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BokuLoader – A Proof-Of-Concept Cobalt Strike Reflective Loader Which Aims To Recreate, Integrate, And Enhance Cobalt Strike’s Evasion Features!

A proof-of-concept User-Defined Reflective Loader (UDRL) which aims to recreate, integrate, and enhance Cobalt Strike’s evasion features!


UDRL Usage Considerations

The built-in Cobalt Strike reflective loader is robust, handling all Malleable PE evasion features Cobalt Strike has to offer. The major disadvantage to using a custom UDRL is Malleable PE evasion features may or may not be supported out-of-the-box.

The objective of the public BokuLoader project is to assist red teams in creating their own in-house Cobalt Strike UDRL. The project aims to support all worthwhile CS Malleable PE evasion features. Some evasion features leverage CS integration, others have been recreated completely, and some are unsupported.

Before using this project, in any form, you should properly test the evasion features are working as intended. Between the C code and the Aggressor script, compilation with different versions of operating systems, compilers, and Java may return different results.

Evasion Features

BokuLoader Specific Evasion Features

  • Reflective callstack spoofing via synthetic frames.
  • Custom ASM/C reflective loader code
  • Indirect NT syscalls via HellsGate & HalosGate techniques
  • All memory protection changes for all allocation options are done via indirect syscall to NtProtectVirtualMemory
  • obfuscate "true" with custom UDRL Aggressor script implementation.
  • Loader will not copy headers raw beacon DLL to virtual beacon DLL. First 0x1000 bytes will be nulls.
  • XGetProcAddress for resolving symbols
  • Does not use Kernel32.GetProcAddress
  • xLoadLibrary for resolving DLL’s base address & DLL Loading
  • For loaded DLLs, gets DLL base address from TEB->PEB->PEB_LDR_DATA->InMemoryOrderModuleList
  • Does not use Kernel32.LoadLibraryA
  • Caesar Cipher for string obfuscation
  • 100k UDRL Size
  • Import DLL names and import entry name strings are stomped in virtual beacon DLL.

Supported Malleable PE Evasion Features

allocatorHeapAlloc, MapViewOfFile, VirtualAllocAll supported via BokuLoader implementation
module_x64string (DLL Name)Supported via BokuLoader implementation. Same DLL stomping requirements as CS implementation apply
obfuscatetrue/falseHTTP/S beacons supported via BokuLoader implementation. SMB/TCP is currently not supported for obfuscate true. Details in issue. Accepting help if you can fix 🙂
entry_pointRVA as decimal numberSupported via BokuLoader implementation
cleanuptrueSupported via CS integration
userwxtrue/falseSupported via BokuLoader implementation
sleep_mask(true/false) or (Sleepmask Kit+true)Supported. When using default “sleepmask true” (without sleepmask kit) set “userwx true”. When using sleepmask kit which supports RX beacon.text memory (src47/Ekko) set “sleepmask true” && “userwx false”.
magic_mz_x644 char stringSupported via CS integration
magic_pe2 char stringSupported via CS integration
transform-x64 prependescaped hex stringBokuLoader.cna Aggressor script modification
transform-x64 strrepstring stringBokuLoader.cna Aggressor script modification
stomppetrue/falseUnsupported. BokuLoader does not copy beacon DLL headers over. First 0x1000 bytes of virtual beacon DLL are 0x00
checksumnumberExperimental. BokuLoader.cna Aggressor script modification
compile_timedate-time stringExperimental. BokuLoader.cna Aggressor script modification
image_size_x64decimal valueUnsupported
namestringExperimental. BokuLoader.cna Aggressor script modification
rich_headerescaped hex stringExperimental. BokuLoader.cna Aggressor script modification


Project Origins


  1. Compile the BokuLoader Object file with make
  2. Start your Cobalt Strike Team Server
  3. Within Cobalt Strike, import the BokuLoader.cna Aggressor script
  4. Generate the x64 beacon (Attacks -> Packages -> Windows Executable (S))
  5. Use the Script Console to ensure BokuLoader was implemented in the beacon build

  6. Does not support x86 option. The x86 bin is the original Reflective Loader object file.

  7. Generating RAW beacons works out of the box. When using the Artifact Kit for the beacon loader, the stagesize variable must be larger than the default.
  8. See the Cobalt Strike User-Defined Reflective Loader documenation for additional information

Detection Guidance

Hardcoded Strings

  • BokuLoader changes some commonly detected strings to new hardcoded values. These strings can be used to signature BokuLoader:
Original Cobalt Strike StringBokuLoader Cobalt Strike String
Microsoft Base Cryptographic Provider v1.012367321236742382543232341241261363163151d

Memory Allocators

DLL Module Stomping

  • The Kernel32.LoadLibraryExA is called to map the DLL from disk
  • The 3rd argument to Kernel32.LoadLibraryExA is DONT_RESOLVE_DLL_REFERENCES (0x00000001)
  • the system does not call DllMain
  • Does not resolve addresses in LDR PEB entry as detailed by MDSec here
  • Detectable by scanning process memory with pe-sieve tool

Heap Allocation

  • Executable RX or RWX memory will exist in the heap if sleepmask kit is not used.

Mapped Allocator

  • The Kernel32.CreateFileMappingA & Kernel32.MapViewOfFile is called to allocate memory for the virtual beacon DLL.

Sleepmask Detection

Indirect Syscalls

  • BokuLoader calls the following NT systemcalls to setup the loaded executable beacon memory: NtAllocateVirtualMemory, NtProtectVirtualMemory
  • These are called indirectly from the BokuLoader executable memory.
  • Setting userland hooks in ntdll.dll will not detect these systemcalls.
  • It may be possible to register kernelcallbacks using a kernel driver to monitor for the above system calls and detect their usage.
  • The BokuLoader itself will contain the mov eax, r11d; mov r11, r10; mov r10, rcx; jmp r11 assembly instructions within its executable memory.

Virtual Beacon DLL Header

  • The first 0x1000 bytes of the virtual beacon DLL are zeros.

Source Code Available

  • The BokuLoader source code is provided within the repository and can be used to create memory signatures.
  • If you have additional detection guidance, please feel free to contribute by submitting a pull request.

Credits / References

Reflective Call Stack Spoofing

Reflective Loader

HalosGate SysCaller

  • Reenz0h from @SEKTOR7net
  • Checkout Reenz0h’s awesome courses and blogs!
  • Best classes for malware development I have taken.
  • Creator of the halos gate technique. His work was initially the motivation for this work.
  • Sektor7 HalosGate Blog

HellsGate Syscaller

Aggressor Scripting

Cobalt Strike User Defined Reflective Loader


Great Resource for learning Intel ASM

ETW and AMSI Bypass

Implementing ASM in C Code with GCC


Cobalt Strike C2 Profiles

Source : KitPloit – PenTest Tools!

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