Attack chain leads to XWORM and AGENTTESLA

この記事のポイント

Threat actors are deploying known malware using their own custom .NET loaders
The threat actors employ simple and well-known tactics such as bypassing AMSI through patching and a basic custom .NET loader
The threat actors are abusing legitimate free file hosting services

前文

Our team has recently observed a new malware campaign that employs a well-developed process with multiple stages. The campaign is designed to trick unsuspecting users into clicking on the documents, which appear to be legitimate, but are in fact fake, the adversary leverages weaponized word documents to execute malicious PowerShell scripts, and also utilizes a custom obfuscated .NET loader to load various malware strains, including XWORM and AGENTTESLA.

RTF loader code analysis

Overview

During a recent investigation, we discovered a malicious word document named Card & Booking Details.docx. This document has been designed with the intent to deceive the victim and includes two falsified scanned documents, namely a credit card and a passport.

Upon opening the document, an RTF object hosted at www.mediafire[.]com/file/79jzbqigitjp2v2/p2.rtf is fetched.

This RTF object contains a macro-enabled Excel object. When opened, this macro downloads an obfuscated powerShell script which in turn deploys different malware families.

At the time of this writing, we have observed two distinct malware families, namely XWORM and AGENTTESLA, have been deployed through this execution chain. Both malware families mentioned above are loaded into the compromised system's memory by the same custom .NET loader. Once loaded, the malicious payload can carry out a range of functions, such as stealing sensitive data and executing commands on the compromised system.

In this research post, we will walk through the initial execution of the malware and detail the capabilities we discovered.

Extracting the malicious VBA

The RTF document contains multiple embedded objects, including an interesting one that caught our attention: Excel.SheetMacroEnabled.

We can use rtfdumpy.py, a script developed by Didier Stevens to analyze RTF files, to dump the object and olevba.py, a script developed by Philippe Lagadec, to extract any embedded VBA scripts from an OLE object. The extracted VBA script shown below downloads and executes a malicious powershell script from https://www.mediafire[.]com/file/xnqxmqlcj51501d/7000m.txt/file.

Powershell script analysis

The malicious PowerShell script is obfuscated using string substitution to evade detection and make analysis more difficult.

It contains additional powershell script blocks in hex format that will be deployed in the infected machine designed to prepare the environment by setting up persistence, bypassing AMSI, disabling Windows defender and creating a mechanism to update the malware. The ultimate objective is to install two .NET binaries, namely a loader and a payload (XWORM / AGENTTESLA).

Deleting the malicious document

The malware starts by deleting the original Word document, first killing the process Winword.exe and then deleting all .DOCX files located in the default Downloads and Desktop folders of every user. This initial step shows the malware's destructive nature and how it can potentially harm the user's data.

永続化

The malware creates a directory in the path C:\ProgramData\MinMinons , which is used to store other Powershell scripts and binaries. The currently running Powershell script is then copied to C:\ProgramData\MinMinons\Candlegraphy.\_\_\_.

Next, the malware deobfuscates the first embedded Powershell script which is used to create persistence. It first writes a JScript file that invokes the original Powershell script saved in C:\ProgramData\MinMinons\Candlegraphy.\_\_\_ through the activeXObject shell, then a scheduled task named “MOperaChrome” is created to run the JScript file using the Microsoft signed Windows Script Host (WSH) utility, wscript.exe.

AMSI bypass

The second embedded powershell script is responsible for bypassing AMSI by patching the amsiInitFailed flag. In doing so, the initialization of AMSI fails, leading to the prevention of any scan being initiated for the ongoing process. Furthermore, the PowerShell script proceeds to disable the Microsoft Windows Defender service.

User creation

The script creates a local administrator account named “System32” and adds it to the Remote Desktop Users group. This enables the attacker to log in via Remote Desktop Protocol (RDP). Next, the script disables the machine's firewall to allow inbound RDP connection attempts which aren’t filtered by edge controls.

Malware update persistence

The third embedded script stores a secondary JScript file, whose purpose is downloading a revised or updated version of the malware. This file is saved to a predetermined location at C:\ProgramData\MinMinons\miguan.js. Furthermore, a scheduled task with the name (“miguaned”) is created to execute the JScript file through wscript.exe , similar to the previously described task.

The JScript creates an instance of WScript.Shell object by calling ActiveXObject with the following CLSID {F935DC22-1CF0-11D0-ADB9-00C04FD58A0B} which corresponds to Shell Object, then downloads from the URL https://billielishhui.blogspot[.]com/atom.xml the update powershell malware.

.NET loader

The custom DOTNET loader employs the P/INVOKE technique to call the native Windows API and inject a payload into a signed microsoft binary via process hollowing.

The loader’s code employs various obfuscation techniques to hinder analysis, including the use of dead instruction, renamed symbols to make the code less readable and more confusion and encoded strings. Fortunately a tool like de4dot can be used to output a human-readable version of it.

