The cybercrime group FIN8 is using a revamped version of the Sardonic backdoor to deliver the BlackCat ransomware.
Sardonic is a sophisticated backdoor that supports a wide range of features that was designed to evade detection. According to the experts, Sardonic is a project still under development and includes several components, some of which were compiled just before the attack.
The researchers from Symantec’s Threat Hunter Team reported that most of the backdoor’s features have been modified by the group.
On December 2022, Symantec observed the group attempting to deploy the ALPHV/BlackCat ransomware.
“The Syssphinx attack observed by Symantec in December 2022, in which the attackers attempted to deploy the Noberus ransomware, involved similar techniques as a Syssphinx attack described by Bitdefender researchers in 2021.” reads the post published by Symantec. “However, the most recent attack had some key differences, including the final payload being the Noberus ransomware and the use of a reworked backdoor. The revamped Sardonic backdoor analyzed in this blog shares a number of features with the C++-based Sardonic backdoor analyzed by Bitdefender. However, most of the backdoor’s code has been rewritten, such that it gains a new appearance.”
The new variant of the Sardonic backdoor is written in C instead of C++ used for the previous variant.
In the attack analyzed by Symantec, the backdoor was embedded (indirectly) into a PowerShell script used to infect target machines. Unlike the previous variant, it doesn’t use an intermediate downloader shellcode to download and execute the backdoor.
The PowerShell script decodes a .NET Loader binary and loads it into the current process. The,loader decrypts and executes the injector and the backdoor.
“The purpose of the injector is to start the backdoor in a newly created WmiPrvSE.exe process. When creating the WmiPrvSE.exe process, the injector attempts to start it in session-0 (best effort) using a token stolen from the lsass.exe process.” continues Symantec.
The researchers reported that the backdoor supports interactive sessions, where the attacker runs cmd.exe or other interactive processes on the infected system. The sample analyzed by Symantec can run up to 10 such sessions at the same time.
The backdoor supports three different formats to extend its functionality, the PE DLL plugins, shellcode, and shellcode but with a different convention to pass the arguments.
The backdoor supports multiple commands, such as dropping arbitrary attacker’s files, exfiltrating content of arbitrary files to the remote attacker, loading a DLL plugin supplied by the remote attacker, and executing shellcode supplied by the remote attacker.
“Syssphinx continues to develop and improve its capabilities and malware delivery infrastructure, periodically refining its tools and tactics to avoid detection. The group’s decision to expand from point-of-sale attacks to the deployment of ransomware demonstrates the threat actors’ dedication to maximizing profits from victim organizations.” concludes the report. “The tools and tactics detailed in this report serve to underscore how this highly skilled financial threat actor remains a serious threat to organizations.”
(SecurityAffairs – hacking, BlackCat ransomware)
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