Thursday, September 26, 2019

Divergent: "Fileless" NodeJS Malware Burrows Deep Within the Host


Update (09/27/2019): Additional information regarding the malware interaction with various online advertisements has been included to highlight the click-fraud related network communications associated with Divergent.


Executive summary


Cisco Talos recently discovered a new malware loader being used to deliver and infect systems with a previously undocumented malware payload called "Divergent." We first dove into this malware after we saw compelling data from Cisco Advanced Malware Protection's (AMP) Exploit Prevention.

This threat uses NodeJS — a program that executes JavaScript outside of a web browser — as well as the legitimate open-source utility WinDivert to facilitate some of the functionality in the Divergent malware. The use of NodeJS is not something commonly seen across malware families.

The observed malware campaigns associated with Divergent feature the use of persistence techniques most commonly associated with "fileless" malware, leaving behind few artifacts for researchers to look at. This malware can be leveraged by an attacker to target corporate networks and appears to be primarily designed to conduct click-fraud. It also features several characteristics that have been observed in other click-fraud malware, such as Kovter.

Technical Details


Talos has identified a new moduler malware that is being used to facilitate the installation of a previously undocumented malware family, which we are referring to as Divergent, due to the naming convention used by the malware during variable declaration and the creation of environment variables. While we were unable to determine the delivery mechanism used, we were able to perform analysis of the malware loader as well as the Divergent malware that it is used to install on victim systems. Divergent is a malware family designed to generate revenue for attackers via the use of click-fraud, similar to other click-fraud malware such as Kovter. Technical details associated with both the installation and operation of the Divergent malware are described in the following sections.

Installation


The malware has many similarities with other popular fileless malware families, particularly Kovter. Like Kovter, it relies heavily on the registry for staging and storage of configuration data while avoiding more traditional on-access endpoint scanning of files on disk. It also uses a key in the registry to maintain persistence, and relies on PowerShell to install itself on the infected host.

When first delivered and executed on a victim's machine, the malware is in the portable executable (PE) format. Its first task, however, is to install itself to the system in a less suspicious form, namely as an HTML Application (HTA) that will load the malware from the registry.

Installation begins by creating several registry keys containing the different parts of the loader as well as the data of the malware PE. The malware reads all the information embedded in its data section and creates three new randomly named registry keys, each holding a different stage of the loader code needed to execute the malware PE using reflective injection.

Next, the HTA loader is written to the CSIDL_COMMON_APPDATA folder (typically C:\ProgramData\) and set to execute each time the user logs on by adding an entry to the "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run" registry key.

An example of an HTA loader and the accompanying registry entries necessary to execute the malware once installed are:
  • 17T21vYHOb.hta :: e4a49af295d6e61877a458a014fe63b733be942c506496b53070aa3d9ca421d8
  • ZfjrAilGdH.reg :: 5863f35959aa542a27319e098f40166f3ace09d265f4ec6d739318c0b739745e

This registry file contains the following subkeys in the key HKLM\Software\ZfjrAilGdH which are set by the installation process (the key names are randomly generated and will be different for each installation):
  • Lvt4wLGLMZ :: JScript executed by ZfjrAilGdh.hta
  • kCu2DZ9WI0 :: PowerShell used to reflectively inject the malware
  • 4FLJBnefsN :: hex data representing the malware PE

Fileless malware loader

The HTA is heavily obfuscated but when cleaned up, evaluates to an eval of the JScript in the registry key "HKLM\Software\ZfjrAilGdh\Lvt4wLGLMZ" via a "ActiveXObject.WScript.Shell.RegRead" (shown here as pseudo code):
The JScript in the reg key executes the following powershell (shown here deobfuscated):
This code will execute the code found in the registry location HKLM\Software\ZfjrAilGdH\kCu2DZ9WI0 after setting the variable regkeyname to the data found in the registry location HKLM\Software\ZfjrAilGdH\4FLJBnefsN. The registry key 4FLJBnefsN contains the bytes of the malicious portable executable (PE) with a modified DOS header, namely the MZ has been replaced with null bytes.

The code from kCu2DZ9WI0 is a version of PowerShell Empire's reflective PE injection script that will inject the malware:
At this point, the malware executes.

Divergent malware

There are two main parts of this threat: one to receive and execute commands from a C2 server and another to execute external component scripts. The configuration for each part is stored in the registry in JSON format.

