Sophos analysts just lately encountered a brand new EDR-killing utility being deployed by a prison group who had been attempting to assault a company 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 device designed to terminate endpoint safety software program. We’re calling this device EDRKillShifter.
Since 2022, we’ve seen a rise within the sophistication of malware designed to disable EDR techniques on an contaminated system, as prospects more and more undertake EDR tooling to guard endpoints. Sophos beforehand printed analysis about AuKill, an EDR killer device Sophos X-Ops found final yr that was being offered commercially inside prison marketplaces.
Through the incident in Could, the risk actors – we estimate with reasonable confidence that this device is being utilized by a number of attackers — tried to make use of EDRKillShifter to terminate Sophos safety on the focused laptop, however the device 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 device is a “loader” executable – a supply mechanism for a authentic driver that’s weak to abuse (also called a “carry your personal weak driver,” or BYOVD, device). Relying on the risk actor’s necessities, it may ship quite a lot of totally different driver payloads.
There are three steps to the execution means 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 closing payload, written within the Go programming language, drops and exploits one in every of quite a lot of totally different weak, authentic drivers to realize privileges ample to unhook an EDR device’s safety.
Peeling off the primary layer
A superficial evaluation reveals that each one samples share the identical model knowledge. The unique filename is Loader.exe and its product identify is ARK-Recreation. (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 Developed.)
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 novel 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 knowledge 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 following 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 by way of using a self-modifying code method. Throughout runtime, the second layer alters its personal directions. Because the precise executed directions are solely revealed throughout execution, extra tooling or emulation is required for evaluation.
The determine under additional illustrates the method. The primary part reveals 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 unique set of directions. The primary name modifies the following set of directions, which then modifies the following set of directions, and so forth.
The only real goal of the ultimate, decoded layer is to load the ultimate payload dynamically into reminiscence and execute it.
Evaluation of the final word payload
All the samples we analyzed executed a unique EDR killer variant in reminiscence. They’re all written in Go and obfuscated (probably by way of using an open-source device named gobfuscate). Obfuscators are instruments designed to hinder reverse engineering. There could also be authentic causes for software program engineers to obscure the software program, akin to to forestall rivals 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 knowledge when analyzing malware written in Go, however on this case, this key knowledge 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. A whole lot 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 had been capable of extract useful data utilizing the GoReSym device from Mandiant.
Similarities between the ultimate payloads
All the unpacked EDR killers embed a weak driver within the .knowledge part. Their habits is simple, like different EDR killers we’ve analyzed[1][2][3]. The one main distinction between the 2 variants we checked out is the weak driver being loaded and exploited.
Upon execution, each variants purchase the mandatory 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 working processes, terminating processes if their identify seems in a hardcoded record of targets. This habits is constant for each variants.
It’s also price noting that each variants exploit authentic (although weak) 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 frequent development we’ve additionally noticed in different EDR killers, akin to Terminator.
Similar loader, totally different closing payloads
The pattern with SHA256 451f5aa55eb207e73c5ca53d249b95911d3fad6fe32eee78c58947761336cc60 abuses a weak driver that has additionally been seen abused in assaults and calls itself RentDrv2. A proof-of-concept for exploiting this driver is accessible on Github.
The variant also can obtain an extra command line argument “–record”, permitting adversaries to move an extra record of course of names as targets.
The variant with SHA256 d0f9eae1776a98c77a6c6d66a3fd32cee7ee6148a7276bc899c1a1376865d9b0 in distinction, abuses a known-vulnerable driver known as ThreatFireMonitor, a part 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 modifications 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 goal is to deploy the ultimate BYOVD payload, and that it might 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 substantiate this speculation presently.
Mitigations and recommendation
Sophos at the moment detects EDRKillShifter as Troj/KillAV-KG. Moreover, behavioral safety guidelines that shield in opposition to protection evasion and privilege escalation block these system calls from going by way of. Companies and particular person folks also can take extra steps to defend their machines in opposition to driver abuse:
- Sophos X-Ops strongly suggests that you simply examine whether or not your endpoint safety product implements and allows tamper safety. This function gives a robust layer in opposition to such kind of assaults. In case you use Sophos merchandise however don’t at the moment have Sophos tamper safety enabled, flip it on right now.
- Observe sturdy hygiene for Home windows safety roles. This assault is barely potential if the attacker escalates privileges they management, or if they’ll receive administrator rights. Separation between person and admin privileges can assist forestall attackers from simply loading drivers.
- Hold your system up to date. Since final yr, Microsoft has begun to push updates that de-certify signed drivers identified to have been abused previously.
