The SunWalker Incident: Netwalker and SunCrypt Ransomware Double-Double Exploitation

Summary:

After eight long hours attempting to siege an online education institution – including initial access, lateral movement, and ransomware deployment - an unidentified threat actor withdraws. Through the combined effort of proactive and reactive security measures, the learning institution and eSentire’s defense teams worked together to deflect hands-on attack paths through a synthesis of automatic and manual defense actions. Eight hours of siege telemetry gives interesting insights into threat actor choices and behaviors, key among them: what brings both the SunCrypt and Netwalker ransomware families together in one attack? After a look at the timeline of interesting behaviors in this eight-hour attempt at Double Extortion, we briefly cover candidate hypotheses.

Introduction:

Public events through August report observations of the SunCrypt ransomware group infiltrating educational institutions [1], while the FBI reported similar targeting of educational institution for Netwalker in July [2]. Like the Maze ransomware group, both groups follow the recent underground market business model in ransomware tactics: Double Extortion. Double Extortion occurs when threat actors successfully a) exfiltrate sensitive data from the targeted organization and b) encrypt critical files within the organization. Both actions can give threat actors considerable leverage for extorting ransom funds from their victims. SunCrypt recently claimed they were part of the Maze Cartel – a social contract between various ransomware groups working together to optimize the many phases of a ransomware deployment attack – but the Maze ransomware group has denied their claim [1]. Observations of SunCrypt and Netwalker ransomwares in the same session does raise the question of whether the Maze Cartel, another ransomware group, or an independent threat actor are behind the attack. The threat actor’s tactics – from initial access to deployment – appear to fall in line with NetWalker’s observed playbook [3]. However, given the public knowledge of such tactics and availability of tools involved, we cannot completely rule out an independent actor. We will briefly explore each hypothesis after disclosing the timeline of the attack, reflecting on winning factors in the resulting battle of attrition.

Key Events in Timeline:

Minutes past the zeroth hour (labeled 12PM), the VPN pool of the organization and one of their workstation endpoints (labeled system zero) received successful connections via an unknown IP address. An hour later (1 PM), a member of the VPN pool successfully logged in to workstation zero via RDP using organization account A (initial access: valid account [4]). Using a service account (B) that had recently been created (+), they re-established RDP with the new account on machine zero (privilege escalation: valid account [4]) and executed mimikatz [5] via PowerShell (credential theft: credentials from password stores [6]). This activity was detected at eSentire’s Security Operations Center and the host was put into isolation. A broader investigation was opened to assess initial access and active response actions. Once the VPN was identified as a vector, the organization shut down the compromised VPN tunnel. Minutes later, the threat actor switched to another tunnel and, once again, the second tunnel was shut down. At this point, no active traces of intrusion were present. Four hours later, (approx. 7:30 PM) likely using the valid credentials they had retrieved through credential theft, the intruder quietly logged into a second machine (two) with account B and dropped SunCrypt ransomware to disk, followed by an attempt to deploy it to other hosts (two->many) on the network (deployment: lateral tool transfer [7]) via psexec [8]. This raised an alert and activated standby defense teams. In the moments before host two was isolated, the intruder attempted to enable psexec via services. Experiencing interference from endpoint defense, they downloaded and used an AV uninstaller program – often used to resolve conflicts between AV software and attempted to execute it. The also used account D to get on the domain controllers (DC) and perform more domain recon before, finally, attempting to execute (execution [9]) an instance of Netwalker Ransomware on the lone endpoint – which also failed. The host was isolated, and the defense teams remained on alert, actively investigating for artifacts and return attempts. After execution, the intruder attempted to delete ransomware samples but wasn‘t able to empty the recycle bin before host isolation, leaving samples for analysis.

Fig 1. Timeline of events. Timeline has been shifted to obscure details. Each system has its own row in the graph, while each account is represented by a distinct marker type. Action performed by adversaries is colored red, Defensive actions are colored blue, and actions that occurred independent of the incident are colored green.

