BeatRev Version 2

Disclaimer/Liability

The work that follows is a POC to enable malware to “key” itself to a particular victim in order to frustrate efforts of malware analysts.

I assume no responsibility for malicious use of any ideas or code contained within this project. I provide this research to further educate infosec professionals and provide additional training/food for thought for Malware Analysts, Reverse Engineers, and Blue Teamers at large.

TLDR

The first time the malware runs on a victim it AES encrypts the actual payload(an RDLL) using environmental data from that victim. Each subsequent time the malware is ran it gathers that same environmental info, AES decrypts the payload stored as a byte array within the malware, and runs it. If it fails to decrypt/the payload fails to run, the malware deletes itself. Protection against reverse engineers and malware analysts.

Updated 6 JUNE 2022

I didn’t feel finished with this project so I went back and did a fairly substantial re-write. The original research and tradecraft may be found Here.

Major changes are as follows:

1. I have released all source code
2. I integrated Stephen Fewer’s ReflectiveDLL into the project to replace Stage2
3. I formatted some of the byte arrays in this project into string format and parse them with UuidFromStringA. This Repo was used as a template. This was done to lower entropy of Stage0 and Stage1
4. Stage0 has had a fair bit of AV evasion built into it. Thanks to Cerbersec’s Project Ares for inspiration
5. The builder application to produce Stage0 has been included

There are quite a few different things that could be taken from the source code of this project for use elsewhere. Hopefully it will be useful for someone.

Problems with Original Release and Mitigations

There were a few shortcomings with the original release of BeatRev that I decided to try and address.

Stage2 was previously a standalone executable that was stored as the alternate data stream(ADS) of Stage1. In order to acheive the AES encryption-by-victim and subsequent decryption and execution, each time Stage1 was ran it would read the ADS, decrypt it, write back to the ADS, call CreateProcess, and then re-encrypt Stage2 and write it back to disk in the ADS. This was a lot of I/O operations and the CreateProcess call of course wasn’t great.

I happened to come upon Steven Fewer’s research concerning Reflective DLL’s and it seemed like a good fit. Stage2 is now an RDLL; our malware/shellcode runner/whatever we want to protect can be ported to RDLL format and stored as a byte array within Stage1 that is then decrypted on runtime and executed by Stage1. This removes all of the I/O operations and the CreateProcess call from Version1 and is a welcome change.

Stage1 did not have any real kind of AV evasion measures programmed in; this was intentional, as it is extra work and wasn’t really the point of this research. During the re-write I took it as an added challenge and added API-hashing to remove functions from the Import Address Table of Stage1. This has helped with detection and Stage1 has a 4/66 detection rate on VirusTotal. I was comfortable uploading Stage1 given that is is already keyed to the original box it was ran on and the file signature constantly changes because of the AES encryption that happens.

I recently started paying attention to entropy as a means to detect malware; to try and lower the otherwise very high entropy that a giant AES encrypted binary blob gives an executable I looked into integrating shellcode stored as UUID’s. Because the binary is stored in string representation, there is lower overall entropy in the executable. Using this technique The entropy of Stage0 is now ~6.8 and Stage1 ~4.5 (on a max scale of 8).

Finally it is a giant chore to integrate and produce a complete Stage0 due to all of the pieces that must be manipulated. To make this easier I made a builder application that will ingest a Stage0.c template file, a Stage1 stub, a Stage2 stub, and a raw shellcode file (this was build around Stage2 being a shellcode runner containing CobaltStrike shellcode) and produce a compiled Stage0 payload for use on target.

Technical Details

The Reflective DLL code from Stephen Fewer contains some Visual Studio compiler-specific instructions; I’m sure it is possible to port the technique over to MingW but I do not have the skills to do so. The main problem here is that the CobaltStrike shellcode (stageless is ~265K) needs to go inside the RDLL and be compiled. To get around this and integrate it nicely with the rest of the process I wrote my Stage2 RDLL to contain a global variable chunk of memory that is the size of the CS shellcode; this ~265K chunk of memory has a small placeholder in it that can be located in the compiled binary. The code in src/Stage2 has this added already.

Once compiled, this Stage2stub is transfered to kali where a binary patch may be performed to stick the real CS shellcode into the place in memory that it belongs. This produces the complete Stage2.

