GULoader Campaigns: A Deep Dive Evaluation of a extremely evasive Shellcode based mostly loader

Authored by: Anandeshwar Unnikrishnan

Stage 1: GULoader Shellcode Deployment 

In current GULoader campaigns, we’re seeing an increase in NSIS-based installers delivered through E-mail as malspam that use plugin libraries to execute the GU shellcode on the sufferer system. The NSIS scriptable installer is a extremely environment friendly software program packaging utility. The installer conduct is dictated by an NSIS script and customers can lengthen the performance of the packager by including customized libraries (dll) generally known as NSIS plugins. Since its inception, adversaries have abused the utility to ship malware. 

NSIS stands for Nullsoft Scriptable Installer. NSIS installer information are self-contained archives enabling malware authors to incorporate malicious belongings together with junk knowledge. The junk knowledge is used as Anti-AV / AV Evasion method. The picture under exhibits the construction of an NSIS GULoader staging executable archive.

The NSIS script, which is a file discovered within the archive, has a file extension “.nsi” as proven within the picture above. The deployment technique employed by the menace actor will be studied by analyzing the NSIS script instructions supplied within the script file. The picture proven under is an oversimplified view of the entire shellcode staging course of. 

The file that holds the encoded GULoader shellcode is dropped on to sufferer’s disc based mostly on the script configuration together with different knowledge. Junk is appended at first of the encoded shellcode. The encoding type varies from pattern to pattern. However in all most all of the instances, it’s a easy XOR encoding. As talked about earlier than, the shellcode is appended to junk knowledge, due to this, an offset is used to retrieve encoded GULoader shellcode. Within the picture, the FileSeek NSIS command is used to do correct offsetting. Some samples have unprotected GULoader shellcode appended to junk knowledge. 

A plugin utilized by the NSIS installer is nothing however a DLL which will get loaded by the installer program at runtime and invokes capabilities exported by the library.  Two DLL information are dropped in person’s TEMP listing, in all analyzed samples one DLL has a constant identify of system.dll and identify of the opposite one varies.   

The system.dll is liable for allocating reminiscence for the shellcode and its execution. The next picture exhibits how the NSIS script calls capabilities in plugin libraries.

The system.dll has the following exports as proven the in the picture under. The operate named “Name” is getting used to deploy the shellcode on sufferer’s system. 

• The Name operate exported by system.dll resolves following capabilities dynamically and execute them to deploy the shellcode. 
• CreateFile – To learn the shellcode dumped on to disk by the installer. As a part of installer arrange, all of the information seen within the installer archive earlier are dumped on to disk in new listing created in C: drive. 
• VirtualAlloc – To carry the shellcode within the RWX reminiscence. 
• SetFilePointer – To hunt the precise place of the shellcode within the dumped file. 
• ReadFile – To learn the shellcode.  
• EnumResourceTypesA – Execution through callback mechanism. The second parameter is of the sort ENUMRESTYPEPROCA which is just a pointer to a callback routine. The tackle the place the shellcode is allotted within the reminiscence is handed because the second argument to this API resulting in execution of the shellcode. Callback capabilities parameters are good sources for oblique execution of the code.   

Vectored Exception Dealing with in GULoader 

The implementation of the exception dealing with by the Working System supplies a possibility for the adversary to take over execution circulate. The Vectored Exception Dealing with on Home windows supplies the person with capacity to register customized exception handler, which is just a code logic that will get executed on the occasion of an exception. The attention-grabbing factor about dealing with exceptions is that the way in which during which the system resumes its regular execution circulate of this system after the occasion of exception. Adversaries exploit this mechanism and take possession of the execution circulate. Malware can divert the circulate to the code which is underneath its management when the exception happens. Usually it’s employed by the malware to attain following targets: 

• Hooking 
• Covert code execution and anti-analysis 

The GuLoader employs the VEH primarily for obfuscating the execution circulate and to decelerate the evaluation. This part will cowl the internals of Vectored exception dealing with on Home windows and investigates how GUloader is abusing the VEH mechanism to thwart any evaluation efforts.  