Attribution Hypotheses

Hypotheses are presented in order of apparent evidence, but keep in mind that strong circumstantial evidence can sometimes be a result of deception. Social engineering and reputation attacks are, after all, a common tactic in organized crime and conflict. eSentire Threat Intelligence draws no final conclusions about attribution given the limited ground truth available about adversaries involved.

• Netwalker entrance exam for SunCrypt ransomware developers
  1. Netwalker delivery path
     1. SunCrypt claim to just be developers
  2. Suncrypt delivered first
     1. SunCrypt actors might try to use their own ransomware first to prove themselves, then fallback to NetWalker, who have a more proven method.
     2. Kept desperately trying to get in despite a lot of defense resistance.
• An Independent actor or ransomware group
  1. SunCrypt group could simply be adopting NetWalker and Maze techniques [10]
  2. No evidence of Netwalker and SunCrypt relationship
  3. Threat Actor continued to disclose their own operational security the more they attempted to bypass security controls
     1. Cartel might be more careful and recognize when they’re in a monitored situation
• Maze or another cartel
  1. Communication from ransomware groups can just as well be taken with a grain of salt. Deception is a common practice in cybercrime and there’s nothing stopping cartels from mimicking each other and pointing fingers.

Looking at Microsoft’s Q2 Ransomware report [3], the attack path is reminiscent of NetWalker Ransomware group:

Battle of Attrition

A multi-layered defense went a long way to deflecting the intruder’s numerous access, lateral movement, and execution attempts. When threat actors find organizations with poor perimeter controls or have driven motives to compromise a particular target, they can make several attempts at compromise. If they have collected credentials and performed domain reconnaissance in any of their attempts, they can return through different accounts and systems in future attempts with valid credentials which require no exploits or malspam-based execution abuses and do not, therefore, raise alerts. However, a multi-layered security posture can provide organizational resilience against the final goal of these attacks by interrupting malicious actions as they are performed by the threat actor. Such threat actors may theoretically achieve their objectives given enough time and resources. The job of the defense team is to make it more trouble than it's worth. In the meantime, eSentire advises keeping an eye on the latest remote vulnerabilities – often targeted by ransomware groups [11].

In this case, pro-defensive attrition actions include:

• Actively monitoring the situation – deeper investigation into threat actor actions allows more context to be gained and leads to speedier decision making and pro-active defense
• Host Isolation – isolating an endpoint from internet communications prevents it from being further leveraged as a foothold in the organization.
• Shutting down abused VPN tunnels – when active investigations reveals access nodes, the access nodes can be disabled
• Next Generation Anti-Virus – NGAV prevented harmful ransomware files from being executed by the hands-on ransomware operator

Detection, investigation, and response tools used:

• Managed investigation – Multi-disciplinary team of security investigators, researchers, and engineers bring together tools, skillsets, and experience in one place.
• Endpoint monitoring and investigation – CarbonBlack Response
• Infrastructure and indicators research – Open Source Intelligence (OSINT)
• Next-Generation Ant-Virus Response – CarbonBlack Defense
• Network Monitoring and Investigations – pcap
• Account monitoring and investigation – Windows and VPN logs
• Exfiltration investigation – Argus flow data

References

[1] https://www.bleepingcomputer.com/news/security/suncrypt-ransomware-sheds-light-on-the-maze-ransomware-cartel/

[2] https://healthitsecurity.com/news/fbi-alerts-to-rise-in-targeted-netwalker-ransomware-attacks

[3] https://www.microsoft.com/security/blog/2020/04/28/ransomware-groups-continue-to-target-healthcare-critical-services-heres-how-to-reduce-risk/

[4] https://attack.mitre.org/techniques/T1078/003/

[5] https://attack.mitre.org/software/S0002/

[6] https://attack.mitre.org/techniques/T1555/

[7] https://attack.mitre.org/techniques/T1570/

[8] https://attack.mitre.org/software/S0029/

[9] https://attack.mitre.org/tactics/TA0002/

[10] https://www.acronis.com/en-us/blog/posts/suncrypt-adopts-attacking-techniques-netwalker-and-maze-ransomware

[11] https://www.esentire.com/security-advisories/ransomware-groups-exploit-remote-access-services