To avoid the I/O and CreateProcess fiasco previously described, the complete Stage2 must also be patched into the compiled Stage1 by Stage0; this is necessary in order to allow Stage2 to be encrypted once on-target in addition to preventing Stage2 from being stored separately on disk. The same concept previously described for Stage2 is conducted by Stage0 on target in order to assemble the final Stage1 payload. It should be noted that the memmem function is used in order to locate the placeholder within each stub; this function is no available on Windows, so a custom implementation was used. Thanks to Foxik384 for his code.

In order to perform a binary patch, we must allocate the required memory up front; this has a compounding effect, as Stage1 must now be big enough to also contain Stage2. With the added step of converting Stage2 to a UUID string, Stage2 balloons in size as does Stage1 in order to hold it. A stage2 RDLL with a compiled size of ~290K results in a Stage0 payload of ~1.38M, and a Stage1 payload of ~700K.

The builder application only supports creating x64 EXE’s. However with a little more work in theory you could make Stage0 a DLL, as well as Stage1, and have the whole lifecycle exist as a DLL hijack instead of a standalone executable.

Instructions

These instructions will get you on your way to using this POC.

1. Compile Builder using gcc -o builder src/Builder/BeatRevV2Builder.c
2. Modify sc_length variable in src/Stage2/dll/src/ReflectiveDLL.c to match the length of raw shellcode file used with builder ( I have included fakesc.bin for example)
3. Compile Stage2 (in visual studio, ReflectiveDLL project uses some VS compiler-specific instructions)
4. Move compiled stage2stub.dll back to kali, modify src/Stage1/newstage1.c and define stage2size as the size of stage2stub
5. Compile stage1stub using x86_64-w64-mingw32-gcc newstage1.c -o stage1stub.exe -s -DUNICODE -Os -L /usr/x86_64-w64-mingw32/lib -l:librpcrt4.a
6. Run builder using syntax: ./builder src/Stage0/newstage0_exe.c x64 stage1stub.exe stage2stub.dll shellcode.bin
7. Builder will produce dropper.exe. This is a formatted and compiled Stage0 payload for use on target.

BeatRev Original Release

Introduction

About 6 months ago it occured to me that while I had learned and done a lot with malware concerning AV/EDR evasion, I had spent very little time concerned with trying to evade or defeat reverse engineering/malware analysis. This was for a few good reasons:

1. I don’t know anything about malware analysis or reverse engineering
2. When you are talking about legal, sanctioned Red Team work there isn’t really a need to try and frustrate or defeat a reverse engineer because the activity should have been deconflicted long before it reaches that stage.

Nonetheless it was an interesting thought experiment and I had a few colleagues who DO know about malware analysis that I could bounce ideas off of. It seemed a challenge of a whole different magnitude compared to AV/EDR evasion and one I decided to take a stab at.

Premise

My initial premise was that the malware, on the first time of being ran, would somehow “key” itself to that victim machine; any subsequent attempts to run it would evaluate something in the target environment and compare it for a match in the malware. If those two factors matched, it executes as expected. If they do not (as in the case where the sample had been transfered to a malware analysts sandbox), the malware deletes itself (Once again heavily leaning on the work of LloydLabs and his delete-self-poc).

This “key” must be something “unique” to the victim computer. Ideally it will be a combination of several pieces of information, and then further obfuscated. As an example, we could gather the hostname of the computer as well as the amount of RAM installed; these two values can then be concatenated (e.g. Client018192MB) and then hashed using a user-defined function to produce a number (e.g. 5343823956).

There are a ton of choices in what information to gather, but thought should be given as to what values a Blue Teamer could easily spoof; a MAC address for example may seem like an attractive “unique” identifier for a victim, however MAC addresses can easily be set manually in order for a Reverse Engineer to match their sandbox to the original victim. Ideally the values chosen and enumerated will be one that are difficult for a reverse engineer to replicate in their environment.

With some self-deletion magic, the malware could read itself into a buffer, locate a placeholder variable and replace it with this number, delete itself, and then write the modified malware back to disk in the same location. Combined with an if/else statement in Main, the next time the malware runs it will detect that it has been ran previously and then go gather the hostname and amount of RAM again in order to produce the hashed number. This would then be evaluated against the number stored in the malware during the first run (5343823956). If it matches (as is the case if the malware is running on the same machine as it originally did), it executes as expected however if a different value is returned it will again call the self-delete function in order to remove itself from disk and protect the author from the malware analyst.