The component configuration describes which components should be executed and how. This example configuration will execute three different JScript components:
Each entry is parsed, and each filename and args value is passed along for execution by the malware. Detailed analysis of the call_03 (see Now I See You), all_socks_05 (see Click Fraud), and block_av_01 (see Block AV Component) components can be found below.

The network configuration stores two classes of C2: "accl" is a list of URLs that the malware should attempt to contact for system information delivery and for commands to execute, while "acll" is a list of URLs the malware should attempt to contact for possible updates to either configuration file. The default values from ZfjrAilGdH.reg are:
Notice that "version" is an epoch timestamp, converting to Saturday, March 30, 2019 7:14:29 PM GMT. The earliest reference we found containing several IOCs from this malware sample dates back to February 2019.

Once executed, the malware begins with five anti-analysis checks. If any of these checks fail, a beacon is sent to a static URL containing a direct IP and sleeps indefinitely:
The beacon message indicates which anti-analysis check failed to pass. The malware checks for unwanted processes and loaded modules by hashing process file names and module names respectively, then comparing each hash against two separate pre-computed lists for each. The lists contain hashes for endpoint security software and hypervisor services as running the sample under both scenarios failed these checks. It also checks for a host CPU with at least two cores, the presence of a debugger, and finally compares system uptime intervals to determine if the sample is running within a sandbox or virtual machine.

If the process is running with the appropriate privileges, it uses WMI (Windows Management Instrumentation) to query recognized anti-virus software installed on the host. In particular, it's looking for the antivirus software Windows Defender. If found, it proceeds to disable various components of Windows Defender and Windows Updates.
Once completed, it attempts to bypass UAC using CMSTP, if needed. Additional code is executed to check for any updates to the malware's two configuration files. Empty POST requests are sent to each of the URLs in the first configuration's accl key. These URLs are later contacted with a comprehensive set of sensitive information from the host. Most are non-responsive but still online. Many appear to be, or once were, compromised hosts used for the C2 network.

The set of direct IP URLs found in the first configuration's acll key are contacted until one server responds back with a configuration update. A response containing an RC4 encrypted update to the first of the malware's configurations in the Registry. In this sample, the RC4 key "seiC4aimaish9zah8kah" is static, and decryption results in a lengthier update for the first configuration:
The data is stored in the pre-existing registry subkey to update the value (in this example, HKLM\SOFTWARE\ZfjrAilGdH\194956). As of Sept. 9, 2019, the version timestamp for the latest config pulled was Thursday, Aug. 29, 2019 11:50:19 a.m.

The second configuration is stored in the last remaining value in the ZfjrAilGdH registry subkey (HKLM\SOFTWARE\ZfjrAilGdH\2177774). Default values mentioned near the beginning of this section remained in use during runtime.

The primary flow of the malware code reaches an end but repeats select tasks every 90 minutes. It continues to check for configuration updates, continues to send encrypted sensitive information from the infected host, and continues to process any C2 responses that might contain additional commands to execute. The following commands and parameters are supported by the C2 protocol used by Divergent:
  • killall: Terminate all processes initiated by the malware, delete corresponding files
  • kill: Find process of specified component, terminate process, and delete the file
  • stop: Find process of specified component, terminate process
  • resume: Given pre-existing component, execute file
  • modules: From the same response data, pull additional configuration data from the following keys:
    • name
    • filename
    • args
    • version
    • type
    • download
    • Key
Using this data, the malware sends additional requests to download each specified file. The files are written to disk and executed.

  • update: From the same response data, pull additional configuration data from the following keys:
    • filename
    • download
    • Key
Same as the modules command, the malware sends additional requests to download each specified file. Each file is written to disk and executed. Finally, the task deletes the Registry data stored in its subkey (handles both HKCU and HKLM).

The command update_interval modifies the main thread's sleep counter for the ending loop (the default time is 90 minutes).

Components

Block AV component


The block_av_01 component attempts to block anti-virus software from receiving updates by blocking all outbound TCP connections on port 80 and port 443. With older revisions of this malware package, this functionality was delivered via a JScript file named bav01.js but in newer versions, this has been seen delivered by PowerShell in the fake PNG trpl.png (see Fake PNG PowerShell Delivery).

This installation script starts by creating a new folder (e.g. SystemConfigInfo000) to hold the files necessary for execution. The two files to be installed are WinDivert.dll and either WinDivert32.sys or WinDivert64.sys, depending on the host CPU architecture. These are the legitimate WinDivert binaries and are used by the malware to create its packet filter.