• The Vectored Exception Dealing with (VEH) is an extension of Structured Exception Dealing with (SEH) with which we are able to add a vectored exception handler which can be referred to as regardless of of our place in a name body, merely put VEH just isn’t frame-based. 
• VEH is abused by malware, both to control the management circulate or covertly execute person capabilities. 
• Home windows supplies AddVectoredExceptionHandler Win32 API so as to add customized exception handlers. The operate signature is proven under. 

The Handler routine is of the kind PVECTORED_EXCEPTION_HANDLER. Additional checking the documentation, we are able to see the handler operate takes a pointer to _EXCEPTION_POINTERS kind as its enter as proven within the picture under. 

The _EXCEPTION_POINTERS kind holds two essential constructions; PEXCEPTION_RECORD and PCONTEXT. PEXCEPTION_RECORD incorporates all the knowledge associated to exception raised by the system like exception code and so forth. and PCONTEXT construction maintains CPU register (like RIP/EIP, debug registers and so forth.) values or state of the thread captured when exception occurred. 

• This implies the exception handler can entry each ExceptionRecord and ContextRecord. Right here from inside the handler one can tamper with the info saved within the ContextRecord, thus manipulating EIP/RIP to regulate the execution circulate when person utility resumes from exception dealing with.    
• There’s one attention-grabbing factor about exception dealing with, the execution to the applying is given again through NtContinue native routine. Exception dispatch routines name the handler and when handler returns to dispatcher, it passes the ContextRecord to the NtContinue and execution is resumed from the EIP/RIP within the document. On a aspect be aware, that is an oversimplified rationalization of the entire exception dealing with course of. 

Vectored Handler in GULoader 

• GULoader registers a vectored exception handler through RtlAddVectoredExceptionHandler native routine.  The under picture exhibits the management circulate of the handler code. Curiously a lot of the code blocks current listed below are junk added to thwart the evaluation efforts.  

• The GULoader’s handler implementation is as follows (disregarding the junk code). 
• Reads ExceptionInfo handed to the handler by the system. 
• Reads the ExceptionCode from ExceptionRecord construction. 
• Checks the worth of ExceptionCode subject in opposition to the computed exception codes for STATUS_ACCESS_VIOLATION, STATUS_BREAKPOINT and STATUS_SINGLESTEP. 
• Primarily based on the exception code, malware takes a department and executes code that modifies the EIP. 

The GULoader units the entice flag to set off single stepping deliberately to detect evaluation. The handler code will get executed as mentioned earlier than, a block of code is executed based mostly on the exception code. If the exception is single stepping, standing code is 0x80000004, following actions happen:

• The GULoader reads the ContextRecord and retrieves EIP worth of the thread. 
•  Increments the present EIP by 2 and reads the one byte from there. 
• Performs an XOR on the one-byte knowledge fetched from step earlier than and a static worth. The static worth adjustments with samples. In our pattern worth is 0x1A. 
• The XOR’ed worth is then added to the EIP fetched from the ContextRecord. 
• Lastly, the modified EIP worth from prior step is saved within the ContextRecord and returns the management again to the system(dispatcher). 
• The malware has the identical logic for the entry violation exception. 

• When the shellcode is executed with out debugger, INT3 instruction invokes the vectored exception handler routine, with an exception of EXCEPTION_BREAKPOINT, handler computes EIP by incrementing the EIP by 1 and fetching the info from incremented location. Later XORing the fetched knowledge with a continuing in our case 0x1A. The result’s added to present EIP worth. The logic applied for dealing with INT3 exceptions additionally scan this system code for 0xCC directions put by the researchers. If 0xCC are discovered which can be positioned by researchers then EIP just isn’t calculated correctly. 

