Sophos analysts lately encountered a brand new EDR-killing utility being deployed by a prison group who have been attempting to assault a corporation with ransomware known as RansomHub. Whereas the ransomware assault finally was unsuccessful, the postmortem evaluation of the assault revealed the existence of a brand new software designed to terminate endpoint safety software program. We’re calling this software EDRKillShifter. 

Since 2022, we’ve seen a rise within the sophistication of malware designed to disable EDR techniques on an contaminated system, as clients more and more undertake EDR tooling to guard endpoints. Sophos beforehand revealed analysis about AuKill, an EDR killer software Sophos X-Ops found final 12 months that was being bought commercially inside prison marketplaces. 

Through the incident in Might, the risk actors – we estimate with average confidence that this software is being utilized by a number of attackers — tried to make use of EDRKillShifter to terminate Sophos safety on the focused laptop, however the software failed. They then tried to run the ransomware executable on the machine they managed, however that additionally failed when the endpoint agent’s CryptoGuard function was triggered. 

How EDRKillShifter works 

The EDRKillShifter software is a “loader” executable – a supply mechanism for a legit driver that’s susceptible to abuse (often known as a “deliver your individual susceptible driver,” or BYOVD, software).  Relying on the risk actor’s necessities, it could actually ship quite a lot of totally different driver payloads. 

There are three steps to the execution strategy of this loader. The attacker should execute EDRKillShifter with a command line that features a password string. When run with the proper password, the executable decrypts an embedded useful resource named BIN and executes it in reminiscence. 

The BIN code unpacks and executes the ultimate payload. This last payload, written within the Go programming language, drops and exploits one among quite a lot of totally different susceptible, legit drivers to realize privileges ample to unhook an EDR software’s safety. 

Peeling off the primary layer 

A superficial evaluation reveals that every one samples share the identical model information. The unique filename is Loader.exe and its product identify is ARK-Sport. (Some members of the analysis crew speculated that the risk actor tries to masquerade the ultimate payload as a well-liked laptop sport named ARK: Survival Advanced.)  

The binary’s language property is Russian, indicating that the malware creator compiled the executable on a pc with Russian localization settings. 

All samples require a singular 64-character password handed to the command line. If the password is incorrect (or not offered), it received’t execute. 

When executed, EDRKillShifter masses an encrypted useful resource named BIN, embedded inside itself, into reminiscence. It additionally copies that information into a brand new file named Config.ini and writes that file to the identical filesystem location the place the binary was executed.  

The loader code then allocates a brand new reminiscence web page utilizing VirtualAlloc, and writes the encrypted content material into the newly allotted web page. The malware then deletes the config.ini file and proceeds with decrypting the subsequent set of payloads – the abusable driver and a Go binary. The loader makes use of a SHA256 hash of the enter password because the decryption key of the second-layer payloads. 

If the malware efficiently decrypts the second-layer payloads, it creates a brand new thread and begins execution in that thread. 

Loading the ultimate EDR killer into reminiscence 

The second stage is obfuscated via using a self-modifying code approach. Throughout runtime, the second layer alters its personal directions. Because the precise executed directions are solely revealed throughout execution, further tooling or emulation is required for evaluation.  

The determine under additional illustrates the approach. The primary part exhibits the start of the self-modifying code layer. All directions after the primary name within the disassembly are nonsense at this level. If we revisit the identical instruction block after executing the primary name, we see a special set of directions. The primary name modifies the subsequent set of directions, which then modifies the subsequent set of directions, and so forth.  

The only objective of the ultimate, decoded layer is to load the ultimate payload dynamically into reminiscence and execute it. 

Evaluation of the final word payload 

The entire samples we analyzed executed a special EDR killer variant in reminiscence. They’re all written in Go and obfuscated (presumably via using an open-source software named gobfuscate). Obfuscators are instruments designed to hinder reverse engineering. There could also be legit causes for software program engineers to obscure the software program, reminiscent of to forestall opponents from stealing mental property. Nonetheless, malware authors additionally use obfuscators to make it tougher for safety researchers to investigate malware. 

