Threat actors are employing a previously undocumented “defense evasion tool” dubbed AuKill that’s designed to disable endpoint detection and response (EDR) software by means of a Bring Your Own Vulnerable Driver (BYOVD) attack.
“The AuKill tool abuses an outdated version of the driver used by version 16.32 of the Microsoft utility, Process Explorer, to disable EDR processes before deploying either a backdoor or ransomware on the target system,” Sophos researcher Andreas Klopsch said in a report published last week.
Incidents analyzed by the cybersecurity firm show the use of AuKill since the start of 2023 to deploy various ransomware strains such as Medusa Locker and LockBit. Six different versions of the malware have been identified to date. The oldest AuKill sample features a November 2022 compilation timestamp.
The BYOVD technique relies on threat actors misusing a legitimate, but out-of-date and exploitable, driver signed by Microsoft (or using a stolen or leaked certificate) to gain elevated privileges and turn off security mechanisms.
By using legitimate, exploitable drivers, the idea is to bypass a key Windows safeguard known as Driver Signature Enforcement that ensures kernel-mode drivers have been signed by a valid code signing authority before they are allowed to run.
“The AuKill tool requires administrative privileges to work, but it cannot give the attacker those privileges,” Sophos researchers noted. “The threat actors using AuKill took advantage of existing privileges during the attacks, when they gained them through other means.”
This is not the first time the Microsoft-signed Process Explorer driver has been weaponized in attacks. In November 2022, Sophos also detailed LockBit affiliates’ use of an open source tool called Backstab that abused outdated versions of the driver to terminate protected anti-malware processes.
Then earlier this year, a malvertising campaign was spotted utilizing the same driver to distribute a .NET loader named MalVirt to deploy the FormBook information-stealing malware.
The development comes as the AhnLab Security Emergency response Center (ASEC) revealed that poorly managed MS-SQL servers are being weaponized to install the Trigona ransomware, which shares overlaps with another strain referred to as CryLock.
It also follows findings that the Play ransomware (aka PlayCrypt) actors have been observed using custom data harvesting tools that make it possible to enumerate all users and computers on a compromised network and copy files from the Volume Shadow Copy Service (VSS).
Grixba, a .NET-based information stealer, is designed to scan a machine for security programs, backup software, and remote administration tools, and exfiltrate the gathered data in the form of CSV files that are then compressed into ZIP archives.
Also used by the cybercriminal gang, tracked by Symantec as Balloonfly, is a VSS Copying Tool written in .NET that makes use of the AlphaVSS framework to list files and folders in a VSS snapshot and copy them to a destination directory prior to encryption.
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Play ransomware is notable for not only utilizing intermittent encryption to speed up the process, but also for the fact that it’s not operated on a ransomware-as-a-service (RaaS) model. Evidence gathered so far points to Balloonfly carrying out the ransomware attacks as well as developing the malware themselves.
Grixba and VSS Copying Tool are the latest in a long list of proprietary tools such as Exmatter, Exbyte, and PowerShell-based scripts that are used by ransomware actors to establish more control over their operations, while also adding extra layers of complexity to persist in compromised environments and evade detection.
Another technique increasingly adopted by financially-motivated groups is the use of the Go programming language to develop cross-platform malware and resist analysis and reverse engineering efforts.
Indeed, a report from Cyble last week documented a new GoLang ransomware called CrossLock that employs the double-extortion technique to increase the likelihood of payment from its victims, alongside taking steps to sidestep event tracing for Windows (ETW).
“This functionality can enable the malware to avoid detection by security systems that depend on event logs,” Cyble said. “CrossLock Ransomware also performs several actions to reduce the chances of data recovery while simultaneously increasing the attack’s effectiveness.”