EIP Calculation Logic Abstract 

*The worth 0x1A adjustments with samples 

Detecting Irregular Execution Movement through VEH 

• The shellcode is structured in such a approach that the malware can detect irregular execution circulate by the order during which exception occurred at runtime. The pushfd/popfd directions are adopted by the code that when executed throws STATUS_ACCESS_VIOLATION. When program is executed usually, the execution is not going to attain the code that follows the pushfd/popfd instruction block, thus elevating solely STATUS_SINGLESTEP. When accidently stepped over the pushfd/popfd block in debugger, the STATUS_SINGLESTEP just isn’t thrown on the debugger because it suppreses this as a result of the debugger is already single stepping by the code, that is detected by the handler logic after we encounter code that follows the pushfd/popfd instruction block wich throws a STATUS_ACCESS_VIOLATION. Now it runs right into a nested exception state of affairs (the entry violation adopted by suppressed single stepping exception through entice). Due to this, at any time when an entry violation happens, the handler routine checks for nested exception info in _EXCEPTION_POINTERS construction as mentioned to start with. 

Beneath picture exhibits this the fastidiously laid out code to detect evaluation. 

The Egg looking: VEH Assisted Runtime Padding 

One attention-grabbing characteristic seen in GULoader shellcode within the wild is runtime padding. Runtime padding is an evasive conduct to beat automated scanners and different safety checks employed at runtime. It delays the malicious actions carried out by the malware on the goal system.  

• The egg worth within the analyzed pattern is 0xAE74B61.  
• It initiates a seek for this worth in its personal knowledge phase of the shellcode. 
• Don’t neglect the truth that that is applied through VEH handler. This search itself provides 0.3 million of VEH iteration on prime of standard VEH management manipulation employed within the code. 
• The loader ends this search when it retrieves the tackle location of the egg worth. To ensure the worth just isn’t being manipulated by any means by the researcher, it performs two further checks to validate the egg location. 
• If the verify fails, the search continues. The method of retrieving the situation of the egg is proven within the picture under.  

• As talked about above, the validity of the egg location is checked by retrieving byte values from two offsets: one is 4 bytes away from the egg location and the worth is 0xB8. The opposite is at 9 bytes from the egg location and the worth is 0xC3. This verify must be handed for the loader to proceed to the subsequent stage of an infection. Core malware actions are carried out after this runtime padding loop. 

 The next photos present the egg location validity checks carried out by GULoader. The values 0xB8 and 0xC3 are checked through the use of correct offsets from the egg location. 

Stage 2: Setting Examine and Code Injection  

Within the second stage of the an infection chain, the GULoader performs anti-analysis and code injection. Main anti-analysis vectors are listed under. After ensuring that shellcode just isn’t working in a sandbox, it proceeds to conduct code injection right into a newly spawned course of the place stage 3 is initiated to obtain and deploy precise payload. This payload will be both commodity stealer or RAT.  

Anti-analysis Strategies  

• Employs runtime padding as mentioned earlier than. 
• Scans entire course of reminiscence for evaluation instrument particular strings 
• Makes use of DJB2 hashing for string checks and dynamic API tackle decision. 
• Strings are decoded at runtime 
• Checks if qemu is put in on the system by checking the set up path: 
• C:Program Informationqqaqqa.exe 
• Patches the next APIs: 
• DbgUIRemoteBreakIn 
• The operate’s prologue is patched with ExitProcess name 
• LdrLoadDll 
• The preliminary bytes are patched with instruction “mov edi edi” 
• DbgBreakPoint 
• Patches with instruction nop 
• Clears hooks positioned in ntdll.dll by safety merchandise or researcher for the evaluation. 
• Window Enumeration through EnumWindows 
• Hides the shellcode thread from the debugger through ZwSetInformationThread by passing 0x11 (ThreadHideFromDebugger) 
• System driver enumeration through EnumDeviceDrivers andGetDeviceDriverBaseNameA 
• Put in software program enumeration through MsiEnumProductsA and MsiGetProductInfoA 
• System service enumeration through OpenSCManagerA and EnumServiceStatusA 
• Checks use of debugging ports by passing ProcessDebugPort (0x7) class to NtQueryInformationProcess 
• Use of CPUID and RDTSC directions to detect digital environments and instrumentation. 

