REF2731 intrusion set, campaigns, and malware observations

The Elastic Security Labs team has been tracking REF2731, an intrusion set involving the PARALLAX loader which deploys the NETWIRE RAT. This activity has managed to stay under the radar with low detection rates and continues to incorporate interesting techniques such as DLL side-loading, syscall adoption, process injection, and leveraging steganography.

PARALLAX is a full-featured modal backdoor and loader featuring defense evasion and information on stealing capabilities, first observed in 2020 and associated with COVID-19 malspam campaigns. NETWIRE is a mature and cross-platform RAT that was first observed in 2012

In this research publication, we will go through the execution flow of one of the observed campaigns, the different features of the PARALLAX loader, technical analysis around the campaigns, campaign intersections, detection logic, and atomic indicators.

Execution Flow (PARALLAX loader)

The Elastic Security Labs team has been monitoring multiple campaigns over the past year leveraging the PARALLAX loader. PARALLAX has multiple capabilities and use cases. This analysis observed the PARALLAX loader being used to load other remote access tools (the NETWIRE RAT). Using our PARALLAX payload extractor, we have also observed the PARALLAX loader being used to load the PARALLAX RAT for interactive remote access. These infections typically start through email spam campaigns delivering macro-enabled lure documents.

High-level summary of the execution flow:

1. An email is sent to a victim with a macro-enabled Microsoft Word document attachment.
2. The macro downloads malicious files used for DLL-side loading and injection.
3. The Microsoft developer tool (MsiDb.exe) sideloads the malicious (msi.dll).
4. This malicious DLL drops and decrypts a WAV file (cs16.wav) before injecting the contents (shellcode) into cmd.exe.
5. The injected shellcode is used to extract the NETWIRE RAT and set up the PARALLAX loader from a dropped image (paper.png) and inject into cmd.exe.
6. A scheduled task is used to establish persistence for the PARALLAX RAT.
7. The NETWIRE payload is then executed and sets up its own persistence mechanism.

Execution flow diagram

First Stage (lure/macro)

The first stage in these campaigns involves macro-enabled lure documents typically with themes around United States tax filings.

Image from lure document walking victim through enabling macros

In this lure, we observed legitimate code lifted from the GLPK  (GNU Linear Programming Kit) used to bypass static analysis of the macro. The malicious code is then interwoven within the macro making it look very genuine and more deceptive.

Legitimate code from GLPK used in macro

This approach to obfuscation is also observed when critical components used for the next stage are not stored in the macro itself but called from text buried several pages deep within the lure document.

Download components embedded as text

The macro parses the embedded paragraph text on page three of the lure document and locates the object names and next stage components based on their string length. This is a clever technique to avoid detection based on static analysis of the macro (green text comments added to the images below by ESL for clarity).

VBA code parsing

The macro then uses the CreateObject  function to create the required objects and download each of the malware components, saving them to the AppData directory of the current user.

VBA code to create objects

It then executes AppData\MsiDb.exe through the created wscript.shell object.

For this observed lure, the five components that are downloaded for the next stage as identified in the embedded text image above are:

Second Stage (MsiDb.exe)

One of the key strengths in these campaigns is its ability to bypass static detection by modifying legitimate DLLs, a common trend previously reported with the BLISTER loader analysis [1, 2]. Once all the components are retrieved, the macro executes the signed Microsoft development tool (MsiDb.exe) to load the previously downloaded malicious library (msi.dll).

When the campaign began in September of 2022, this DLL had zero detections in VirusTotal due to its DLL tampering technique where a slight modification of a benign function is overwritten with the second stage.

0 detection rate on initial upload in VirusTotal

When (MsiDb.exe) sideloads the malicious (msi.dll) module, we can see the difference between the patched and unpatched version of msi.dll.

Comparison between the original and the patched msi.dll function

During this loading stage, the malicious code is heavily obfuscated and leverages dynamic API resolution to bypass static analysis tools and processes. It performs this  using two functions:

• One function is used to retrieve library addresses using the CRC32 checksum hash of the requested library name.
• Another function is used to take the address of the library and the hash of the API name.