This seemed like a fine idea in theory until I spoke with a colleague who has real malware analysis and reverse engineering experience. I was told that a reverse engineer would be able to observe the conditional statement in the malware (if ValueFromFirstRun != GetHostnameAndRAM()), and seeing as the expected value is hard-coded on one side of the conditional statement, simply modify the registers to contain the expected value thus completely bypassing the entire protection mechanism.

This new knowledge completely derailed the thought experiment and seeing as I didn’t really have a use for a capability like this in the first place, this is where the project stopped for ~6 months.

Overview

This project resurfaced a few times over the intervening 6 months but each time was little more than a passing thought, as I had gained no new knowledge of reversing/malware analysis and again had no need for such a capability. A few days ago the idea rose again and while still neither of those factors have really changed, I guess I had a little bit more knowledge under my belt and couldn’t let go of the idea this time.

With the aforementioned problem regarding hard-coding values in mind, I ultimately decided to go for a multi-stage design. I will refer to them as Stage0, Stage1, and Stage2.

Stage0: Setup. Ran on initial infection and deleted afterwards

Stage1: Runner. Ran each subsequent time the malware executes

Stage2: Payload. The malware you care about protecting. Spawns a process and injects shellcode in order to return a Beacon.

Lifecycle

Stage0

Stage0 is the fresh executable delivered to target by the attacker. It contains Stage1 and Stage2 as AES encrypted byte arrays; this is done to protect the malware in transit, or should a defender somehow get their hands on a copy of Stage0 (which shouldn’t happen). The AES Key and IV are contained within Stage0 so in reality this won’t protect Stage1 or Stage2 from a competent Blue Teamer.

Stage0 performs the following actions:

1. Sandbox evasion.
2. Delete itself from disk. It is still running in memory.
3. Decrypts Stage1 using stored AES Key/IV and writes to disk in place of Stage0.
4. Gathers the processor name and the Microsoft ProductID.
5. Hashes this value and then pads it to fit a 16 byte AES key length. This value reversed serves as the AES IV.
6. Decrypts Stage2 using stored AES Key/IV.
7. Encrypts Stage2 using new victim-specific AES Key/IV.
8. Writes Stage2 to disk as an alternate data stream of Stage1.

At the conclusion of this sequence of events, Stage0 exits. Because it was deleted from disk in step 2 and is no longer running in memory, Stage0 is effectively gone; Without prior knowledge of this technique the rest of the malware lifecycle will be a whole lot more confusing than it already is.

In step 4 the processor name and Microsoft ProductID are gathered; the ProductID is retreived from the Registry, and this value can be manually modified which presents and easy opportunity for a Blue Teamer to match their sandbox to the target environment. Depending on what environmental information is gathered this can become easier or more difficult.

Stage1

Stage1 was dropped by Stage0 and exists in the same exact location as Stage0 did (to include the name). Stage2 is stored as an ADS of Stage1. When the attacker/persistence subsequently executes the malware, they are executing Stage1.

Stage1 performs the following actions:

1. Sandbox evasion.
2. Gathers the processor name and the Microsoft ProductID.
3. Hashes this value and then pads it to fit a 16 byte AES key length. This value reversed serves as the AES IV.
4. Reads Stage2 from Stage1’s ADS into memory.
5. Decrypts Stage2 using the victim-specific AES Key/IV.
6. Checks first two bytes of decryted Stage2 buffer; if not MZ (unsuccessful decryption), delete Stage1/Stage2, exit.
7. Writes decrypted Stage2 back to disk as ADS of Stage1
8. Calls CreateProcess on Stage2. If this fails (unsuccessful decryption), delete Stage1/Stage2, exit.
9. Sleeps 5 seconds to allow Stage2 to execute + exit so it can be overwritten.
10. Encrypts Stage2 using victim-specific AES Key/IV
11. Writes encrypted Stage2 back to disk as ADS of Stage1.