Anti-dump Safety 

Every time GULoader invokes a Win32 api, the decision is sandwiched between two XOR loops as proven within the picture under.  The loop previous to the decision encoded the energetic shellcode area the place the decision is happening to stop the reminiscence from getting dumped by the safety merchandise based mostly on occasion monitoring or api calls. Following the decision, the shellcode area is decoded once more again to regular and resumes execution. The XOR key used is a phrase current within the shellcode itself. 

String Decoding  

This part covers the method undertaken by the GUloader to decode the strings on the runtime. 

• The NtAllocateVirtualMemory is known as to allocate a buffer to carry the encoded bytes. 
• The encoded bytes are computed by performing numerous arithmetic and logical operations on static values embedded as operands of meeting directions. Beneath picture exhibits the restoration of encoded bytes through numerous mathematical and logical operations. The EAX factors to reminiscence buffer, the place computed encoded values get saved. 

The primary byte/phrase is reserved to carry the dimensions of the encoded bytes. Beneath exhibits a 12 byte lengthy encoded knowledge being written to reminiscence. 

Later, the primary phrase will get changed by the primary phrase of the particular encoded knowledge. Beneath picture exhibits the buffer after changing the primary phrase. 

The encoded knowledge is absolutely recovered now, and malware proceeds to decode it. For decoding the easy XOR is employed, and secret’s current within the shellcode. The meeting routine that does the decoding is proven in the picture under. Every byte within the buffer is XORed with the important thing. 

The results of the XOR operation is written to identical reminiscence buffer that holds the encoded knowledge. A last view of the reminiscence buffer with decoded knowledge is proven under. 

The picture exhibits the decoding the string “psapi.dll”, later this string is utilized in fetching the tacklees of numerous capabilities to make use of anti-evaluation.  

The stage 2 culminates in code injection, to be particular GULoader employs a variation of the method hollowing method, the place a benign course of is spawned in a suspended state by the malware stager course of and proceeds to overwrite the unique content material current within the suspended course of with malicious content material, later the state of the thread within the suspended course of is modified by modifying processor register values like EIP and eventually the method resumes its execution. By controlling EIP, malware can now direct the management circulate within the spawned course of to a desired code location. After a profitable hollowing, the malware code can be working underneath the duvet of a legit utility.  

The variation of hollowing method employed by the GULoader doesn’t substitute the file contents, however as a substitute injects the identical shellcode and maps the reminiscence within the suspended course of. Curiously, GULoader employs an extra method if the hollowing try fails. Extra particulars are coated within the following part.  

Listed under Win32 native APIs are dynamically resolved at runtime to carry out the code injection. 

• NtCreateSection 
• ZwMapViewOfSection 
• NtWriteVirtualMemory 
• ZwGetContetThread 
• NtSetContextThread 
• NtResumeThread   

Overview of Code Injection 

• Initially picture “%windirpercentMicrosoft.NETFrameworkversion on 32-bit techniques<model>CasPol.exe” is spawned in suspended mode through CreateProcessInternalW native API. 
• The Gu loader retrieves a deal with to the file “C:WindowsSysWOW64iertutil.dll” which is utilized in part creation. The part object created through NtCreateSection can be backed by iertutil.dll.  
• This conduct is principally to keep away from suspicion, a bit object which isn’t backed by any file could draw undesirable consideration from safety techniques.  
• The subsequent part within the code injection is the mapping of the view created on the part backed by the iertutil.dll into the spawned CasPol.exe course of. As soon as the view is efficiently mapped to the method, malware can inject the shellcode within the mapped reminiscence and resume the method thus initiating stage 3. The native api ZwMapViewOfSection is used to carry out this activity. Following the execution of the above API, the malware checks the results of the operate name in opposition to the under listed error statuses. 
• C0000018 (STATUS_CONFLICTING_ADDRESS) 
• C0000220 (STATUS_MAPPED_ALIGNMENT) 
• 40000003 (STATUS_IMAGE_NOT_AT_BASE). 
• If the mapping is unsuccessful and standing code returned by ZwMapViewOfSection matches with any of the code talked about above, it has a backup plan. 
• The GuLoader calls NtAllocateVirtualMemory by immediately calling the system name stub which is often present in ntdll.dll library to bypass EDR/AV hooks. The reminiscence is allotted within the distant CasPol.exe course of with an RWX reminiscence safety. Following picture exhibits the direct use of NtAllocateVirtualMemory system name. 

