GuLoader is one of the most widely used loaders to distribute malware throughout 2020. Among the malware families distributed by GuLoader, we can find FormBook, AgentTesla and other commodity malware. A recent research performed by Check Point suggests that GuLoader code is almost identical to a loader named as CloudEye and sold “legitimately” as a protection mechanism for binaries.
At Blueliv we have been keeping track of it and have observed that some of the methods it uses to detect virtual machine execution and hook detection, although not new, are still quite effective, as we have seen that in several online sandboxes this loader does not run correctly. Our own sandbox was detected by GuLoader, but it is correctly handled now, executing this malware family without problems.
In this blogpost, we will explain the techniques that this loader uses to thwart the analysts and their virtual machines/sandboxes, offering an application to check if a sandbox is detected by this technique or not.
Anti-Analysis & Anti-VM techniques
GuLoader uses the following techniques to make analysis tasks more difficult and to detect if it is running in a virtual machine:
- Using ZwQueryVirtualMemory to locate pages containing vm-related strings.
- Enumeration of windows (EnumWindow)
- Hooking ntdll_DbgBreakPoint and ntdll_DbgUiRemoteBreakin
- Checking breakpoints
- Hiding the thread (NtSetInformatioThread with 0x11)
- Checking the existence of
RDTSCVirtual machine detection
- Hook detection
Due to the fact that many of these techniques have been documented, we will focus on the following ones throughout this article:
- Hook detection
- RDTSC Virtual machine detection
Sandbox hook detection
GuLoader uses the following code fragment to detect if it’s running in a sandbox with hooks installed.
_ANTI_HOOK fnop pop ebx cld cmp dword ptr [ebx], 0 jnz short _jmp_copied_NtAllocateVirtualMemory clc xor ecx, ecx _copy: clc push dword ptr [eax+ecx] pop dword ptr [ebx+ecx] add ecx, 4 cmp ecx, 18h jnz short _copy cld _jmp_copied_NtAllocateVirtualMemory: fnop jmp ebx _ANTI_HOOK
With the push
dword ptr [eax+ecx] and
pop dword ptr [ebx+ecx] the NtAllocateVirtualMemory function is copied till
ntdll_NtAllocateVirtualMemory arg_0= byte ptr 4 mov eax, 15h xor ecx, ecx lea edx, [esp+arg_0] call large dword ptr fs:0C0h add esp, 4 retn 18h
Then it jumps to the code where it has copied the function, using
jmp ebx and executes it.
If it is running in an environment without hooks, the function will be executed correctly. But in the case that there is a hook in the function and this hook makes a relative jump, for example with opcode E9 (such as those performed by the cuckoo monitor), when the function that has been copied to another position is executed, it will jump to an unknown position and an exception will occur, causing an abrupt termination of the program.
RDTSC Virtual machine detection
Even though this technique is widely known, the interesting thing is the way that it has been implemented.
GuLoader uses the following algorithm to detect if it is inside a virtual machine:
_VM_DETECT proc near cld _VM_DETECT_START: fnop xor edi, edi nop mov ecx, 186A0h cld nop _VM_DETECT_CONTINUE: push ecx call _RDTSC_OPS pop ecx cmp edx, 32h jl short _VM_DETECT_CONTINUE add edi, edx dec ecx cmp ecx, 0 jnz short _VM_DETECT_CONTINUE cmp edi, 0 jl short _VM_DETECT_START cmp edi, 68E7780h jge short _VM_DETECT_START mov eax, edi retn _VM_DETECT endp
1. The value
0x186A0 is stored in
ECX. This value indicates the number of times
EDI will be incremented with the result of the
_RDTSC_OPS function as long as the result of the operation is greater than
2. After that it will perform a call to the
_RDTSC_OPS function, explained later in detail. For now, it is only necessary to know that this function will return a value higher than 0.
3. Then it checks if the value is higher than 0x32. If that’s the case it adds the result in the
EDI register and decreases the
ECX value. Otherwise it returns to
_VM_DETECT_CONTINUE to call
_RDTSC_OPS again. As a note, if the value returned by
_RDTSC_OPS at this point is greater than 0 but less than 0x32 continuously, the program will remain in an infinite loop.
4. This will be done until
ECX is 0, so the result of the function will be added to the
EDI value with the
add edi, edx operation
5. Finally, it will check if the result of those increments is greater or equal to
0x68E7780. The result must be lower to pass the virtual machine check. If not, the execution will return to
_VM_DETECT_START. It is important to highlight that in a virtual machine without any modification or hook over
RDTSC this is the point where the program will remain running in an infinite loop.
Basically, the malware developer estimated that the addition of values returned by the execution of the function
0x186A0 times within a virtualized environment will result in a value above
0x68E7780 due to the overhead generated by the
_RDTSC_OPS function. If the value of
RDTSC has been artificially lowered below
0x32 to attempt to bypass similar techniques, the analysis will be stuck in this Anti-VM check forever.
