Cyble – Rhadamanthys: New Stealer Spreading Through Google Ads

Evasive Infostealer leveraging Phishing and Spam Campaigns for its Delivery

Threat Actors (TAs) are increasingly using spam emails and phishing websites to trick users into downloading malware such as Stealer and Remote Access Trojan (RAT) to infect users’ machines and steal sensitive information.

Cyble Research & Intelligence Labs (CRIL) is actively monitoring various stealer malware and publishing blogs about them to inform and educate its readers.

Recently, we came across a new strain of malware called “Rhadamanthys Stealer.” This stealer variant is active, and the TA behind the malware stealer is selling this under the Malware as a Service (MaaS) model.

Rhadamanthys stealer spreads by using Google Ads that redirect the user to phishing websites that mimic popular software such as Zoom, AnyDesk, Notepad++, Bluestacks, etc. It can also spread via spam email containing an attachment for delivering the malicious payload.

Spam Email

The Rhadamanthys stealer infection starts through spam emails containing a PDF attachment named “Statement.pdf” as shown in the figure below.

When opening the attachment present in the spam email, it displays a message indicating it is an “Adobe Acrobat DC Updater” and includes a download link labelled “Download Update,” as shown below.

When a user clicks the “Download Update” link, it downloads a malware executable from an URL “https[:]zolotayavitrina[.]com/Jan-statement[.]exe” into the Downloads folder.

Upon execution of the “Jan-statement.exe” file, it runs the stealer and allows it to steal sensitive information from the victim’s machine. The figure below illustrates the process tree of the Rhadamanthys stealer that was delivered via a spam email.

Phishing Sites

The TAs behind this campaign also created a highly convincing phishing webpage impersonating legitimate websites to trick users into downloading the stealer malware, which carries out malicious activities. The link to these phishing websites spreads through Google ads. We have observed several phishing domains created to spread this malware. Some of the following:

• bluestacks-install[.]com
• zoomus-install[.]com
• install-zoom[.]com
• install-anydesk[.]com
• install-anydeslk[.]com
• zoom-meetings-install[.]com
• zoom-meetings-download[.]com
• anydleslk-download[.]com
• zoomvideo-install[.]com
• zoom-video-install[.]com
• istaller-zoom[.]com
• noteepad.hasankahrimanoglu[.]com[.]tr

The phishing websites further downloads an installer file disguised as a legitimate installer downloading the respective applications. When installing the respective application, it also silently installs the stealer malware without the user’s knowledge. The below figure shows the process tree of the malicious AnyDesk installer deploying Rhadamanthys stealer.

 Payload Analysis

Upon execution of the installer file, it creates a folder named “ST” in the %temp% location and drops two hidden binary executable files. 

• Initialize 4.exe
• Runtime Broker.exe

The loader “Runtime Broker.exe” is a 32-bit PyInstaller executable with SHA256: db66fc58c07ba0ccbe1b9c2db770179d0d931e5bf73838da9c915581661d4c1a.

The additional information is shown in the figure below.

Upon execution of “Runtime Broker.exe”, it drops multiple Python-supporting files in the %temp% folder.

These files include “.pyc”, “.pyd”, and “.dll” files, which were extracted from the PyInstaller executable as shown below.

The “Binary_Stub_Replacer.pyc” is a python compiled file which contains obfuscated raw data that will be de-obfuscated using replace function and then converted into Binary and ASCII format for getting the second stage malicious python code as shown below.

The decoded python code contains an embedded base64-encoded content which is a shellcode. When executed, this python code decodes the base64-encoded stub, creating a new Portable Executable (PE) payload file. The PE file is then injected into a new “Runtime Broker.exe” process using the CreateThread() API function, as shown in the image below.

The below image shows the details of the shellcode, which is a 32-bit executable file compiled with Microsoft visual C/C++ compiler, as shown below.

Upon execution, the shellcode begins by creating a mutex object to ensure that only one copy of the malware is running on the victim’s system at any given time. It then checks if it is running on a virtual machine, such as VMware or VirtualBox, by searching for strings associated with virtual machine environments, as shown in the figure below.