After reminiscence allocation, it writes itself into distant course of through NtWriteVirtualMemory as mentioned above. GULoader shellcodes taken from the subject are larger in measurement,  samples taken for this evaluation are all better than 20 mb. In samples analyzed, the buffer measurement allotted to carry the shellcode is 2950000 bytes. The under picture exhibits the GuLoader shellcode within the reminiscence. 

Deceptive Entry level  

• The GULoader is extremely evasive in nature, if irregular execution circulate is detected with assist of employed anti-analysis vectors, the EIP and EBX fields of thread context construction (of CasPol.exe course of) can be overwritten with a decoy tackle, which is required for the stage 3 of malware execution. The placement ebp+4 is used to carry the entry level regardless of of the very fact whether or not program is being debugged or not. 
• The Gu loader makes use of ZwGetContextThread and NtSetContextThread routines to perform modification of the thread state. The CONTEXT construction is retrieved through ZwGetContextThread, the worth [ebp+14C] is used because the entry level tackle. The present EIP worth held within the EIP subject within the context construction of the thread can be modified to a recalculated tackle based mostly on worth at ebp+4. Beneath picture exhibits the RVA calculation.  The bottom tackle of the executing shellcode (stage 2) is subtracted from the digital tackle [ebp+4] to acquire RVA.  

The RVA is added to the base tackle of the newly allotted reminiscence within the CasPol.exe course of to acquire new VA which can be utilized within the distant course of. The brand new VA is written into EIP and EBX subject within the thread context construction of the CasPol.exe course of retrieved through ZwGetContextThread. Beneath picture exhibits the modified context construction and worth of EIP.  

Lastly, by calling ZwSetContextThread, the changes made to the CONTEXT construction is dedicated within the goal thread of CasPol.exe course of. The thread is resumed by calling NtResumeThread. The CasPol.exe resumes execution and performs stage 3 of the an infection chain. 

Stage 3: Payload Deployment  

The GULoader shellcode resumes execution from inside a brand new host course of, on this report, analyzed samples inject the shellcode both into the identical course of spawned as a toddler course of or caspol.exe. Stage3 performs all of the anti-analysis as soon as once more to ensure this stage just isn’t being analyzed. In spite of everything checks, GUloader proceeds to carry out stage3 actions by decoding the encoded C2 string within the reminiscence as proven within the picture under. The decoding methodology is identical as mentioned earlier than. 

Later the addresses of following capabilities are resolved dynamically by loading wininet.dll: 

• InternetSetOptionA 
• InternetOpenUrlA 
• InternetReadFile 
• InternetCloseHandle. 

The under picture exhibits the response from the content material supply community (cdn) server the place the ultimate payload is saved. On this evaluation, a payload of measurement 0x2E640 bytes is shipped to the loader. Curiously, the primary 40 bytes are ignored by the loader. The precise payload begins from the offset 40 which is highlighted within the picture. 

The cdn server is properly protected, it solely serves to shoppers with correct headers and cookies. If these are usually not current within the HTTP request, the next message is proven to the person. 