Malicious code performing dynamic API resolution

The malware then builds its own import table, storing it on the stack. An interesting aspect is that the malicious code performs an anti-analysis check to see if the current process name matches the targeted application (MsiDb.exe), if it doesn’t match, the malware will stop at this stage. This check will hinder automated dynamic analysis systems that might try to analyze msi.dll in isolation by executing it with other common applications such as rundll32.exe or regsvr32.exe.

Next, the malware will load cs16.wav and XOR-decrypt it using a key embedded in the file. The key resides in the 200 bytes following the first 4 bytes of the file (bytes 5-204).

The malware will then execute the shellcode inside the decrypted WAV file.

Third Stage (shellcode)

To evade user mode hooks utilized by EDR/AV products and as debugger breakpoints, the malware uses direct system calls to low-level APIs used for process injection. It performs this by first mapping a file view of the Windows ntdll.dll library from the System directory.

NTDLL mapping

It then retrieves the API offset by subtracting the API address from the loaded base address of the loaded ntdll.dll, then finally it will use the offset from the mapped ntdll.dll and extract the syscall number.

Extracting the syscall number

After this, the loader uses the Heaven’s Gate technique and performs injection in the suspended cmd.exe process leveraging native Windows ZwAllocateVirtualMemory, ZwWriteVirtualMemory, and ZwResumeThread API functions.

Fourth Stage

One interesting technique observed during this stage is through the use of a dropped file (cs16.cfg). The file is a legitimate Python header file and is prepended with the next stage file name (paper.png). In our observations, these point to local files previously downloaded but also has the flexibility to point to hosted objects. This is another example of using benign code to obfuscate more malicious intent.

Prepending paper.png to cs16.cfg

If the first string of (cs16.cfg) points to a hosted file, it uses the IBackgroundCopyManager Component Object Model (COM) interface to download a PNG file and store it on disk (paper.png in our example).

Downloading the PNG file using BITS

The malware extracts a configuration structure from the stenographically-obfuscated PNG that contains the next PARALLAX loader stage and the final payload; in our sample, we identified the final payload as the NETWIRE RAT, but this process could be used to deliver other payloads.

Stenographically obfuscated PNG file

The malware executes position independent shellcode that reads and decodes the PNG file, it first extracts the red pixel bytes to an array by parsing the PNG, then decompresses the data with the LZMA algorithm.

Decompressing PNG extracted data

Next, it creates a suspended cmd.exe process and injects the NETWIRE payload and the last PARALLAX stage that will set up the environment and execute the NETWIRE payload.

Process tree of the malware’s execution

Below is the memory regions showing the injected process hosting the NETWIRE payload:

Injected NETWIRE payload in a cmd.exe process

Fifth Stage

The fifth and final stage of PARALLAX Loader performs a UAC bypass through CMSTPLUA COM interface, a technique that has been used by ransomware-like LockBit, it then sets persistence on the system before executing the final payload by creating a scheduled task to run Msidb.exe using Component Object Model (COM).

Scheduled task to run MsiDb.exe

Campaign Analysis

Throughout the analysis of the lure documents and malware families, we observed two campaigns associated with their TTPs, malware, network infrastructure, and lure metadata.

The intersections we observed allowed us to observe additional network infrastructure and identify the characteristics of one infrastructure owner in Campaign 1.

In the following sections, we will describe relevant elements and artifacts associated with each campaign, as well as their relationships.

This section will be focused on campaign intersections. As each campaign functioned similarly with respect to their technical implementation (lure document -> macro -> defense evasion techniques -> PARALLAX loader -> NETWIRE RAT), we’ll use the analysis of the five stages for the deployment of the PARALLAX and NETWIRE malware that has been described in detail in the previous Execution Flow section.

While we are not attributing these campaigns to any specific threat actor, we have identified parallel research leveraging the same TTPs that we observed. This research was attributed to the financially motivated threat group, Evilnum [1, 2] and the DarkCasino campaign.

Campaign 1

Overview

This campaign is clustered by shared lure document metadata, network infrastructure, dropped macro, and malicious DLL (msi.dll).

