Pierluigi Paganini
February 26, 2021
Experts from French national cyber-security agency ANSSI have spotted a new Ryuk ransomware variant that implements worm-like capabilities that allow within local networks.
“On top of its usual functions, this version holds a new attribute allowing it to self replicate over the local network.” reads the report published by the ANSSI. “Through the use of scheduled tasks, the malware propagates itself – machine to machine – within the Windows domain. Once launched, it will thus spread itself on every reachable machine on which Windows RPC accesses are possible.”
This Ryuk ransomware variant doesn’t include any mechanism for blocking the execution of the ransomware (MUTEX like or else), it copies itself with a rep.exe or lan.exe suffix.
The ransomware generates every possible IP address on local networks and sends them an ICMP ping. It lists the IP addresses of the local ARP cache and sends them a packet, then it lists all the sharing resources opened on the found IPs, mounts each of them, and attempts to encrypt their content. This variant is also able to remotely create a scheduled task to execute itself on this host.
Scheduled tasks are created using the Windows native tool schtasks.exe.
“The Ryuk variant analyzed in this document does have self-replication capabilities. The propagation is achieved by copying the executable on identified network shares. This step is followed by the creation of a scheduled task on the remote machine.” continues the report. “The content of this scheduled task is described in the analysis present in this document. Some filenames were identified for this copy: rep.exe and lan.exe. Finally, Ryuk deletes the Volume Shadow Copies to prevent file recovery.”
The ransomware achieves persistence by setting the registry key HKEY_CURRENT_USER\SOFTWARE\Microsoft
\Windows\CurrentVersion\Run\svchost with its filepath.
ANSSI report revealed that the ransomware does not check if a machine has already been infected, the malicious code uses a privileged account of the domain for its propagation. French experts pointed out that even if the user’s password is changed, the replication will continue as long as the Kerberos tickets are not expired.
“One way to tackle the problem could be to change the password or disable the user account (according to the used account) and then proceed to a double KRBTGT domain password change. This would induce many disturbances on the domain – and most likely require many reboots but would also immediately contain the propagation.” continues the report.
ANSSI also provided Indicators of compromise (IOCs) associated with this new Ryuk ransomware variant.
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(SecurityAffairs – hacking, Ryuk)
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