The malware leverages the LoadLibrary and GetProcAddress APIs to access the required Windows APIs. To obscure the names of these APIs, the loader stores them in an encoded format within the binary file, utilizing a sequence of substitution and string reversal methods.

The loader then starts a process in a suspended state using CreateProcessA API. The following is the list of executables it uses as a host for it’s malicious code:

C:\Windows\Microsoft.NET\Framework\v4.0.30319\RegSvcs.exe
C:\Windows\Microsoft.NET\Framework\v2.0.50727\RegSvcs.exe
C:\Windows\Microsoft.NET\Framework\v3.5\Msbuild.exe

These binaries are signed and trusted by the system and can evade detection by security software that relies on whitelisting system processes. It then uses Zwunmapviewofsection to unmap the memory of the target process, writes the payload to the suspended process and then resume the thread using ResumeThread API.

Final payload

During our research we discovered that the threat actor has been deploying different payloads. Namely, we observed 2 families: XWORM and AGENTTESLA.

XWORM has gained notoriety in the underground criminal marketplace due to its ability to employ sophisticated capabilities like virtualization and sandbox detection, used to avoid detection and support persistence within an infected system.

Of particular concern is the fact that XWORM is readily available on the internet as a cracked version, with version 2.1 being especially prevalent. This highlights the dangers of underground cybercrime markets and the ease with which malicious actors can access and utilize powerful tools.

Two different versions of the XWORM family were observed versions 2.2 and 3.1. The following is the configuration of a XWORM sample in plain text.

AGENTTESLA is a trojan and credential stealer written in .NET. While it first emerged in 2014, it is now among the most active and malicious software. AGENTTESLA is affordably priced and includes support from the developers, making it easily accessible to cybercriminals with limited technical skills.

The sample we analyzed was heavily obfuscated, masqueraded as an AVG installer,and leverages discord for C2. It uploads stolen information to the attacker’s Discord channel via the following webhook: https://discord[.]com/api/webhooks/1089956337733087274/uYNA_D8Ns1z9NZ3B1mGp0XXyGq-785KLGIfEAZsrz3TJd5fvOjXA927F7bUTTzbNT6Zk.

観察された敵対者の戦術と手法

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

戦術（Tactics）

Tactics represent the “why” of a technique or sub-technique. They represent the adversary’s tactical goals: the reason for performing an action.

Techniques/subtechniques

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

検知のロジック

ヤラ

Elastic Security has created YARA rules to identify this activity. Below are YARA rules to identify XWORM and AGENTTESLA malware families.

rule Windows_Trojan_Xworm_732e6c12 {
meta:
    author = "Elastic Security"
    id = "732e6c12-9ee0-4d04-a6e4-9eef874e2716"
    fingerprint = "afbef8e590105e16bbd87bd726f4a3391cd6a4489f7a4255ba78a3af761ad2f0"
    creation_date = "2023-04-03"
    last_modified = "2023-04-03"
    os = "Windows"
    arch = "x86"
    category_type = "Trojan"
    family = "Xworm"
    threat_name = "Windows.Trojan.Xworm"
    source = "Manual"
    maturity = "Diagnostic"
    reference_sample = "bf5ea8d5fd573abb86de0f27e64df194e7f9efbaadd5063dee8ff9c5c3baeaa2"
    scan_type = "File, Memory"
    severity = 100

strings:
    $str1 = "startsp" ascii wide fullword
    $str2 = "injRun" ascii wide fullword
    $str3 = "getinfo" ascii wide fullword
    $str4 = "Xinfo" ascii wide fullword
    $str5 = "openhide" ascii wide fullword
    $str6 = "WScript.Shell" ascii wide fullword
    $str7 = "hidefolderfile" ascii wide fullword
condition:
    all of them}

rule Windows_Trojan_AgentTesla_d3ac2b2f {
meta:
    author = "Elastic Security"
    id = "d3ac2b2f-14fc-4851-8a57-41032e386aeb"
    fingerprint = "cbbb56fe6cd7277ae9595a10e05e2ce535a4e6bf205810be0bbce3a883b6f8bc"
    creation_date = "2021-03-22"
    last_modified = "2022-06-20"
    os = "Windows"
    arch = "x86"
    category_type = "Trojan"
    family = "AgentTesla"
    threat_name = "Windows.Trojan.AgentTesla"
    source = "Manual"
    maturity = "Diagnostic, Production"
    reference_sample = "65463161760af7ab85f5c475a0f7b1581234a1e714a2c5a555783bdd203f85f4"
    scan_type = "File, Memory"
    severity = 100

strings:
    $a1 = "GetMozillaFromLogins" ascii fullword
    $a2 = "AccountConfiguration+username" wide fullword
    $a3 = "MailAccountConfiguration" ascii fullword
    $a4 = "KillTorProcess" ascii fullword
    $a5 = "SmtpAccountConfiguration" ascii fullword
    $a6 = "GetMozillaFromSQLite" ascii fullword