Executive Summary

Threat

• Threat actors attack VMware ESXi servers using a custom  Ransomware script.
• The ransomware exploits an old RCE vulnerability (CVE-2021-21974) in ESXi versions:7.0 before ESXi70U1c-173255516.7 before ESXi670-202102401-SG6.5 before ESXi650-202102101-SG

Impact

• The ransomware encrypts files stored on the servers using RSA and Sosemanuk stream cipher.
• This can lead to loss of data, business continuity, and revenue.

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Mitigation

• Update servers to the latest versions and install the necessary patches.
• Regularly back-up all data stored on the servers.

Analysis and Attribution

On 03 February 2023, there was a surge in attacks targeting VMware ESXi servers. Threat actors were leveraging an old vulnerability (CVE-2021-21974) in order to gain RCE (Remote Code Execution) and launch ransomware attacks. 

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The OpenSLP version used in ESXi versions 7.0 before ESXi70U1c-17325551, 6.7 before ESXi670-202102401-SG, and 6.5 before ESXi650-202102101-SG are vulnerable to a heap-overflow attack, which allowed attackers to perform Remote Code Execution.

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On 03 February 2023, a member on the Bleeping Computer forum thread, with access to a targeted server, shared a script and an executable, found on their ESXi servers. These files were reportedly used to encrypt the contents of their ESXi servers.

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Overview of the files

encrypt.sh (The script)

• The script is used to prepare the target for encryption, and load the ransomware payload that is used to encrypt the server.
• The script looks for specific file extensions (.vmdk, .vmx, .vmxf, .vmsd, .vmsn, .vswp, .vmss, .nvram, and .vmem) on the server, calculates their sizes, and starts encryption.
• It is also responsible for editing the configuration of the server, and renaming important files. This is done in order to make it difficult to restore the system.
• Once all operations are done, the script performs a cleanup and leaves a ransom note on the victim server.

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encrypt binary (Ransomware Payload)

• Binary used by the script to encrypt the target files on the server.
• Uses an RSA public key and the SOSEMANUK stream cipher to encrypt files.

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Technical Analysis (encrypt.sh)

The script is written in Bash. It is responsible for preparing the system, and calling the Ransomware payload in order to encrypt files. Once done, it performs a cleanup on the system.

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Pre-Encryption Operations

The script starts by setting its working directory as /tmp/. Files that it uses for the attack are stored here. After this, it uses the command esxcli vm process list | grep "Config File" | awk '{print $3}' to find the configuration files associated with virtual machines on the server. 

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It then replaces the filenames of the disk image and swap file in all config files with 1.vmdk and 1.vswp. This is done to make it difficult for victims to recover and reconfigure the virtual machines, since identifying the disk images and swap files will be nearly impossible.

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After this, the script kills the running VM processes using the kill command. The script is now ready to start encryption of all files on the server.

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Encryption

The script starts its encryption operations by making the encrypt binary executable for all users and groups, using the command chmod +x. Then, it iterates over the filesystem contents, looking for all files with the extensions .vmdk, .vmx, .vmxf, .vmsd, .vmsn, .vswp, .vmss, .nvram, and .vmem. All these file extensions are used by virtual machines, for example, disk images, configuration files, swap files, etc.

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The script then calculates the sizes of the files that have the target extensions, and gives the target filename extensions and sizes as arguments to the encrypt binary, along with a .pem key file. 

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Upon investigating the usage of the binary, it is found that the .pem file is an RSA public key which is used in the encryption process.

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This commences the encryption process. Note that there is a .args file created for every file that is encrypted, containing metadata about the file. This could be needed for the decryption process.

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Post-Encryption Operations and Clean-Up

Once the files are encrypted, the script first replaces the Message of The Day (.motd) file with the ransom note. It then deletes all log files in order to cover its tracks. It makes sure that the encryption process is over before it proceeds to do so.

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It then deletes the file /sbin/hostd-probe, which is a file used by the hostd daemon to manage virtual machines. In the event that a backup of this file exists, it modifies the timestamp of the file, to hide any changes made to the file.

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Depending on the VMware build, the script executes different operations. If the VMware build version is not 7.0 it executes the following steps to hide any cron jobs run by the root user:

• The script modifies the contents of the /var/spool/cron/crontabs/root file. It deletes the first 8 lines of the file.
• Renames the new file to the name of the original file.
• Deletes the original file.
• Changes the timestamp of the new file.