This check is designed to prevent the malware from being detected and analyzed in a virtual environment. If the malware detects that it is running in a controlled environment, it will terminate its execution. Otherwise, it will continue and perform the stealer activity as intended.

After the check, the shellcode further drops a DLL file named “nsis_unsibcfb0.dll” in the %temp% folder and launches it using the “rundll32.exe” with specific parameters shown in the figure below.

While investigating this malware, we observed that a steganography image was downloaded from the remote server. We suspect the shellcode decrypts the steganography image to get the actual Rhadamanthys payload. The memory of rundll32.exe contains all the malicious code responsible for stealer activities.

The Rhadamanthys stealer now starts collecting system information by executing a series of Windows Management Instrumentation (WMI) queries. The collected information includes the computer name, username, OS version, RAM, CPU information, HWID, time zone, user and keyboard language, and others.

After gathering system details, the malware queries the directories of the installed browsers on the victim’s machine and searches for browser-related files such as browsing history, bookmarks, cookies, auto-fills, login credentials, etc. It targets different browsers such as Brave, Edge, Chrome, Firefox, Opera Software, Sleipnir5, Pale Moon, CocCoc, etc.

Crypto Wallets

This stealer malware is also designed to target various crypto wallets and collects information from them. While the malware can target a wide range of crypto wallets, the observed stealer samples were found to have specific functionality to target the following crypto wallets:

• Armory
• Binance
• Bitcoin
• Bytecoin
• Electron
• Qtum-Electrum
• Solar wallet
• WalletWasabi
• Zap
• Zecwallet Lite
• Zcash

Also, the Rhadamanthys stealer steals data from the following crypto wallet browser extensions, which are hard coded in the stealer binary, as shown in the image below.

The stealer also targets various applications such as FTP clients (CoreFTP, WinSCP), email clients (Foxmail, Thunderbird, Outlook, TrulyMail, GmailNotifierPro), File managers (Total commanders), password managers (RoboForm, KeePass), VPN services (NordVPN, ProtonVPN, Windscribe VPN, OpenVPN), messaging applications (Tox, Discord, Telegram) and others. Additionally, it captures screenshots of the victim’s machine using the BitBlt() API function. Finally, it sends all the collected stolen information to the attacker’s C&C server.

C&C Panel

The below figure shows the Rhadamanthys stealer’s active C&C panel.

Conclusion

Information stealers are malicious software used to gain unauthorized access to corporate networks, which has become a serious concern. Threat Actors use various techniques to deploy their malicious payloads into the victim’s system. In this case, we observed that the TAs used spam email and phishing websites to deliver the Rhadamanthys Stealer, designed to steal sensitive information from the victim’s machine. Additionally, it was also noticed that the malware spreads via Google Ads. It is crucial for users to exercise caution when receiving spam emails or to visit phishing websites and to verify the source before downloading any applications.

Cyble Research and Intelligence Labs will continue monitoring the new malware strains in the wild and update blogs with actionable intelligence to protect users from such notorious attacks.

Our Recommendations

• The initial infection may happen via spam emails or phishing websites, so enterprises should use security products to detect phishing emails and websites.
• Avoid downloading pirated software from Warez/Torrent websites. The “Hack Tool” present on sites such as YouTube, Torrent sites, etc., contains such malware. 
• Use strong passwords and enforce multi-factor authentication wherever possible.   
• Turn on the automatic software update feature on your computer, mobile, and other connected devices.  
• Use a reputed anti-virus and internet security software package on your connected devices, including PC, laptop, and mobile.  
• Refrain from opening untrusted links and email attachments without first verifying their authenticity.   
• Educate employees on protecting themselves from threats like phishing/untrusted URLs.  
• Block URLs that could be used to spread the malware, e.g., Torrent/Warez.  
• Monitor the beacon on the network level to block data exfiltration by malware or TAs.

MITRE ATT&CK® Techniques

Indicators of Compromise (IOCs)

Related

Source: https://blog.cyble.com/2023/01/12/rhadamanthys-new-stealer-spreading-through-google-ads/