Two hacker gangs, Shatak (TA551) have partnered with the ITG23 group (AKA TrickBot) to deploy Conti Ransomware on targeted systems
Shatak frequently partners with malware developers to develop phishing campaigns that infect victims with malware.
Researchers from IBM X-Force discovered the alliance, which began in July 2021. The two apparently have had a good working relationship as the campaigns are still active.
A technical report from researchers at Cybereason provided more detail on how the two distinct threat actors partnered to deliver malware, especially Conti Ransomware.
A typical attack chain starts with Shatak delivering phishing emails, which contain password-protected archives holding a malicious document.
According to X-Force, Shatak uses reply-chain emails stolen from previous victims to seem authentic.
The attachments contain scripts that excute encoded code to download and install TrickBot from a remote site. The majority of these are located in Eastern Europe.
After TrickBot has been deployed, ITG23 takes over the operation and deploys a Cobalt Strike beacon on the compromised system.
With the ultimate goal being a deployment of Conti Ransomware, the hackers conduct network reconnaissance, spreading throughout an entire network as they do so.
Then they steal user credentials, password hashes, and Active Directory data, and abuse what they can to spread laterally through the network.
Some signs of this activity include fiddling with registry values that enable the RDP connectivity and modifying Windows Firewall rules with the ‘netsh’ command.
Windows Defender’s real-time monitoring feature is also disabled to prevent alerts or interventions during the encryption process.
The next step is data exfiltration, which is the final stage before the file encryption, with Conti using the ‘Rclone’ tool to send everything to a remote endpoint under their control.
After harvesting all valuable data from the network, the threat actors deploy Conti Ransomware to encrypt devices.
Conti Ransomware Analysis
Note: The Analysis of Conti Ransomware was carried out by researchers at Vipre Labs.
Conti ransomware encrypts the files of their victims and publishes the data on their website similar to what other strains do. This extortion behavior is visible on their ransom note saying “We’ve downloaded your data and are ready to publish it on our news website”.
When executed, it will start to encrypt files and change the file extension of the encrypted files to .ODMUA. Like other ransomware, it will leave a ransom note that has a filename “readme.txt”.
The Conti ransomware website has an instruction on how to upload the README.txt for the decryption and a contact button at the bottom left of the page. Once you click the contact button, a form will appear where you will provide your contact information and question as shown below.
Conti ransomware will perform a known malware technique called process hollowing. It is where the malware will create a process in a suspended state, unmaps or removes the PE image layout from a given process space using ZwUnmapViewofSection function, write it’s malicious code using WriteProcessMemory, set a new entry point using SetThreadContext, and resume the execution of the suspended process using the ResumeThread function.
Upon research, we found out that the use of -p argument is to encrypt a specific directory with a single thread and the -m argument is to encrypt the files with multiple threads. It means that Conti ransomware has a multi-threading capability. Multi-threading is where main ransomware creates child threads to speed up the encryption.
It will use a string “hsfjuukjzloqu28oajh727190” that was decrypted using the decryption of string routine mentioned above for creating a mutex using CreateMutexA function. Then check if there’s an already running mutex. This was commonly used by ransomware to avoid infecting the system more than once.
It will also delete all the shadow volume copies on the infected system to ensure that the victims won’t be able to recover their encrypted files.
After deleting the shadow copies, Conti ransomware will now start its file encryption by first creating the ransom note which will be first drop in C drive using “CreateFileW” and write the content of its ransom note using “WriteFile”.
As with other ransomware, it will utilize the functions “FindFirstFileW” and “FindNextFileW” to find the files they will encrypt. Conti ransomware has a list of files/file extension and directories which will be excluded for the infection.
When Conti finds the file to be encrypted, it will now generate keys that will be used to encrypt the files. It will used the handle returned by calling the function “CryptAcquireContext” that request a cryptographic context from the Microsoft Enhanced Cryptographic Provider, then the “CryptGenRandom” function to generate cryptographically random bytes, and “CryptEncrypt” function. It leverages AES 256 encryption for their infection.
Then it will open the target file using the “CreateFile” function and retrieve the size of the target file using “GetFileSize”. After this the malware will decrypt different file extensions and check if the file extension of the targeted file is in the list.
Conti ransomware will not just encrypt the files of the infected machine but also spreads and infects the other machine on the same network using SMB protocol.
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