Campaign 1 node-link analysis

Lure Documents

The three lure documents used in Campaign 1 were all macro-embedded Microsoft Word documents. The documents were all 153 pages long, with the macro embedded on the 3rd page. The documents all included the H1 Word document header of Как я искал Гантмахера (loosely translated to: “How I searched for Gantmakher”). Vsevolod Gantmakher was a Russian physicist.

Extracting the metadata for all three documents, we can see their relationships based on several fields; most notably:

• The identical HeadingPairs (the names of the Word document header).
• The identical CreationDate dates.
• The identical LastPrinted dates.
• The ModifyDate dates are all within 14-minutes.

Campaign 1 lure document metadata

The H1 document header of the lure documents does not appear relevant to the targeting as the lure document names and lure document content are wholly unrelated: two of the three document names were related to 2021 United States tax filings, all three of the document names are in English, and the contents of the lure documents are in Cyrillic.

Macro

The macro downloads five files, detailed in the Execution Flow section above (cs16.wav, msi.dll, MsiDb.exe, paper.png, and cs16.cfg), from a different domain for each lure document.

Campaign 1 macro-downloaded files

Network Infrastructure

Campaign 1 included three domains contacted by the macro to download artifacts required for stages two through five (described in the “Execution Flow” section above) and three domains used for the NETWIRE RAT C2.

The six domains are:

• digitialrotprevention[.]com - macro-connected.
• internationalmusicservices[.]com - macro-connected.
• globalartisticservices[.]com - macro-connected.
• ohioohioa[.]com - NETWIRE C2.
• ywiyr[.]com - NETWIRE C2.
• septton[.]com - NETWIRE C2.

The macro-connected domains (digitialrotprevention[.]com, internationalmusicservices[.]com, and globalartisticservices[.]com) include metadata that has allowed us to cluster these three domains together in Campaign 1.

Campaign 1 network infrastructure

In the above image, the Admin email address and Admin user name is russnet123@protonmail[.]com and rus fam, respectively. As of this writing, these domains have been suspended.

Once the execution flow reaches the Fourth Stage (described in the Execution Flow section above), the final three domains (ohioohioa[.]com, ywiyr[.]com, and septton[.]com) act as ongoing command and control nodes for the NETWIRE RAT.

While ​​ohioohioa[.]com and ywiyr[.]com are protected by privacy services, septton[.]com has interesting metadata that we were able to collect and is outlined below in the SEPTTON Domain section below.

Campaign 1 Indicators

Campaign 2

Overview

This campaign is clustered through its lure document metadata, network infrastructure, dropped macro, and malicious DLL (msvcr100.dll).

Campaign 2 node-link analysis

Lure Documents

The lure document used in Campaign 2 is a macro-embedded Microsoft Word document. The document metadata differentiates it from Campaign 1 based on the LastModifiedBy field and the macro network infrastructure.

Campaign 2 lure document metadata

The document name was also related to 2021 United States tax filings.

Macro

Like Campaign 1, the macro downloads several files. Beyond the DLL file (msvcr100.dll), all files were offline before they could be collected. Based on the TTPs observed in this campaign, we assess with high confidence that they (java.exe, Fruit.png, idea.cfg, and idea.mp3) function similarly to the files from Campaign 1 and detailed in the Execution Flow section above.

Campaign 2 macro-downloaded files

Additional details about the Campaign 1 and Campaign 2 file relationships are in the “Campaign intersections” section below.

Network Infrastructure

Campaign 2 included one domain contacted by the macro to download artifacts required for stages two through five (described in detail in the “Execution Flow” section above). Additionally, there was one domain used for the NETWIRE RAT C2.

The two domains are:

• solro14.s3.ap-northeast-3.amazonaws[.]com - macro-connected
• ohioohioa[.]com - NETWIRE C2

Once the execution flow reaches stage four, ohioohioa[.]com acts as the ongoing command and control node for the NETWIRE RAT.

Campaign 2 Indicators

Campaign Intersections

Campaign 1 and Campaign 2 intersect in several ways.

