The Conti Ransomware gang is using the Log4Shell exploit to leverage rapid access to VMware vCenter Server instances, where they aim to encrypt virtual machines.
The Conti Ransomware gang quickly adopted the new attack vector into its arsenal, and is the first major operation to make use of the Log4j vulnerability.
A proof-of-concept (PoC) exploit for CVE-2021-44228 – otherwise known as Log4Shell and LogJam – emerged in the public space on December 9.
A day later, mass scanning of the internet started, with multiple actors looking for vulnerable systems. Among the first to leverage the bug were cryptocurrency miners, botnets, and a new ransomware strain called Khonsari.
By the 15th of December, a large gallery of threat actors were known to be using Log4Shell for a range of attacks.
The Conti Ransomware gang, perhaps the most prolific ransomware gang today, took interest in Log4Shell early on, starting on December 12th.
According to information shared between researchers at Advanced Intelligence (AdvIntel) and BleepingComputer, the Conti Ransomware gang started looking for victims with their goal being lateral movement to VMware vCenter networks.
Dozens of vendors have been affected by Log4Shell and rushed to patch their products or provide workarounds and mitigations for customers. VMware is one of them, listing 40 vulnerable products.
While the company provided mitigations or fixes, a patch for vCenter versions impacted has yet to become available.
vCenter servers are not normally exposed to the public internet, there are scenarios where an attacker could exploit the issue:
“A malicious actor with network access to an impacted VMware product may exploit this issue to gain full control of the target system and/or perform a denial of service attack” – VMware
AdvIntel says that Conti ransomware gang members showed interest in leveraging Log4Shell for their operations using the public exploit.
The AdvIntel report says, “this is the first time this vulnerability entered the radar of a major ransomware group.”
“The current exploitation led to multiple use cases through which the Conti group tested the possibilities of utilizing the Log4J exploit” – AdvIntel
While most defenders are focused on blocking Log4Shell attacks on Internet-exposed devices, the Conti ransomware operation shows how the vulnerability can be used to target internal devices that may not receive as much attention.
The researchers confirmed that Conti ransomware affiliates had already compromised the target networks and exploited vulnerable Log4j machines to gain access to vCenter servers.
This means that Conti ransomware members relied on a different initial access vector (RDP, VPN, email phishing) to compromise a network and are currently using Log4Shell to move laterally on the network.
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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