The WinDivert binaries are embedded in bav01.js as comments and written to disk with .b64 extensions. The following is the code to retrieve the embedded resource named arch5 from the script, embedded in a comment block in the format /*[<resource name>[resource data]]*/:
Each WinDivert binary is then decoded from base64 using the Windows Certificate Services utility certutil.exe. For example:
This script is set to execute as a task each time the computer starts. This is accomplished by creating a scheduled task with a random-looking service name that is set to run as the SYSTEM user at the highest run level:
Next, the following PowerShell command is executed (shown here decoded) to execute the base64 encoded PowerShell commands in the environment variable 'nttyuuyt':
The 'nttyuuyt' environment variable was set by bav01.js previously and contains a base64 encoded PE and the PowerShell commands necessary to reflectively inject this executable (truncated for readability):
At this point in the script, installation is complete and the PE-based module to block anti-virus HTTP/HTTPS connections is loaded.

To achieve its anti-virus blocking, the reflectively loaded PE periodically checks the names of all running processes against a predefined list. If any process names appear in the list, the PIDs are added to the filter string passed to WinDivertOpen which will block all traffic to that process on remote ports 80 or 443. An example filter string is:

((processId=620 or processId=736) and (remotePort==80 or remotePort==443))

Example process names that would be blocked include msmpeng.exe (Windows Defender) and svchost.exe.

Click Fraud

The all_socks component is a NodeJS-based Socket.IO client that is commanded to navigate to arbitrary web pages by the attacker ostensibly for monetization and click fraud purposes. With older revisions of this malware package this functionality was delivered via a JScript file named either 04sall.js or 05sall.js, but in newer versions this has been seen delivered by PowerShell in the fake PNG strpk.png (see Fake PNG PowerShell Delivery).

Like the anti-virus blocking component, the click fraud component makes use of the WinDivert library and therefore installs the necessary WinDivert DLL and driver in the same manner as bav01.js, described above. Additionally, the NodeJS executable and a NodeJS Socket.IO client named app.js are part of the installation process for this component. Older versions of this component also installed two executables, divergent.exe and mdivergent.exe, however in later versions, these are executed from memory via reflective PE injection.

The malicious NodeJS application, seen either as app.js or init.js, is a simple Socket.IO client that takes a base64 encoded IP address as its only parameter.

node.exe app.js <base64 encoded IP>

In all samples we have encountered, the IP address has been 176.9.117.194 (encoded as MTc2LjkuMTE3LjE5NA==). Upon execution, the malicious NodeJS app will make a request to the IP passed as a parameter:
The response from this server is the address of the next server which the application will connect to. This new connection uses Socket.IO web sockets to maintain continuous communication between the victim and the server so the server can periodically send commands. The commands sent from this second server contain the host address of an advertisement revenue service and the entire HTTP request that should be made to that server, effectively faking a click on an advertisement link.
To protect themselves from these kinds of fraudulent requests, advertisement monetization services may go to extra lengths to confirm the device making the request is the type of device it claims to be. For example, if the monetization service only expects mobile devices, it may reject requests that have the characteristics of desktop devices. We believe the divergent.exe and mdivergent.exe executables are used by the 04sall.js/05sall.js components to circumvent these kinds of checks (see TCP/IP stack fingerprinting).

The divergent and mdivergent PEs make use of the WinDivert library to intercept and rewrite the first SYN packet of the 3-way TCP handshake for all outgoing connections the infected host attempts to make. The changes made to the SYN packets depend on which executable was used, either divergent.exe or mdivergent.exe; divergent.exe will rewrite the TCP header options to follow the same format as Android devices while mdivergent.exe will rewrite the TCP header options to follow the format as iOS devices. Which version of the divergent executable is used is dependent on the app.js deployment script (either 04sall.js or 05sall.js). These scripts contain code that decides which version should be used depending on a variable named macchance which can be passed to the deployment script as its only parameter. This variable contains the probability that mdivergent.exe will be deployed instead of divergent.exe (the PowerShell parameter, normally base64 encoded, is shown here in its decoded form):
In the code shown above, the variables and and mac correspond to the code necessary to reflectively inject divergent.exe and mdivergent.exe respectively, i.e. and represents Android and mac represents iOS.

For either of these divergent executables to work properly, several changes to the TCP/IP stack on the infected machine must be made; these include setting the TTL to 64, turning on the timestamp TCP header options, and changing the MTU to 1440. Once those changes have been made, the system is forced to reboot with a false message of Critical_Windows_Update:
With these changes made, the divergent executables can perform the necessary modifications to each SYN packet so that they follow the standards of the device the host should be disguised as.