As illustrated in the image below, each campaign relied on a lure document (or documents) to execute a macro that contacted adversary-owned or controlled domains; downloaded artifacts used to install and protect the PARALLAX and NETWIRE RAT implants. Additionally, in both campaigns we analyzed, there is a shared network infrastructure used for the NETWIRE C2.

Campaign 1 and Campaign 2 intersections

The Pyramid of Pain

In 2013 (and updated in 2014), security researcher David Bianco released an analytical model called the Pyramid of Pain. The model is intended to understand how uncovering different parts of an intrusion can impact a campaign. As you can see in the model below, the identification of hash values is useful, but easily changed by an adversary whereas identifying TTPs is very difficult for an adversary to change.

The Pyramid of Pain

The goal of using the Pyramid of Pain is to understand as much about the intrusion as possible and project the impact (read: the amount of "pain") you can inflict.

When analyzing the two campaigns, we can put the Pyramid of Pain into action.

• Hash values - each lure document had a unique hash.
• IP addresses - each network connection leveraged a different IP address.
• Domain names - each network connection leveraged exclusive domains for the macro components but shared a NETWIRE C2 domain (ohioohioa[.]com).
• Network/host artifacts
• Identically-named host artifacts observed in Campaign 1.
• Renamed from Campaign 1, but functionally identical, host artifacts observed in Campaign 2.
• Artifact bundles from both campaigns include similarly formatted and functionally identical files.
• Tools - macro-enabled Word document lures, and PARALLAX and NETWIRE RATs.
• TTPs - complex and defensive five-staged execution chain.

Looking across both campaigns, we can see there is some shared infrastructure at the Domain Names tier in the NETWIRE C2 domain (ohioohioa[.]com). In the Network/host artifacts tier we can see additional intersections between the campaigns.

Network/host artifacts

In both campaigns, we can see a PE file (MsiDb.exe and java.exe), a DLL file (msi.dll and msvcr100.dll), a PNG file (paper.png and Fruit.png), an audio-format named file (cs16.wav and idea.mp3), and a configuration file (cs16.cfg and idea.cfg) at the Network/host artifact tier. All downloaded files in Campaign 1 are named the same across all three lure documents. In both campaigns, the audio-format named files have the same base name as the configuration files (cs16.wav/cs16.cfg and idea.mp3/idea.cfg). In both campaigns, we assess with high confidence that all host artifacts are functionally identical as described in the Execution Flow section above.

The SEPTTON Domain

As reported in the Campaign 1 section, most of the network infrastructure was either well-used across multiple intrusions unrelated to our campaigns or protected by domain privacy services.

An exception to that is the seppton[.]com domain, which was used as the C2 node for a NETWIRE RAT implant in our sampling. Continuing to analyze this domain, we observed several other associated malicious files. While we did not independently verify the family of malware that is communicating with this domain, signature names in VirusTotal include NETWIRE.

septton[.]com file relationships

Looking through the registration information for the domain, we observed two elements of note, both email addresses - marketforce666@yandex[.]com and chisholm.i@aol[.]com.

septton[.]com domain registration

In the next two sections, we’ll discuss the resource development for domains used in campaigns.

marketforce666

Searching for marketforce666 in a search engine did not return results of value from the United States; however, when changing to an Internet egress point within Russia and using the Yandex search engine (Yandex is a Russian Internet services provider), we identified 802 results that show this term has been associated with multiple abuse reports.

marketforce666 Yandex search engine results

When expanding our search for domains registered by marketforce666@yandex[.]com, we identified three additional domains. We did not observe these additional domains in our campaigns, but we are including them as indicator artifacts. Below are the four total domains (one from Campaign 1 and three additional) that were registered by, either as the admin, tech, or registrant address, marketforce666@yandex[.]com.

Domains registered to marketforce666@yandex[.]com

gaza666

Looking at the other email address, chisholm.i@aol[.]com, we were able to connect this email address with a moniker of gaza666 from the online forum and marketplace, Infected Zone.