Last Payload 

Quasi Key Technology 

Step one in decoding the the downloaded last payload by the GUloader is producing a quasi key which can be later utilized in decoding the precise key embeded within the GULoader shellcode. The encoded embeded key measurement is 371 bytes in analysed pattern. The method of quasi key technology is as follows: 

• The 40th and 41st bytes (phrase) are retrived from the obtain buffer within the reminiscence. 
• The above phrase is XORed with the primary phrase of the encoded embeded key alongside and a counter worth. 
• The method is repeated untill the the phrase taken from the downloaded knowledge absolutely decodes and have a worth of 0x4D5A “MZ”. 
• The worth current within the counter when the 4D5A will get decoded is taken because the quasi key. This secret’s proven as “key-1” within the picture under. Within the analysed pattern the worth of this secret’s “0x5448” 

Decoding Precise Key 

The embedded key within the GULoader shellcode is of the dimensions 371 bytes as mentioned earlier than. The quasi secret’s used to decode the embeded key as proven within the picture under. 

• Every phrase within the embeded secret’s XORed with quasi key key-1. 
• When the interation counter exceeds the dimensions worth of 371 bytes, it stops and proceeds to decode the downloaded payload with this new key. 

The decoded 371 bytes of embeded secret’s proven under within the picture under. 

Decoding File 

A byte stage decoding occurs after embeded secret’s decoded within the reminiscence. Every byte of the downloaded knowledge is XORed with the important thing to acquire the precise knowledge, which is a PE file. The decoded knowledge is overwritten to the identical buffer used to obtain the decoded knowledge. 

The ultimate decoded PE file residing within the reminiscence is proven within the picture under: 

Lastly, the loader masses the PE file by allocating the reminiscence with RWX permission within the stage3 course of, based mostly on analyzing a number of samples it’s both the identical course of in stage 2 because the baby course of, or casPol.exe. The loading concerned code relocation and IAT correction as anticipated in such a state of affairs. The ultimate payload resumes execution from inside the hollowed stage3 course of. Beneath malware households are often seen deployed by the GULoader: 

• Vidar (Stealer) 
• Raccoon (Stealer) 
• Remcos RAT 

Beneath picture exhibits the injected reminiscence areas in stage3 course of caspol.exe on this report. 

Conclusion  

The position performed by malware loaders popularly generally known as “crypters” is important within the deployment of Distant Administration Instruments and stealer malwares that concentrate on client knowledge. The exfiltrated Private Identifiable Info (PII) extracted from the compromised endpoints are largely collected and funneled to numerous underground knowledge promoting marketplaces. This additionally impacts companies as numerous vital info used for authentication functions are getting leaked from the private techniques of the person resulting in preliminary entry on the corporate networks. The GuLoader is closely utilized in mass malware campaigns to contaminate the customers with standard stealer malware like Raccoon, Vidar, and Redline. Commodity RATs like Remcos are additionally seen delivered in such marketing campaign actions. On the intense aspect, it’s not tough to fingerprint malware specimens used within the mass campaigns due to the quantity its quantity and relevance, detection guidelines and techniques will be constructed round this actual fact. 

Following desk summarizes all of the dynamically resolved Win32 APIs  

IOC 

889fddcb57ed66c63b0b16f2be2dbd7ec0252031cad3b15dfea5411ac245ef56 

59b71cb2c5a14186a5069d7935ebe28486f49b7961bddac0a818a021373a44a3 

4d9cdd7526f05343fda35aca3e0e6939abed8a037a0a871ce9ccd0e69a3741f2 

c8006013fc6a90d635f394c91637eae12706f58897a6489d40e663f46996c664 

c69e558e5526feeb00ab90efe764fb0b93b3a09692659d1a57c652da81f1d123 

45156ac4b40b7537f4e003d9f925746b848a939b2362753f6edbcc794ea8b36a 

e68ce815ac0211303d2c38ccbb5ccead144909d295230df4b7a419dfdea12782 

b24b36641fef3acbf3b643967d408b10bf8abfe1fe1f99d704a9a19f1dfc77e8 

569aa6697083993d9c387426b827414a7ed225a3dd2e1e3eba1b49667573fdcb 

60de2308ebfeadadc3e401300172013be27af5b7d816c49696bb3dedc208c54e 

23458977440cccb8ac7d0d05c238d087d90f5bf1c42157fb3a161d41b741c39d 

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