ARCrypter Ransomware Expands Its Operations From Latin America to the World

Summary

On August 25, 2022, Chile's government computer systems were attacked by a previously unseen ransomware variant. CSIRT of Chile’s government published a report which contained some Indicators of Compromise (IoCs) and recommendations for prevention measures.

On October 3, 2022, Invima — The Colombia National Food and Drug Surveillance Institute — reported a cyberattack that led to a temporary shutdown of the organization's web services.

Through our threat hunting efforts, BlackBerry has identified additional samples of interest for this ransomware. Given the timeframe of the attack and the contents of the ransom note mentioning Invima, we believe with a high degree of certainty that this ransomware was used in the Invima cyberattack that took place in early October.

Based on the unique strings identified during the analysis, we have named this unknown ransomware variant “ARCrypter.”

Figure 1 –  Invima reports a cyber-attack via Twitter.
(Translation: “Following the contingency of the cyberattack, and according to the established monitoring framework, we have enabled the following temporary links for users to fill out the different reports.")

While live and retro hunting on VirusTotal, we found submissions by apparently real victims from China and Canada. Based on the original file names, timelines, and type of submissions, it's highly likely that ARCrypter is no longer confined to LatAm only. It has now expanded its operations worldwide.

Weaponization and Technical Overview

Technical Analysis

Context

Through effective hunting efforts, we were able to locate samples associated with the first ARCrypter campaign from early August 2022, which was three weeks before the incident that occurred in Chile. Unlike other ransomware variants, where a ransom note is dropped after the file encryption stage, this variant drops the ransom note before the files are encrypted, making this particular delivery mechanism unusual compared to what we have seen in other ransomware samples. Upon ransom note delivery, the dropper then proceeds to drop two batch scripts and the main payload encrypter.

Since this was an undocumented ransomware family and the unique strings of “ARC” were found in all the samples we analyzed, we named this ransomware variant “ARCrypter.”

Figure 2 – ARCrypter strings

Attack Vector

The attack vector of the infection is unknown.

Weaponization

During our investigation, we uncovered two AnonFiles URLs facilitating the download of “win.exe” and “win.zip”. The “win.exe” file is a dropper file which we will discuss in more detail next. The “win.zip” file is a password-protected archive containing the “win.exe” file. It is important to note that AnonFiles provides anonymous file upload while hiding the IP address of the submitter. This method of file distribution is favored by many threat actors; however, it is unclear whether this particular method was used by the threat actor behind ARCrypter, or somebody else.

Figure 3 – Dropper file "win.exe" download link

First Stage – Dropper

Upon deeper analysis of the dropper file, we found that it contains two resources of interest – BIN and HTML. The HTML resource stores ransom note contents, and the BIN resource contains the encrypted data.

In order to decrypt the BIN resource, the dropper expects an argument “-p” followed by a password. Once the password is provided, the dropper proceeds by creating a random directory under one of the following environment variables:

• %TMP%
• %APPDATA%
• %ALLUSERSPROFILE%
• %HOMEPATH%

The purpose of this newly created directory is to store the second stage payload using alphanumeric characters, matching the following regular expression:

C:\Users\<username>\AppData\Local\Temp\[a-zA-Z0-9]{9}\[a-zA-Z0-9]{9}.exe

Figure 4 – Creation of the second stage payload

While we were unable to identify the correct decryption key used for decryption of the BIN resource, we believe with a high degree of certainty that the second payload is the ARCrypter ransomware.

The ransom note is then extracted from the embedded resource of HTML type, and saved in %TMP%/readme_for_unlock.txt:

C:\Users\<username>\AppData\Local\Temp\readme_for_unlock.txt

Figure 5 – ARCrypter ransom note.

The ransom note contains the username and password required for the victim to log into the communication panel with the threat actors, which is hosted on the .onion site.

Figure 6 – Victims' communication panel with the threat actor

To perform a cleanup for the dropper, two bat files are created in the following location under the %TMP% directory:

C:\Users\<username>\AppData\Local\Temp\[a-zA-Z0-9]{9}.bat

The first bat file terminates the dropper process, overwrites its original file, and executes the second bat file.  

Second Stage – Payload

Upon execution, the malware proceeds in establishing persistence by invoking the reg.exe process. The purpose of this is to add the registry key “HKLM\Software\Microsoft\Windows\CurrentVersion\Run\SecurityUpdate” with value of path to the malware.

It also modifies two registry keys used to display the short date by adding a value called "ALL YOUR FILES HAS BEEN ENCRYPTED".

• HKCU\Control Panel\International\sShortDate
• HKLM\SYSTEM\ControlSet001\Control\CommonGlobUserSettings\Control Panel\International\sShortDate

To quietly delete all Shadow Volume Copies on the system, the malware invokes vssadmin.exe and runs the following command:

vssadmin delete shadows /All /quiet

Since ARCrypter also targets all the network drives that are connected to the system, the following command is used to ensure that they stay connected during downtime:

C:\Windows\system32\cmd.exe /c net config server /autodisconnect:-1

The ARCrypter ransomware will encrypt all the affected files, while skipping some predefined file extensions:

The malware will also avoid encrypting files in certain critical locations of the target system:

• All Users
• Boot
• Microsoft
• Local Settings
• $Win Reagent
• Application Data
• Windows
• Boot
• All Users

All of the files that are not on the whitelisted file extension list or whitelisted location get encrypted, and the “.crypt” file extension is appended.

Figure 7 – Files encrypted by ARCrypter

Targets

• Chilean government organizations
• The Colombian National Food and Drug Surveillance Institute

Attribution

We have not identified any evidence to attribute this malware to any previously known threat actor or threat group. While both campaigns targeted Latin American countries, currently there is no evidence in the code suggesting that a local threat actor could be the culprit.

Conclusions

In early August 2022, an unknown ransomware variant which BlackBerry named ARCrypter was found targeting Latin American institutions. At this point, we have witnessed that the threat actor behind it has expanded its operations to other countries like China and Canada. The research conducted for this report has helped uncover two sets of files – a malware dropper and file encryptor. A comprehensive list of IoCs as well as MITRE TTPs have been listed below, to help organizations and defenders detect this threat.

APPENDIX 1 – Indicators of Compromise (IoCs)

APPENDIX 2 – Applied Countermeasures

Sigma Rules

Payload

APPENDIX 3 – Detailed MITRE ATT&CK® Mapping