On this forum, the user gaza666 attempted to purchase (https://infected-zone[.]com/threads/2814/) an “Office 365 Complete Package” from the online seller rzkyo. gaza666 and the seller rzkyo engaged in a dispute on the forum where gaza666 did not believe they received what they purchased - which was a package for email spamming and four United States Office 365 accounts but received three nonfunctional and non-Office 365 Phillipino accounts. The seller, rzkyo, responded and the two debated what was purchased and what was delivered. The dispute was responded to by a moderator who attempted to resolve the issue.

Invoice email where gaza666 purchased infrastructure from rzkyo

Invoice where gaza666 is linked to chisholm.i@aol[.]com

The results of the dispute were not in the forum, but there were several screenshots where rzkyo  showed gaza666 and the moderators that the services they sold were functional.

SMTP credentials from rzkyo to gaza666

Setup of online SMTP testing service

Test from email spam accounts purchased from rzkyo

While it is unknown if the infrastructure above that gaza666 attempted to purchase from rzkyo was used in our observed campaigns (or ever used at all), but gaza666 is associated with chisholm.i@aol[.]com, which was used to register septton[.]com, and septton[.]com was used as a NETWIRE C2 node in Campaign 1.

gaza666 involvement in NETWIRE C2 in Campaign 1

marketforce666 (marketforce666@yandex[.]com) and gaza666 (chisholm.i@aol[.]com) share a relationship in that both emails were used in the registration of septton[.]com, which was used as a NETWIRE C2 domain for Campaign 1. The 666 term appended to marketforce and gaza could be another indicator of their relationship, but this could not be confirmed.

Diamond Model

Elastic Security utilizes the Diamond Model to describe high-level relationships between adversaries and victims of intrusions.

REF2731 diamond model

Observed Adversary Tactics and Techniques

Elastic uses the MITRE ATT&CK framework to document common tactics, techniques, and procedures that advanced persistent threats use against enterprise networks.

Tactics

Tactics represent the why of a technique or sub-technique. It is the adversary’s tactical goal: the reason for performing an action.

• Resource Development
• Initial Access
• Execution
• Persistence
• Privilege Escalation
• Defense Evasion
• Command and Control

Techniques / Sub techniques

Techniques and Sub techniques represent how an adversary achieves a tactical goal by performing an action.

• Acquire Infrastructure: Domains
• Phishing: Attachment
• Hijack Execution Flow: DLL Side-Loading
• Process Injection
• Scheduled Task
• Native API
• Obfuscated Files or Information: Steganography
• Abuse Elevation Control Mechanism: Bypass User Account Control

Detection

Detection Logic

The following detection rules and behavior prevention events were observed throughout the analysis of this intrusion set.

Behavioral Rules

• NetWire RAT Registry Modification
• Remcos RAT Registry or File Modification

Detection Rules

• Persistence via Scheduled Job Creation
• Command Prompt Network Connection

Signatures

• Windows.Trojan.Parallax
• Windows.Trojan.Netwire
• Windows.Trojan.Remcos

YARA

Elastic Security has created YARA rules to identify this activity.

rule Windows_Trojan_Parallax_1 {
    meta:
        author = “Elastic Security”
        creation_date = "2022-09-05"
        last_modified = "2022-09-15"
        license = “Elastic License v2”
        os = "Windows"
        arch = "x86"
        category_type = "Trojan"
        family = "Parallax"
        threat_name = "Windows.Trojan.Parallax"
    strings:
        $COM_png = { B9 01 00 00 00 6B D1 00 C6 44 15 D4 83 B8 01 00 00 00 C1 E0 00 C6 44 05 D4 B6 B9 01 00 00 00 D1 E1 C6 44 0D D4 33 BA 01 00 00 00 6B C2 03 C6 44 05 D4 28 B9 01 00 00 00 C1 E1 02 C6 44 0D D4 36 BA 01 00 00 00 6B C2 05 C6 44 05 D4 6B B9 01 00 00 00 6B D1 06 C6 44 15 D4 90 B8 01 00 00 00 6B C8 07 C6 44 0D D4 97 }
        $png_parse = { 8B 4D ?? 8B 04 B8 85 C9 74 ?? 8B F1 90 8A 08 8D 40 ?? 88 0C 1A 42 83 EE ?? 75 ?? 8B 4D ?? 8B 45 ?? 47 3B 7D ?? 72 ?? }
        $config_func = { C7 45 F8 68 74 74 70 8B ?? ?? 8B 02 89 ?? ?? 6A 08 8D ?? ?? 51 E8 ?? ?? ?? ?? 83 C4 08 8B ?? ?? 52 8D ?? ?? 50 8B ?? ?? 8B 51 0C FF D2 }
        $winnet_function = { B8 77 00 00 00 66 89 ?? ?? B9 69 00 00 00 66 89 ?? ?? BA 6E 00 00 00 66 89 ?? ?? B8 69 00 00 00 66 89 ?? ?? B9 6E 00 00 00 66 89 ?? ?? BA 65 00 00 00 66 89 ?? ?? B8 74 00 00 00 66 89 ?? ?? 33 C9 66 89 ?? ?? 8D ?? ?? 52 8B ?? ?? 8B 48 1C FF D1 }
    condition:
        $config_func or $winnet_function or $COM_png or $png_parse
}

