Executive summary

On April 21st, 2023, AT&T Managed Extended Detection and Response (MXDR) investigated an attempted ransomware attack on one of our clients, a home improvement business. The investigation revealed the attacker used AuKill malware on the client’s print server to disable the server’s installed EDR solution, SentinelOne, by brute forcing an administrator account and downgrading a driver to a vulnerable version.

AuKill, first identified by Sophos X-Ops researchers in June 2021, is a sophisticated malware designed to target and neutralize specific EDR solutions, including SentinelOne and Sophos. Distributed as a dropper, AuKill drops a vulnerable driver named PROCEXP.SYS (from Process Explorer release version 16.32) into the system’s C:WindowsSystem32drivers folder. This malware has been observed in the wild, utilized by ransomware groups to bypass endpoint security measures and effectively spread ransomware variants such as Medusa Locker and Lockbit on vulnerable systems.

In this case, SentinelOne managed to isolate most of the malicious files before being disabled, preventing a full-scale ransomware incident. As a result, AT&T MXDR found no evidence of data exfiltration or encryption. Despite this, the client opted to rebuild the print server as a precautionary measure. This study provides an in-depth analysis of the attack and offers recommendations to mitigate the risk of future attacks.

Investigating the first phase of the attack

Initial intrusion

The targeted asset was the print server, which we found unusual. However, upon further investigation we concluded the attacker misidentified the asset as a Domain Controller (DC), as it had recently been repurposed from a DC to a print server. The attacker needed both local administrator credentials and kernel-level access to successfully run AuKill and disable SentinelOne on the asset. To gain those local administrator credentials, the attacker successfully brute-forced an administrator account. Shortly after the compromise, this account was observed making unauthorized registry changes.

  

Establishing a beachhead

After compromising the local administrator account, the attackers used the “UsersAdministratorMusicaSentinel” folder as a staging area for subsequent phases of their attack. All AuKill-related binaries and scripts were executed from this path, with the innocuous “Music” folder name helping to conceal their malicious activities.

AuKill malware has been found to operate using two Windows services named “aSentinel.exe” and “aSentinelX.exe” in its SentinelOne variant. In other variants, it targets different EDRs, such as Sophos, by utilizing corresponding Windows services like “aSophos.exe” and “aSophosX.exe”. 

Establishing persistence

We also discovered “aSentinel.exe” running from “C:Windowssystem32”, indicating that the attackers attempted to establish a foothold on the compromised server. Malware authors frequently target the system32 folder because it is a trusted location, and security software may not scrutinize files within it as closely as those in other locations. This can help malware bypass security measures and remain hidden. It is likely that the malware was initially placed in the “UsersAdministratorMusicaSentinel” directory and later copied to the system32 directory for persistence.

Network reconnaissance

Our investigation also revealed that PCHunter, a publicly accessible utility previously exploited in ransomware incidents like Dharma, was running from the “UsersAdministratorMusicaSentinel” directory. This suggests that the attackers used PCHunter as a reconnaissance tool to survey the client’s network before deploying the EDR killer malware. Additionally, PCHunter enables threat actors to terminate programs and interface directly with the Windows kernel, which aligns with the needs of the attacker. We observed PCHunter generating several randomly named .sys files, as illustrated below:

Preventing data recovery

We found that the attacker deleted shadow volume copies from the print server. Windows creates these copies to restore files and folders to previous versions in case of data loss. By removing the shadow copies, the attacker was attempting to make it more challenging for our client to recover their files if they were successfully encrypted. Although no ransomware was deployed, the deletion of shadow copies reveals the attackers’ intentions. This information, together with the usage of PCHunter and the staging of the EDR killer malware, paints a more complete picture of the attacker’s objectives and tactics.