During our analysis of systems actively infected with Divergent, we observed several web requests initiated by the malware attempting to interact with various online advertisements and advertising platforms, indicative of the previously described click-fraud process performed by the malware. Similar to what was observed related to the modification of the TCP configuration, the User-Agent field was modified to make the web requests appear as if they had originated from a mobile device. Web requests were made to various online advertising services and were similar to the following:
We also observed additional web requests similar to the following, which included a standard desktop User-Agent:
Additionally, the malware attempts to interact with “in-app” advertising platforms such as Mobfox.

Now I See You


The component named call_03 by the malware's configuration file, which is delivered by the em_02.js and em_03.js scripts, appears to be a means of installing a remote assist tool on the infected machine that would allow the attacker to view and possibly control the victim's computer.

Like the previous components, the PE associated with em_03.js is executed via reflective PE injection, this time with the environment variable fdghjgfdhj.
The PE to be injected is a DLL which, according to its export table, was originally called now_i_see_you.dll. This DLL has a single exported function named VoidFunc which contains all of its functionality. When VoidFunc is executed, it makes an HTTP GET request to the hxxps://uoibppop[.]tk/. It then takes the response from this server and treats it as a new URL to navigate to. Using COM objects, the DLL launches an instance of Internet Explorer, resizes the window to fit the entire screen and navigates to the URL in the response. At the time of analysis, the server was active but did not respond with any data so we were unable to confirm what was being hosted there.

Additionally, the DLL hides the Windows taskbar so the user is more compelled to comply with any instructions on the page that is presented to them. Next, the malware enters a loop looking for a process containing the string gotoassist (older versions also looked for teamviewer), ostensibly for confirming that the user followed the instructions in Internet Explorer by downloading and running the attacker's malware. Once this process is running, the Windows Taskbar is restored to view. A process list is gathered and sent to the URL hxxps://uoibppop[.]tk/clean; no response is expected from the server. The registry key `HKEY_CURRENT_USER\Software\fbsjbdfhsv` is created and the key value `weqr` is set to 1, indicating that execution was successful, then the process exits. Instead of a URL, the attacker has the option to send the word stop to the victim which will cause the DLL to forcefully reboot the infected machine.

While we do not know what URL the victim is intended to navigate to and therefore which program they are to be tricked into running, based on the process names gotoassist and teamviewer and the original DLL name of now_i_see_you.dll, it is likely that the victim is intended to install one of these remote administration software.

Fake PNG PowerShell delivery


Newer versions of the Divergent malware package no longer deliver and execute components as JScript, instead multi-stage PowerShell scripts are used. The first stage will retrieve the second stage from a static URL. The first-stage PowerShell is heavily obfuscated:
Here is our deobfuscated version:
While the requested resource features the extension normally associated with PNG images, it is actually malicious Powershell that has been encrypted using RC4 with the encryption key "raimeey2nu," which was stored in the previous PowerShell (the particular implementation of RC4 used can be found here).

We have encountered following URLs to retrieve the encrypted PowerShell:
  • hxxp://1292172017[.]rsc.cdn77[.]org/images/trpl.png
  • hxxp://1292172017[.]rsc.cdn77[.]org//imtrack/strkp.png
The first URL delivers the PowerShell version of bav01.js and the second delivers the PowerShell version of 05sall.js.

Taking a deeper look at the PowerShell version of 05sall.js, we see Base64 encoded blobs corresponding to binaries associated with WinDivert. Like its JScript counterpart, the Powershell decodes these blobs and saves them to the filesystem location defined by the environment variable %ALLUSERSPROFILE%.
Shellcode stored within the PowerShell is loaded into a new memory region using the Windows API function VirtualAlloc and then executed to continue the infection process:

Conclusion


The malware loader described is currently under active development. Talos has observed multiple versions of the loader being used to install the Divergent malware. Attackers are attempting to monetize these infections through the use of click fraud. The threat landscape is constantly evolving as attackers test new techniques and methodologies to maximize their revenue generation capabilities. Organizations should be aware of these changes and ensure that their security programs are able to remain effective against these changing tactics, techniques, and procedures. This threat is successfully stopped by the Cisco Advanced Malware Protection (AMP) Exploit Prevention engine, and the resulting event data assisted with our analysis of the threat. Talos will continue to monitor the threat landscape to ensure that customers remain protected.