rule Windows_Trojan_Parallax_2 {
    meta:
        author = “Elastic Security”
        creation_date = "2022-09-08"
        last_modified = "2022-09-08"
        license = “Elastic License v2”
        os = "Windows"
        arch = "x86"
        category_type = "Trojan"
        family = "Parallax"
        threat_name = "Windows.Trojan.Parallax"
    strings:
        $parallax_payload_strings_0 = "[Ctrl +" ascii wide fullword
        $parallax_payload_strings_1 = "[Ctrl]" ascii wide fullword
        $parallax_payload_strings_2 = "Clipboard Start" ascii wide fullword
        $parallax_payload_strings_3 = "[Clipboard End]" ascii wide fullword
        $parallax_payload_strings_4 = "UN.vbs" ascii wide fullword
        $parallax_payload_strings_5 = "lt +" ascii wide fullword
        $parallax_payload_strings_6 = "lt]" ascii wide fullword
        $parallax_payload_strings_7 = ".DeleteFile(Wscript.ScriptFullName)" ascii wide fullword
        $parallax_payload_strings_8 = ".DeleteFolder" ascii wide fullword
        $parallax_payload_strings_9 = ".DeleteFile " ascii wide fullword
        $parallax_payload_strings_10 = "Scripting.FileSystemObject" ascii wide fullword
        $parallax_payload_strings_11 = "On Error Resume Next" ascii wide fullword
        $parallax_payload_strings_12 = "= CreateObject" ascii wide fullword
        $parallax_payload_strings_13 = ".FileExists" ascii wide fullword
    condition:
        7 of ($parallax_payload_strings_*)}

PARALLAX Payload Extractor

Automating the payload extraction from PARALLAX is a key aspect when it comes to threat hunting as it gives visibility of the campaign and the malware deployed by the threat actors which enable us to discover new unknown samples in a timely manner.

Our extractor takes either a directory of samples with -d option or -f for a single sample, You can use the -o switch to set the output directory of the payloads.

Payload extraction from a PARALLAX sample

To enable the community to further defend themselves against existing and new variants of the PARALLAX loader, we are making the payload extractor open source under the Apache 2 License. The payload extractor documentation and binary download can be accessed here.

Conclusion

In the above research, we have analyzed the two campaigns that we’ve tracked using macro-embedded lure documents that download seemingly benign artifacts from the staging hosts on the Internet, and weaponize those artifacts to perform persistence, command and control, and remote access of an infected host.

We also highlighted the elements used to cluster the two campaigns together and how the campaigns can be used with analytical models to impose costs on the campaign owners.

References

The following were referenced throughout the above research:

• https://blog.morphisec.com/parallax-rat-active-status
• https://malpedia.caad.fkie.fraunhofer.de/details/win.parallax
• https://attack.mitre.org/software/S0198/
• https://attack.mitre.org/groups/G0120/
• https://malpedia.caad.fkie.fraunhofer.de/actor/evilnum
• http://blog.nsfocus.net/darkcasino-apt-evilnum/

Indicators

Artifacts are also available for download in both ECS and STIX format in a combined zip bundle.