Most reverse engineers depend on this obfuscated information when analyzing malware written in Go, however on this case, this key information is obscured within the compiled code. A few of this data contains: 

• Strings are encrypted. They are going to be decrypted throughout runtime. 

• The Go model data is gone. Lots of open-source reverse engineering instruments depend on this Go model data to rebuild buildings within the disassembly. 

• Helpful bundle data, or bundle paths, are encrypted or stripped from the ultimate malware. 

Nonetheless, we have been in a position to extract invaluable data utilizing the GoReSym software from Mandiant. 

Similarities between the ultimate payloads 

The entire unpacked EDR killers embed a susceptible driver within the .information part. Their habits is easy, like different EDR killers we’ve analyzed[1][2][3]. The one main distinction between the 2 variants we checked out is the susceptible driver being loaded and exploited. 

Upon execution, each variants purchase the required privileges to load a driver and drop the exploitable sys file into the AppDataLocalTemp folder. The malware generates a random filename for the driving force each time it’s run. 

After the malware creates a brand new service for the driving force, begins the service, and masses the driving force, it enters an countless loop that constantly enumerates the operating processes, terminating processes if their identify seems in a hardcoded checklist of targets. This habits is constant for each variants. 

It is usually price noting that each variants exploit legit (although susceptible) drivers, utilizing proof-of-concept exploits out there on Github. We suspect that the risk actors copied parts of those proofs-of-concept, modified them, and ported the code to the Go language. It is a widespread pattern we’ve additionally noticed in different EDR killers, reminiscent of Terminator.  

Similar loader, totally different last payloads 

The pattern with SHA256 451f5aa55eb207e73c5ca53d249b95911d3fad6fe32eee78c58947761336cc60 abuses a susceptible driver that has additionally been seen abused in assaults and calls itself RentDrv2. A proof-of-concept for exploiting this driver is offered on Github.  

The variant may also obtain a further command line argument “–checklist”, permitting adversaries to go a further checklist of course of names as targets. 

The variant with SHA256 d0f9eae1776a98c77a6c6d66a3fd32cee7ee6148a7276bc899c1a1376865d9b0 in distinction, abuses a known-vulnerable driver known as ThreatFireMonitor, a element of a deprecated system-monitoring bundle. A proof of idea for this particular driver can also be out there on Github. 

Mapping EDRKillShifter into the bigger risk panorama 

The ultimate payload embedded into the loader adjustments from incident to incident (and, presumably, creator to creator). If we attempt to map EDRKillShifter to the bigger risk panorama, additionally it is believable that the loader and the ultimate payloads are developed by separate risk actors. 

Promoting loaders or obfuscators is a profitable enterprise on the darkish internet. Sophos X-Ops suspects that the loader’s sole objective is to deploy the ultimate BYOVD payload, and that it may need been acquired on the darkish internet. The ultimate EDR killer payloads are then merely being delivered by the loader itself, which consists of the layer 1 and a pair of we described in our evaluation above. 

It’s worthwhile to notice that we’re unable to verify this speculation right now. 

Mitigations and recommendation

Sophos presently detects EDRKillShifter as Troj/KillAV-KG. Moreover, behavioral safety guidelines that defend towards protection evasion and privilege escalation block these system calls from going via. Companies and particular person folks may also take further steps to defend their machines towards driver abuse: 

• Sophos X-Ops strongly suggests that you just verify whether or not your endpoint safety product implements and allows tamper safety. This function offers a robust layer towards such sort of assaults. For those who use Sophos merchandise however don’t presently have Sophos tamper safety enabled, flip it on immediately. 

• Apply robust hygiene for Home windows safety roles. This assault is barely potential if the attacker escalates privileges they management, or if they will receive administrator rights. Separation between consumer and admin privileges might help stop attackers from simply loading drivers. 

• Maintain your system up to date. Since final 12 months, Microsoft has begun to push updates that de-certify signed drivers identified to have been abused prior to now.