Coverage

Additional ways our customers can detect and block this threat are listed below.


Advanced Malware Protection (AMP) is ideally suited to prevent the execution of the malware used by these threat actors.

Cisco Cloud Web Security (CWS) or Web Security Appliance (WSA) web scanning prevents access to malicious websites and detects malware used in these attacks.

Email Security can block malicious emails sent by threat actors as part of their campaign.

Network Security appliances such as Next-Generation Firewall (NGFW), Next-Generation Intrusion Prevention System (NGIPS), and Meraki MX can detect malicious activity associated with this threat.

AMP Threat Grid helps identify malicious binaries and build protection into all Cisco Security products.

Umbrella, our secure internet gateway (SIG), blocks users from connecting to malicious domains, IPs, and URLs, whether users are on or off the corporate network.

Open Source Snort Subscriber Rule Set customers can stay up to date by downloading the latest rule pack available for purchase on Snort.org.

Indicators of Compromise (IOCs)


The following IOCs have been observed as being associated with these malware campaigns.

HTA Hashes:

47b5dac9152220fbbf122eff89ac93d42e9196f5ab665a2a6d99594246ab8a81
062688aec1bdf1208bd72a77696e1fbcd1076f54bd6e59141ed12b6f8e3ba32c

PE32 Hashes:


c7052f4676102bfe39ab19c227832861caa2959933e296ee1806973619948624
781adc919a705ca3e8a82fe1d1eac68f651c50ba402172aea033eaec7879e932
05fbd38ea0b99621d22ce5f057173fdec40f3dccd63f887e1c301766c6597714
2135acda2d2739773fbb827e8d180ac901c040d2f071127bb597a714591672cd
72b6a8bf9598bd445e26a04ab58be62ed3941fb1fe4cf4a094a6272a77b66009
ba04eacaa80bb5da6b02e1e7fdf3775cf5a44a6179b2c142605e089d78a2f5b6
a82dd93585094aeba4363c5aeedd1a85ef72c60a03738b25d452a5d895313875
2f4a9ef2071ee896674e3da1a870d4efab4bb16e2e26ea3d7543d98b614ceab9
77498f0ef4087175aa85ce1388f9d02d14aaf280e52ce7c70f50d3b8405fea9f
b2d29bb9350a0df93d0918c0208af081f917129ee46544508f2e1cf30aa4f4ce
bf2cdd1dc2e20c42d2451c83b8280490879b3515aa6c15ab297419990e017142
ba04eacaa80bb5da6b02e1e7fdf3775cf5a44a6179b2c142605e089d78a2f5b6
a7656ccba0946d25a4efd96f4f4576494d5f1e23e6ad2acc16d2e684656a2d4f
607b2f3fd1e73788a4d6f5a366c708dbb12d174eba9863ade0af89ca40e1fdba

URLs:

hxxps://1292172017[.]rsc[.]cdn77[.]org/images/trpl.png
hxxps://1292172017[.]rsc[.]cdn77[.]org/imtrack/strkp.png

Mutexes:

Global\Divergent
Global\CreatorsPatch
Global\LocalLow7

IP Addresses:


95[.]70[.]244[.]209
13[.]228[.]224[.]121
54[.]241[.]31[.]99
103[.]31[.]4[.]11
103[.]31[.]4[.]54
198[.]41[.]128[.]74
198[.]41[.]128[.]55
131[.]0[.]72[.]36
131[.]0[.]72[.]59
188[.]114[.]96[.]87
188[.]114[.]96[.]116
43[.]250[.]192[.]98
43[.]250[.]192[.]87
217[.]160[.]231[.]125
208[.]91[.]197[.]25
184[.]168[.]221[.]42
103[.]224[.]248[.]219
31[.]31[.]196[.]120
217[.]160[.]223[.]93
103[.]224[.]248[.]219
184[.]168[.]221[.]45
119[.]28[.]87[.]235
23[.]227[.]38[.]32
50[.]63[.]202[.]39
216[.]239[.]34[.]21
83[.]243[.]58[.]172
5[.]9[.]41[.]178
88[.]198[.]26[.]25
62[.]75[.]189[.]110
109[.]239[.]101[.]62
107[.]186[.]67[.]4
184[.]168[.]221[.]63
45[.]55[.]154[.]177
104[.]28[.]2[.]169
202[.]56[.]240[.]5
89[.]163[.]255[.]171
185[.]243[.]114[.]111

No comments:

Post a Comment