Note that Stage2 MUST exit in order for it to be overwritten; the self-deletion trick does not appear to work on files that are already ADS’s, as the self-deletion technique relies on renaming the primary data stream of the executable. Stage2 will ideally be an inject or spawn+inject executable.

There are two points that Stage1 could detect that it is not being ran from the same victim and delete itself/Stage2 in order to protect the threat actor. The first is the check for the executable header after decrypting Stage2 using the gathered environmental information; in theory this step could be bypassed by a reverse engineer, but it is a first good check. The second protection point is the result of the CreateProcess call- if it fails because Stage2 was not properly decrypted, the malware is similiary deleted. The result of this call could also be modified to prevent deletion by the reverse engineer, however this doesn’t change the fact that Stage2 is encrypted and inaccessible.

Stage2

Stage2 is the meat and potatoes of the malware chain; It is a fully fledged shellcode runner/piece of malware itself. By encrypting and protecting it in the way that we have, the actions of the end state malware are much better obfuscated and protected from reverse engineers and malware analysts. During development I used one of my existing shellcode runners containing CobaltStrike shellcode, but this could be anything the attacker wants to run and protect.

Impact, Mitigation, and Further Work

So what is actually accomplished with a malware lifecycle like this? There are a few interesting quirks to talk about.

Alternate data streams are a feature unique to NTFS file systems; this means that most ways of transfering the malware after initial infection will strip and lose Stage2 because it is an ADS of Stage1. Special care would have to be given in order to transfer the sample in order to preserve Stage2, as without it a lot of reverse engineers and malware analysts are going to be very confused as to what is happening. RAR archives are able to preserve ADS’s and tools like 7Z and Peazip can extract files and their ADS’s.

As previously mentioned, by the time malware using this lifecycle hits a Blue Teamer it should be at Stage1; Stage0 has come and gone, and Stage2 is already encrypted with the environmental information gathered by stage 0. Not knowing that Stage0 even existed will add considerable uncertainty to understanding the lifecycle and decrypting Stage2.

In theory (because again I have no reversing experience), Stage1 should be able to be reversed (after the Blue Teamers rolls through a few copies of it because it keeps deleting itself) and the information that Stage1 gathers from the target system should be able to be identified. Provided a well-orchestrated response, Blue Team should be able to identify the victim that the malware came from and go and gather that information from it and feed it into the program so that it may be transformed appropriately into the AES Key/IV that decrypts Stage2. There are a lot “ifs” in there however related to the relative skill of the reverse engineer as well as the victim machine being available for that information to be recovered.

Application Whitelisting would significantly frustrate this lifecycle. Stage0/Stage1 may be able to be side loaded as a DLL, however I suspect that Stage2 as an ADS would present some issues. I do not have an environment to test malware against AWL nor have I bothered porting this all to DLL format so I cannot say. I am sure there are creative ways around these issues.

I am also fairly confident that there are smarter ways to run Stage2 than dropping to disk and calling CreateProcess; Either manually mapping the executable or using a tool like Donut to turn it into shellcode seem like reasonable ideas.

Code and binary

During development I created a Builder application that Stage1 and Stage2 may be fed to in order to produce a functional Stage0; this will not be provided however I will be providing most of the source code for stage1 as it is the piece that would be most visible to a Blue Teamer. Stage0 will be excluded as an exercise for the reader, and stage2 is whatever standalone executable you want to run+protect. This POC may be further researched at the effort and discretion of able readers.

I will be providing a compiled copy of this malware as Dropper64.exe. Dropper64.exe is compiled for x64. Dropper64.exe is Stage0; it contains Stage1 and Stage2. On execution, Stage1 and Stage2 will drop to disk but will NOT automatically execute, you must run Dropper64.exe(now Stage1) again. Stage2 is an x64 version of calc.exe. I am including this for any Blue Teamers who want to take a look at this, but keep in mind in an incident response scenario 99& of the time you will be getting Stage1/Stage2, Stage0 will be gone.

Conclusion

This was an interesting pet project that ate up a long weekend. I’m sure it would be a lot more advanced/more complete if I had experience in a debugger and disassembler, but you do the best with what you have. I am eager to hear from Blue Teamers and other Malware Devs what they think. I am sure I have over-complicatedly re-invented the wheel here given what actual APT’s are doing, but I learned a few things along the way. Thank you for reading!