoint Security and the FireEye Network Security detect
and block this attack at several stages of the attack chain.
We would like to thank Frederick House, Arvind Gowda, Nart
Villeneuve and Nick Carr for their valuable feedback.
Go to Source
Author: Sumith Maniath