#1243087: Big Game Hunting with Ryuk: Another Lucrative Targeted Ransomware

Description: GRIM SPIDER is a sophisticated eCrime group that has been operating the Ryuk ransomware since August 2018, targeting large organizations for a high-ransom return. This methodology, known as “big game hunting,” signals a shift in operations for WIZARD SPIDER, a criminal enterprise of which GRIM SPIDER appears to be a cell. The WIZARD SPIDER threat group, known as the Russia-based operator of the TrickBot banking malware, had focused primarily on wire fraud in the past.

Similar to Samas and BitPaymer, Ryuk is specifically used to target enterprise environments. Code comparison between versions of Ryuk and Hermes ransomware indicates that Ryuk was derived from the Hermes source code and has been under steady development since its release. Hermes is commodity ransomware that has been observed for sale on forums and used by multiple threat actors. However, Ryuk is only used by GRIM SPIDER and, unlike Hermes, Ryuk has only been used to target enterprise environments. Since Ryuk’s appearance in August, the threat actors operating it have netted over 705.80 BTC across 52 transactions for a total current value of $3,701,893.98 USD.

CrowdStrike® has conducted multiple incident response (IR) engagements responding to Ryuk infections in which TrickBot was also identified on hosts in the victim environment. CrowdStrike Falcon® Intelligence™ believes that the initial compromise is performed through TrickBot, which is typically distributed either via spam email or, through the use of the Emotet (developed and operated by MUMMY SPIDER) geo-based download function. Falcon Intelligence has been monitoring the geo-based download activity from Emotet and, during 2018, MUMMY SPIDER has been an avid supporter of WIZARD SPIDER, predominantly distributing TrickBot to Emotet victims in the U.K., the U.S., and Canada.

Some of TrickBot’s modules (such as pwgrab) could aid in recovering the credentials needed to compromise environments — the SOCKS module in particular has been observed tunneling PowerShell Empire traffic to perform reconnaissance and lateral movement. Through CrowdStrike IR engagements, GRIM SPIDER has been observed performing the following events on the victim’s network, with the end goal of pushing out the Ryuk binary:

An obfuscated PowerShell script is executed and connects to a remote IP address.
A reverse shell is downloaded and executed on the compromised host.
PowerShell anti-logging scripts are executed on the host.
Reconnaissance of the network is conducted using standard Windows command line tools along with external uploaded tools.
Lateral movement throughout the network is enabled using Remote Desktop Protocol (RDP).
Service User Accounts are created.
PowerShell Empire is downloaded and installed as a service.
Lateral movement is continued until privileges are recovered to obtain access to a domain controller.
PSEXEC is used to push out the Ryuk binary to individual hosts.
Batch scripts are executed to terminate processes/services and remove backups, followed by the Ryuk binary.
Hermes to Ryuk
Hermes ransomware, the predecessor to Ryuk, was first distributed in February 2017. Only one month after its release, a decryptor was written for Hermes, followed by the release of version 2.0 in April 2017, which fixed vulnerabilities in its cryptographic implementation. Since this release, the only way for a victim to recover files is with the private encryption key, which is obtained by paying the ransom. In late August 2017, Hermes version 2.1 was released.

Hermes was originally sold on forums for $300 USD. When purchased, the buyer received a build that supported two email addresses, a decryptor and a unique RSA key pair. If the purchaser desired more email addresses, they were required to purchase another build for an additional $50. The seller of Hermes ransomware appears to have stopped or limited advertising on forums in 2017.

Early versions of Hermes were reportedly installed via internet-accessible RDP servers protected by weak credentials. In October 2017, Hermes was deployed as a destructive distraction for a Society for Worldwide Interbank Financial Telecommunication (SWIFT) compromise at the Far Eastern International Bank (FEIB) in Taiwan. Hermes’ role in the SWIFT attack is described in more detail in the Attribution section at the end of this blog. In March 2018, Hermes was observed targeting users in South Korea via the GreenFlash Sundown exploit kit.

In mid-August 2018, a modified version of Hermes, dubbed Ryuk, started appearing in a public malware repository. Ryuk was tailored to target enterprise environments and some of the modifications include removing anti-analysis checks. These checks include querying the Process Environment Block (PEB) to see if the field is BeingDebugged, or querying the PEB to see if the field NtGlobalFlag has been set; checking to see if the host is running VirtualBox by calling the instruction CPUID; and ensuring that the host language is not Russian, Ukrainian, or Belarusian. From a process and file perspective, Hermes and Ryuk target files in a similar fashion. The core differences are Ryuk’s logic that handles file access, and the use of a second, embedded public RSA key.

The following are characteristics that have not changed:

Encrypts files using RSA-2048 and AES-256
Stores keys in the executable using the proprietary Microsoft SIMPLEBLOB format
Encrypts mounted devices and remote hosts
Uses a file marker of HERMES to mark or check if a file has been encrypted
Another notable difference between Hermes and Ryuk is how the encryption keys are created. Hermes starts the encryption initialization by first generating an RSA public and private key pair — referred to as a “victim key.” An AES-256 key is generated and the victim’s RSA private key is encrypted in AES-CBC mode. The attacker-controlled public RSA key is used to encrypt the AES key (previously used to encrypt the victim’s RSA private key). Then, for each file encrypted, an AES key is generated, which is used to encrypt the file. Finally, the AES key for each file is encrypted with the victim’s RSA public key, then stored at the end of the file.

Ryuk contains the same logic, but no longer generates the victim-specific RSA key pair. Instead, Ryuk has two public RSA keys embedded in the executable, and what was previously the victim’s RSA private key is encrypted and embedded in the executable. Because Ryuk does not generate a victim-specific RSA key pair, all hosts can be decrypted with the same decryption key. This might appear to be a design flaw but is not, since Ryuk has a unique key for each executable. If a single executable is used for a single victim environment, then there are no repercussions if the private keys are leaked because it will only decrypt the damage from a single Ryuk executable. Thus, it is highly likely that Ryuk pre-generates the RSA key pairs for each victim. This is arguably more secure, since the victim’s system will never have access to the unencrypted RSA key pair parameters without paying the ransom. This approach is similar to INDRIK SPIDER’s BitPaymer ransomware, which generates a victim-specific sample with a hard-coded public key.
More info: https://www.crowdstrike.com/blog/big-game-hunting-with-ryuk-another-lucrative-targeted-ransomware/

Date added Jan. 11, 2019, 6:09 a.m.
Source Crowdstrike
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