A threat actor associated with the LockBit 3.0 ransomware-as-a-service (RaaS) operation has been observed abusing the Windows Defender command-line tool to decrypt and load Cobalt Strike payloads.

According to a report published by SentinelOne last week, the incident occurred after obtaining initial access via the Log4Shell vulnerability against an unpatched VMware Horizon Server.

"Once initial access had been achieved, the threat actors performed a series of enumeration commands and attempted to run multiple post-exploitation tools, including Meterpreter, PowerShell Empire, and a new way to side-load Cobalt Strike," researchers Julio Dantas, James Haughom, and Julien Reisdorffer said.

LockBit 3.0 (aka LockBit Black), which comes with the tagline "Make Ransomware Great Again!," is the next iteration of the prolific LockBit RaaS family that emerged in June 2022 to iron out critical weaknesses discovered in its predecessor.

It's notable for instituting what's the first-ever bug bounty for a RaaS program. Besides featuring a revamped leak site to name-and-shame non-compliant targets and publish extracted data, it also includes a new search tool to make it easier to find specific victim data.

The use of living-off-the-land (LotL) techniques by cyber intruders, wherein legitimate software and functions available in the system are used for post-exploitation, is not new and is usually seen as an attempt to evade detection by security software.

Earlier this April, a LockBit affiliate was found to have leveraged a VMware command-line utility called VMwareXferlogs.exe to drop Cobalt Strike. What's different this time around is the use of MpCmdRun.exe to achieve the same goal.

MpCmdRun.exe is a command-line tool for carrying out various functions in Microsoft Defender Antivirus, including scanning for malicious software, collecting diagnostic data, and restoring the service to a previous version, among others.

In the incident analyzed by SentinelOne, the initial access was followed by downloading a Cobalt Strike payload from a remote server, which was subsequently decrypted and loaded using the Windows Defender utility.

"Tools that should receive careful scrutiny are any that either the organization or the organization's security software have made exceptions for," the researchers said.

"Products like VMware and Windows Defender have a high prevalence in the enterprise and a high utility to threat actors if they are allowed to operate outside of the installed security controls."

The findings come as initial access brokers (IABs) are actively selling access to company networks, including managed service providers (MSPs), to fellow threat actors for profit, in turn offering a way to compromise downstream customers.

In May 2022, cybersecurity authorities from Australia, Canada, New Zealand, the U.K., and the U.S. warned of attacks weaponizing vulnerable managed service providers (MSPs) as an "initial access vector to multiple victim networks, with globally cascading effects."

"MSPs remain an attractive supply chain target for attackers, particularly IABs," Huntress researcher Harlan Carvey said, urging companies to secure their networks and implement multi-factor authentication (MFA).