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Netskope Threat Coverage: CrossLock Ransomware

May 02 2023

Summary

CrossLock is a ransomware group that emerged in April 2023, targeting a large digital certifier company in Brazil. This ransomware was written in Go, which has also been adopted by other ransomware groups, including Hive, due to the cross-platform capabilities offered by the language. CrossLock operates in the double-extortion scheme, by threatening to leak stolen data on a website hosted on the deep web if the ransom isn’t paid by the victim.

Deep web site where CrossLock is posting its targets and leaking stolen data.

In this blog post, we will show how the CrossLock ransomware works.

Analysis 

The CrossLock ransomware was developed in the Go programming language, and was likely compiled on April 16, 2023, which is the same day where the attackers added the Brazilian digital certifier in their deep web site.

CrossLock binary details.

Attackers can use CrossLock to infect not only a local system, but also a remote system by using their custom parameters, which are described below:

ParameterDescription
--path, -PEncrypt the specified path
--host, -HRemote system to run CrossLock (which can be a DNS or an IP address)
--domain, -dThe domain name (default is “.”)
--user, -uUsername used to authenticate in the remote system
--pwd, -pPassword of the user specified in “-u” or “--user”
--uac, -ubIf specified, tries to bypass Windows UAC to run with elevated privileges

Like other ransomware families, such as BlackCat, CrossLock offers a “help” menu to provide instructions on how to use these parameters. They even included an example on how to execute CrossLock on a remote system and encrypt the entire file directory.

CrossLock help menu.

When executed, CrossLock outputs what it’s doing to a console. Also, the attacker left a few debug logs enabled, so it’s possible to get more insights of what it’s doing by just looking at the output.

Debug logs outputted by CrossLock ransomware.

To avoid detection, CrossLock bypasses the Event Tracing for Windows (ETW), which is a mechanism that provides system and application logs that can be used by developers for debugging. This is done through a common API hooking technique, by patching functions EtwEventRegister, EtwNotificationRegister, EtwEventWriteFull, and EtwEventWrite on “ntdll.dll”.

CrossLock ransomware patching ETW functions.

CrossLock has a hardcoded list of services and processes that it tries to stop before encrypting files, which is a common practice among ransomware families. First, it tries to stop services related to backup services, databases, and security software. Then, it tries to stop a series of processes. The complete list of services and processes it tries to stop can be found in our GitHub repository.

CrossLock loading the name of processes to stop.

CrossLock also runs additional commands to delete Windows Shadow Copies using vssadmin, and to disable boot recovery using bcdedit.

Additional commands CrossLock ransomware runs.

If no path was specified via “–path” or “-P” parameters, CrossLock starts crawling and encrypting files.

CrossLock ransomware output logs.

Like other ransomware families, CrossLock does not encrypt files that contain specific extensions or that are under specific folders, such as Windows, Program Files, and ProgramData. This is a common practice that avoids encrypting files that can corrupt the system. The list of extensions CrossLock ignores can be found in our GitHub repository.

Some of the extensions CrossLock will ignore.

To encrypt files, CrossLock combines Curve25519 and ChaCha20 algorithms, which are implemented through Go packages.

Go packages related to encryption within CrossLocker.

CrossLock adds the extension “.crlk” to encrypted files.

Files encrypted by CrossLock ransomware.

The ransom note contains the name of the company that was targeted and instructions to contact the attackers via tox chat to negotiate the ransom value.

CrossLocker ransom note.

Conclusion

CrossLock is a ransomware group that emerged in April 2023, having its encryptor developed in the cross-platform language Go. So far CrossLock ransomware has added only one target in its list, and there is only a Windows variant of its encryptor. Since CrossLock was developed in Go, it is possible that samples targeting other OSes may emerge. 

Protection

Netskope Threat Labs is actively monitoring this campaign and has ensured coverage for all known threat indicators and payloads. 

  • Netskope Threat Protection
    • Win64.Ransomware.BlackLockbit
  • Netskope Advanced Threat Protection provides proactive coverage against this threat.
    • Gen.Malware.Detect.By.StHeur indicates a sample that was detected using static analysis
    • Gen.Malware.Detect.By.Sandbox indicates a sample that was detected by our cloud sandbox

IOCs

All the IOCs related to this campaign, and a Yara rule can be found in our GitHub repository.

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Gustavo Palazolo
Gustavo Palazolo is an expert in malware analysis, reverse engineering and security research, working many years in projects related to electronic fraud protection. He is currently working on the Netskope Research Team, discovering and analyzing new malware threats.