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For VMware build version 7.0:

•  It removes the first line of the /sbin/hostd-probe file and overwrites the original file.

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Lastly, the script conducts its clean-up operations. The clean-up operations are as follows:

• Deleting the file /store/packages/vmtools.py. Note that this filename is associated with a backdoor created for ESXi servers in the past.
• Deleting the last line of the file /etc/vmware/rhttpproxy/endpoints.conf, and modifying the timestamp.
• Clear the contents of the /etc/rc.local.d/local.sh file and modify the timestamp.
• Modify the timestamp of the /bin/hostd-probe.sh file.
• Delete all the files used in the attack, which are stored in the working directory.

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After the clean-up operations, the script starts the SSH service, possibly in the event that the threat actor wants to return to the server.

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Technical Analysis (encrypt binary)

The ransomware payload used by the script is a GCC compiled ELF 64-bit binary, which goes by the name encrypt. The arguments that the binary takes are shown below:

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Pre-Encryption Operations

• It first initializes the libssl library using the init_libssl function. This library contains functions that are used in encryption. 
• It then proceeds to get information about the previously supplied RSA public key, and creates an RSA object, in order to RSA encrypt the files. This is done using the get_pk_data and create_rsa_obj function.
• After this, it calls the encrypt_file function

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Encryption

There are two layers of encryption applied:

• Using the RSA public key earlier, the files are first RSA encrypted. This is done using the rsa_encrypt function.
• Then, the files are further encrypted using the Sosemanuk library. Sosemanuk is a stream cipher, which aims for great efficiency on low-cost hardware. This is done using the sosemauk_encrypt function, which is called by the encrypt_simple function.

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OSINT Analysis

OSINT analysis revealed that 3200+ organizations were infected, even though it is tough to ascertain the number of vulnerable servers. However, Shodan and Censys can be used to check for infected organizations.

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A sample query to check on shodan: 

html:"We hacked your company successfully" title:"How to Restore Your Files"

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A similar query can be run on Censys to get infected organizations as well:

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Ransom BTC Addresses

The group demands the ransom amount via the following BTC addresses:

• 1PAFdD9fwqRWG4VcCGuY27VTW8xPZmuF1D 
• 1GequkXF8tEYrfPhpY79TrV7xRTMargC92

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However, there are no transactions on the addresses yet.

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Ransom Note

The ransom note displayed to the victims contains the ransom amount in Bitcoin, and a TOX ID, which the victims are told to reach in order to decrypt their files. Reportedly, the amount of Bitcoin and the TOX ID differ from victim to victim.

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Detecting in the Wild

Researchers at CloudSEK’s Threat Intelligence team have written YARA detection rules for both the files used in this attack. Refer to Appendix.

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Indicators of Compromise (IoCs)

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Appendix

YARA Rule for detecting the script

rule esxiargs_script
{
    meta:
        Version = "1.0"
        author = "MalwareIntel_TRIAD_CloudSEK"
        Date = "07/02/23"
        Description = "YARA rule to identify script used in ESXiArgs Ransomware attack"

    strings:
        $a1 = "START ENCRYPT: $file_e SIZE: $size_kb STEP SIZE: $size_step" fullword nocase 
        $a2 = "nohup $CLEAN_DIR/encrypt $CLEAN_DIR/public.pem " fullword nocase        
        $a3 = "/bin/ps | /bin/grep encrypt | /bin/grep -v grep | /bin/wc -l" fullword nocase       
    
    condition:
        all of ($a*)

}

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YARA Rule for detecting the binary

import "pe"
rule esxiargs_binary
{
    meta:
        Version = "1.0"
        author = "MalwareIntel_TRIAD_CloudSEK"
        Date = "07/02/23"
        Description = "YARA rule to identify Ransomware used in ESXiArgs Ransomware attack"

    strings:
        $a1 = "usage: encrypt   [] [] []" fullword nocase 
        $a2 = "enc_step   -   number of MB to skip while encryption" fullword nocase        
        $a3 = "enc_size   -   number of MB in encryption block" fullword nocase       
        $a4 = "file_size  -   file size in bytes (for sparse files)" fullword nocase
    
    condition:
        uint32(0) == 0x464C457F and all of ($a*)

}

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