Bypassing native Windows protection

With all these pieces in place, the attacker last needed to acquire kernel-level access. Despite gaining administrator rights early on, the attacker did not have enough control over the system to kill SentinelOne at this time. EDR solutions are classified as essential by Windows and are protected from being turned off by attackers when they escalate privileges. To successfully circumvent these safeguards, the attacker would need to travel one level deeper into the operating system and gain kernel-level access to the machine.

Investigating the second phase of the attack

Dropping the vulnerable driver

Our team discovered that AuKill had replaced the current Process Explorer driver, PROCEXP152.sys, with an outdated and vulnerable version named PROCEXP.SYS (from Process Explorer release version 16.32), located in the C:WindowsSystem32drivers directory. The alarm screenshot below demonstrates how AuKill swapped the existing driver with this older version, making the system susceptible to further exploitation.

 

Windows incorporates a security feature called Driver Signature Enforcement, which ensures that kernel-mode drivers are signed by a valid code signing authority before they can run. To bypass this security measure, the attackers exploited the insecure PROCEXP.SYS driver, which was produced and signed by Microsoft at an earlier date. As demonstrated in the SentinelOne screenshot below, the driver is signed and verified by Microsoft. Furthermore, the originating process was aSentinel.exe, an executable created to disable SentinelOne.

Acquiring kernel-level access

Process Explorer, a legitimate system monitoring tool developed by Microsoft’s Sysinternals team, enables administrators to examine and manage applications’ ongoing processes, as well as their associated threads, handles, and DLLs.

Upon startup, Process Explorer loads a signed kernel-mode driver, facilitating interaction with the system’s kernel, which is responsible for managing hardware and resources. Normally, that driver is PROCEXP152.sys. The attacker replaced the PROCEXP152.sys driver on the print server with the exploitable PROCEXP.SYS, employing what is known as a BYOVD (Bring Your Own Vulnerable Driver) attack. The attacker used this method to exploit the now vulnerable kernel mode driver to gain the kernel-level access they needed to successfully kill SentinelOne.

Killing SentinelOne

The kernel-mode driver used by Process Explorer has the unique ability to terminate handles that are inaccessible even to administrators. A handle is an identifier that corresponds to a specific resource opened by a process, such as a file or a registry key. At this point, AuKill hijacked Process Explorer’s kernel driver to specifically target protected handles associated with SentinelOne processes running on the print server. The SentinelOne processes were killed when the protected process handles were closed, rendering the EDR powerless. AuKill then generated several threads to ensure that these EDR processes remained disabled and did not resume. Each thread concentrated on a certain SentinelOne component and regularly checked to see if the targeted processes were active. If they were, AuKill would terminate them. SentinelOne was out of the way and no longer an obstacle to the attacker.

Response

Customer interaction

At this point, the attacker had gained privileged access to the asset, deployed their malware, and successfully killed the endpoint protection solution, SentinelOne. Based on the Cyber Kill Chain methodology developed by Lockheed Martin, we can conclude that the attacker had now successfully reached the “Command and Control” stage. However, the attacker did not reach the “Actions on Objectives” stage, as SentinelOne managed to disrupt ransomware deployment enough before it was killed to prevent any additional damage.

Any attempts to re-deploy malware or move laterally following the disablement of the EDR were thwarted by our team, who swiftly alerted the client to the activity and advised that the asset be taken offline and isolated from the rest of the network. Our team informed the client that the shadow copies had been deleted and SentinelOne had been turned off on their print server. After having our threat hunters thoroughly review their environment, w e reassured the client that no sensitive information was exfiltrated or encrypted. In response to the attack, the client moved to rebuild their print server and reinstall SentinelOne.

Recommendations

As BYOVD attacks to bypass EDR software become more widespread, we strongly advise blacklisting outdated drivers with a known history of exploitation. Furthermore, we encourage our clients to maintain an inventory of the drivers installed on their systems, ensuring they remain current and secure. Lastly, we recommend bolstering the security of administrator accounts to defend against brute force attacks, as the incident detailed in this blog post could not have transpired without the initial privileged user compromise.