11/10/2021 | News release | Distributed by Public on 11/10/2021 10:33
U.S. and European law enforcement agencies revealed new seizures and arrests this week involving the REvil ransomware group, underscoring the intense interest and outrage following in the wake of these malicious campaigns. These developments also highlight the immediate need for security professionals to fully understand the inner workings of this group and the associated malware family, to better protect their organizations and stakeholders. This blog dives deep into REvil's latest tactics, techniques, and procedures (TTPs), drawing insights from a recent incident handled by the BlackBerry Incident Response Team.
Following execution of the Gootkit loader, the final deobfuscated code snippet performs the following actions:
A full analysis of the Gootkit loader and additional actions taken following its execution are included below.
Notice the setting of electric  = third, which sets the next function to run after returning to the calling while loop. In this fashion, we find multiple assignments of functions to various array positions within the electric [ ] array. After another attempted sleep (commented out above), the indicate function sets the knew variable, which is later decoded.
To avoid the consistent iterations of the while loop and speed up analysis, the round ( ) function can be modified to make its calls directly, rather than waiting for the loop to continue incrementing the race value as such:
Above we can see an important host-based Indicator of Compromise (IoC) for the loader, a registry key:
If this key does not exist, it is created. Next, we see another set of obfuscated code that is decoded via a call to multiply ( ). By setting a breakpoint on the return for the multiply ( ) function, we see the final bit of deobfuscated code:
BlackBerry researchers retrieved a copy of the BloodHound output file and began enumerating attack paths that the threat actor may have abused. A path to Domain Admin was found via three "Kerberoastable" accounts. The attack paths looked similar to the following:
After gaining access to multiple highly privileged accounts, the threat actor began pivoting to other hosts on the network via Remote Desktop Protocol (RDP). From there, the PsExec tool was also used to pivot to other hosts to shut down Windows Defender services.
The threat actor utilized two methods of installing Cobalt Strike command-and-control (C2) within memory. The first and simplest method was utilizing a simple encoded PowerShell command to execute a Cobalt Strike stager. Below is a snippet from the discovered Cobalt Strike stager:
BlackBerry extracted the two executable files and determined that they were Cobalt Strike Beacons, configured to reach out to the following two IPs:
The BlackBerry Incident Response Team also discovered a PowerShell command used to stage Cobalt Strike within the registry key: HKLM:\SOFTWARE\Microsoft\PowerShell\info. The threat actor ran the PowerShell via a remote service which executed the following:
The contents of the registry key were extracted for further analysis. Unfortunately, the PowerShell code executed on the system contained undefined variables, such as pdqnas. As such, it did not appear to be decodable as-is.
The REvil group is known to exfiltrate data prior to deploying ransomware. In identifying any potential exfiltration activity, the BlackBerry Incident Response Team searched across a number of forensic artifacts to identify common exfiltration tools, or enumeration of sensitive folders or file shares. The threat actor used the PowerSploit PowerShell module to discover file servers on the network that may contain sensitive data for exfiltration:
After discovering potential file servers, typically a threat actor will begin enumerating available shares. One of the most helpful artifacts in identifying enumeration of sensitive file shares or folders is the Windows Shellbag artifact, located within the USRCLASS.dat registry hive. In this case, the actor navigated through many folders on the primary file server, likely in attempts to identify the "crown jewels."
In addition to this enumeration activity, the FreeFileSync utility was executed from the same system immediately following the Shellbag enumeration event. Unfortunately, the threat actor had deleted the folder containing any logs related to FreeFileSync, and Windows Event logs on the system were unavailable from the timeframe of the file's initial execution. However, through file carving and memory analysis, BlackBerry was able to extract many (but not all) Windows Event logs from the incident timeframe. Windows Event ID 5156, used to track connections allowed by the Windows Filtering Platform, showed connections from FreeFileSync to Google-owned IP addresses, such as that shown in the image below.
Analysis of firewall activity from the system to the Google-owned IPs also revealed several gigabytes worth of network traffic sent from FreeFileSync. BlackBerry determined that the REvil group likely used FreeFileSync to exfiltrate data to Google Drive™.
Prior to deploying ransomware across the environment, the threat actor once again attempted to disable the Windows Defender feature via Powershell and Scheduled Tasks. Rather than deploying ransomware across the entire environment, the group was more selective, instead targeting the Hyper-V hosts, and more specifically the Cluster Shared Volumes (CSVs) containing the virtual machines (VMs). VMs and Hyper-V services were stopped via PowerShell just prior to deploying ransomware to the CSVs.
REvil/Sodinokibi ransomware includes a configuration that is used to determine parameters, such as which file extensions to target for encryption, which processes to kill prior to beginning the encryption routine, and which directories or extensions to exclude to avoid causing damage to the target operating system. The Sodinokibi ransomware is well-documented in the following article by the BlackBerry Threat Research team:
Applying defense in depth is important to both detecting and preventing this sort of intrusion. BlackBerry identified several opportunities at which the threat group may have been detected and eradicated early on in the attack chain. While intrusion prevention is critical, a robust intrusion detection posture is as - if not more - vital to preventing this sort of widespread ransomware event. With 24x7x365 monitoring of antivirus/EDR tools, as well as endpoint event logs and network appliances, this event very likely could have been detected and prevented before becoming a large-scale compromise.
In this incident, BlackBerry identified multiple TTPs that can be monitored for abuse detection, including:
Codi Starksis Senior Professional Services Incident Response Consultant, BlackBerry.
Codi Starks has more than twelve years of IT, cyber security, and incident response experience. During his time in the field he has supported and led difficult incident response engagements for Fortune 500 companies spanning multiple continents. He currently holds several certifications and achievements, including an M.S. in Information Security and Assurance, as well as the OSCP and SANS GCFE certifications. He has won multiple cyber security competitions, including OpenSOC, SANS DFIR Netwars, and SOCX.
Ryan Chapman is Principal Incident Response & Forensics Consultant, BlackBerry.
As an author, instructor, and information security professional with over 18 years' experience, Ryan runs and works incidents for clients to provide response, assessment, and training in the digital forensics and incident response (DFIR) realm at BlackBerry. His primary case types involve digital forensics investigations (e.g. ransomware cases), compromise assessments, business email compromises, tabletop exercises, and more. Ryan loves the fact that the security industry is an ever-evolving creature.