GuLoader uses the following algorithm to obtain the execution times between two
RDTSC calls, after a
CPUID EAX=1 as shown below:
_RDTSC_OPS lfence rdtsc lfence shl edx, 20h or edx, eax mov esi, edx pusha mov eax, 1 cpuid bt ecx, 1Fh jb short $+2 popa lfence rdtsc lfence shl edx, 20h or edx, eax sub edx, esi cmp edx, 0 jle short _RDTSC_OPS retn _RDTSC_OPS
The algorithm performs the following operations:
1. First obtains the elapsed time in
EAX (low-part) and
EDX (high-part) through
2. Performs an
OR operation between the high and the low part, and saves the result in
3. It makes the call to
EAX=1 and then, thanks to the
bt ecx, 1Fh instruction, it checks if it is running in a virtual machine. However, the result of the operation is not relevant, because the call to
CPUID is made with the intention of generating a VM-Exit causing that the hypervisor passes the execution to the Virtual Machine Manager. This allows to differentiate if it is running in a virtual machine, because the call takes more time than in a physical machine.
4. Makes a call to
RDTSC and get again the time that has been spent in
EAX (low-part) and
5. Performs an
OR operation between the high and low parts, and subtracts the previous result stored in
6. If the result is greater than 0, as would be expected in normal execution, the function will return the result in
EDX, otherwise it will return to the start of the function. It is important to highlight that it is possible that in some sandboxes the sample will be locked in an infinite loop at this point., depending on the way the sandbox deals with the detection problem by
Implementation changes in some GuLoader versions
We also found some differences between different GuLoader samples, particularly in the functions responsible for performing the virtual machine check.
|GuLoader Blueliv Article||25018a8ff2a535ed05ebe8a1d2158a79dbeb53fc0be67d4e788bc936cb551b6d|
|GuLoader Checkpoint Shellcode||295cb5b21bafcafa8d770d5ce325893340037c8efe545691b8289aff82315539|
|CloudEyE Checkpoint Shellcode||3ca3f172d222d5f52be734079658e2a141d92e15c8edd4ea7515a72cf03a28a3|
Comparing the sample that we used to write the article with the different shellcodes that can be found in the following Check Point article, it is possible to see how the
_VM_DETECT function has gone through some modifications.
Comparison of the _VM_DETECT function in different samples
The function on the left corresponds to the sample that we have analyzed throughout the article and the function on the right corresponds to the shellcodes mentioned in Check Point’s article.
We can see that the function on the left adds a check after calling
_RDTSC_OPS where it checks if the value is less than
0x32. As mentioned previously, this new check may be due to the fact that there are sandboxes that after the call to the
_RDTSC_OPS function return values in
EDX below the normal. So GuLoader can avoid to be fully executed in those sandboxes where it will stay in an infinite loop at this point.
We can also see that the value compared in
EDI in the function on the left is
0x68E7780, which is greater than
0x66FF300 (function on the right). It is possible that the variation of this value is due to the fact that there are non-virtualized machines whose value after these operations is above
0x66FF300 and in some update they have had to increase the value up to
0x68E7780, although this is just an assumption.
Blueliv Anti-VM detection tool
This technique was quite interesting to us because some of the existent sandboxes were not dealing with the issue. That’s why we thought it was a good idea to create a tool to help to detect the problem. This tool permits to know if a sandbox is detected by the technique described above (the most restrictive version). You can find this tool in our repository. If anyone is interested in discussing these technique or taking a look at the source code in order to fix the problem in a sandbox, be free to reach to us.
The tool has been created from the code of the GuLoader sample without modifications, to avoid modifying its behavior. If the sandbox is detected, it displays by console the
EDI value after the
0x186A0 loop have been completed.
However, if the sandbox is not detected, a message appears on the screen indicating that it does not detect the sandbox.
If the tool does not display any value, there are two possibilities:
- The result of the operation between the
RDTSCsis less than or equal to 0.
- The result returned by the
_RDTSC_OPSfunction is less than 0x32.
In both cases the execution would stay in an infinite loop. It is recommended to analyze the application step by step to find the problem.
We have conducted some tests using the article’s sample in several well-known public sandboxes and have found that the sample does not run correctly in most of them.
In those that allow access to the API log it is possible to see that the last logged API calls are:
- NtCreateFile of
- NtCreateFile of
This is because the
RDTSC and anti-hook checks are performed after those calls, and may be avoiding the correct execution of the sample.
We hope our tool can help to fix those problems.
Blueliv sandbox execution graph
We can see below the execution graph generated by the Blueliv sandbox, using our kernel monitor, during the execution of this sample (
As explained in this article, during the execution of the sample it performs several anti-virtual machine and anti-hook checks in order to thwart the analysis, but our sandbox was able to bypass them and continue with the execution flow. When executed it runs twice (
liTK.exe), then the GuLoader sample creates persistence and changes its name to host.exe and finally downloads from Microsoft OneDrive the final payload. The final payload could not be retrieved if the sandbox would not correctly bypass the mentioned anti-vm techniques.
Cybercriminals and malware authors are always evolving, modifying their tools and using techniques to evade virtual machines and difficult the analysis of the samples. GuLoader is a clear example of this, a really active loader which makes use of some modifications of well-known techniques like the RDTSC check to go further in the detection of sandboxes. The tool we provide will help to confirm if GuLoader is able to detect a sandbox and we hope that the affected online sandboxes can benefit from it and fix this problem.
Keeping a sandbox in a good shape and with all the latest anti-vm techniques under control is crucial to be able to detonate samples and artifacts, and bypass packers and obfuscation layers. At Blueliv we know that and we investigate and fix all the problems that we detect so the number of extracted IOCs and our malware classification keeps having a good quality. This is especially important for our internal investigations, for our customers and for our Threat Exchange Network, where our sandbox can be used free of charge.
This blog post was authored by Carlos Rubio and supported by the Blueliv Labs team.