Sophos analysts just lately encountered a brand new EDR-killing utility being deployed by a legal group who had been making an attempt to assault a corporation with ransomware referred to as RansomHub. Whereas the ransomware assault in the end was unsuccessful, the postmortem evaluation of the assault revealed the existence of a brand new instrument designed to terminate endpoint safety software program. We’re calling this instrument 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 instrument Sophos X-Ops found final yr that was being offered commercially inside legal marketplaces.
In the course of the incident in Could, the menace actors – we estimate with reasonable confidence that this instrument is being utilized by a number of attackers — tried to make use of EDRKillShifter to terminate Sophos safety on the focused laptop, however the instrument 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 instrument is a “loader” executable – a supply mechanism for a reliable driver that’s weak to abuse (also called a “deliver your personal weak driver,” or BYOVD, instrument). Relying on the menace actor’s necessities, it might ship a wide range of completely 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 last payload, written within the Go programming language, drops and exploits considered one of a wide range of completely different weak, reliable drivers to achieve privileges enough to unhook an EDR instrument’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 title is ARK-Recreation. (Some members of the analysis staff speculated that the menace actor tries to masquerade the ultimate payload as a preferred laptop recreation 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 flawed (or not supplied), it gained’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 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 via 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 beneath additional illustrates the method. 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 following set of directions, which then modifies the following 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
All the 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 instrument named gobfuscate). Obfuscators are instruments designed to hinder reverse engineering. There could also be reliable 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 harder for safety researchers to research 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. Loads of open-source reverse engineering instruments depend on this Go model data to rebuild constructions within the disassembly.
- Helpful package deal data, or package deal paths, are encrypted or stripped from the ultimate malware.
Nonetheless, we had been capable of extract useful data utilizing the GoReSym instrument from Mandiant.
Similarities between the ultimate payloads
All the unpacked EDR killers embed a weak driver within the .information part. Their habits is simple, like different EDR killers we have now 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 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 limitless loop that constantly enumerates the working processes, terminating processes if their title seems in a hardcoded listing of targets. This habits is constant for each variants.
Additionally it is price noting that each variants exploit reliable (although weak) drivers, utilizing proof-of-concept exploits out there on Github. We suspect that the menace actors copied parts of those proofs-of-concept, modified them, and ported the code to the Go language. This can be a frequent development we have now additionally noticed in different EDR killers, akin to Terminator.
Similar loader, completely different last 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 offered on Github.
The variant may also obtain a further command line argument “–listing”, permitting adversaries to cross a further listing of course of names as targets.
The variant with SHA256 d0f9eae1776a98c77a6c6d66a3fd32cee7ee6148a7276bc899c1a1376865d9b0 in distinction, abuses a known-vulnerable driver referred to as ThreatFireMonitor, a element of a deprecated system-monitoring package deal. A proof of idea for this particular driver can also be out there on Github.
Mapping EDRKillShifter into the bigger menace 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 menace panorama, it’s also believable that the loader and the ultimate payloads are developed by separate menace actors.
Promoting loaders or obfuscators is a profitable enterprise on the darkish web. Sophos X-Ops suspects that the loader’s sole objective is to deploy the ultimate BYOVD payload, and that it might need been acquired on the darkish web. 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 presently detects EDRKillShifter as Troj/KillAV-KG. Moreover, behavioral safety guidelines that shield towards protection evasion and privilege escalation block these system calls from going via. Companies and particular person individuals may also take extra steps to defend their machines towards driver abuse:
- Sophos X-Ops strongly suggests that you just examine whether or not your endpoint safety product implements and allows tamper safety. This function gives a powerful layer towards such kind of assaults. In the event you use Sophos merchandise however don’t presently have Sophos tamper safety enabled, flip it on in the present day.
- Observe sturdy hygiene for Home windows safety roles. This assault is simply attainable if the attacker escalates privileges they management, or if they’ll receive administrator rights. Separation between consumer and admin privileges can assist stop attackers from simply loading drivers.
- Maintain your system up to date. Since final yr, Microsoft has begun to push updates that de-certify signed drivers identified to have been abused prior to now.