A proof-of-concept User-Defined Reflective Loader (UDRL) which aims to recreate, integrate, and enhance Cobalt Strike’s evasion features!
Contributors:
Contributor Twitter Notable Contributions Bobby Cooke @0xBoku Project original author and maintainer Santiago Pecin @s4ntiago_p Reflective Loader major enhancements Chris Spehn @ConsciousHacker Aggressor scripting Joshua Magri @passthehashbrwn IAT hooking Dylan Tran @d_tranman Reflective Call Stack Spoofing James Yeung @5cript1diot Indirect System Calls
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. NOHEADERCOPY 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
Command Option(s) Supported allocator HeapAlloc, MapViewOfFile, VirtualAlloc All supported via BokuLoader implementation module_x64 string (DLL Name) Supported via BokuLoader implementation. Same DLL stomping requirements as CS implementation apply obfuscate true/false HTTP/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_point RVA as decimal number Supported via BokuLoader implementation cleanup true Supported via CS integration userwx true/false Supported 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_x64 4 char string Supported via CS integration magic_pe 2 char string Supported via CS integration transform-x64 prepend escaped hex string BokuLoader.cna Aggressor script modification transform-x64 strrep string string BokuLoader.cna Aggressor script modification stomppe true/false Unsupported. BokuLoader does not copy beacon DLL headers over. First 0x1000 bytes of virtual beacon DLL are 0x00 checksum number Experimental. BokuLoader.cna Aggressor script modification compile_time date-time string Experimental. BokuLoader.cna Aggressor script modification image_size_x64 decimal value Unsupported name string Experimental. BokuLoader.cna Aggressor script modification rich_header escaped hex string Experimental. BokuLoader.cna Aggressor script modification stringw string Unsupported string string Unsupported
Test
(2/22/23) All 4 allocator methods tested with threatexpress/malleable-c2/master/jquery-c2.4.7.profile
Project Origins
Based on Stephen Fewer’s incredible Reflective Loader project: https://github.com/stephenfewer/ReflectiveDLLInjection Initially created while working through Renz0h’s Reflective DLL videos from the Sektor7 Malware Developer Intermediate (MDI) Course
Usage
Compile the BokuLoader Object file with make Start your Cobalt Strike Team Server Within Cobalt Strike, import the BokuLoader.cna Aggressor script Generate the x64 beacon (Attacks -> Packages -> Windows Executable (S))
Use the Script Console to ensure BokuLoader was implemented in the beacon build
Does not support x86 option. The x86 bin is the original Reflective Loader object file.
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. 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 String BokuLoader Cobalt Strike String ReflectiveLoader BokuLoader Microsoft Base Cryptographic Provider v1.0 12367321236742382543232341241261363163151d (admin) (tomin) beacon bacons
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
If sleepmask kit is used, there exists detection methods for this independent memory allocation as detailed by MDSec here
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
LoudSunRun: Call stack synthetic frame spoofer with indirect syscalls SilentMoonwalk: PoC Implementation of a fully dynamic call stack spoofer Vulcan Raven: PoC implementation for spoofing arbitrary call stacks when making syscalls CallStackMasker: A PoC implementation for dynamically masking call stacks with timers ThreadStackSpoofer: Thread Stack Spoofing PoC Behind the Mask: Spoofing Call Stacks Dynamically with Timers
Reflective Loader
https://github.com/stephenfewer/ReflectiveDLLInjection Checkout these videos if you’re interested in Reflective DLL: Dancing with Import Address Table (IAT) – Sektor 7 MDI Course Walking through Export Address Table – Sektor 7 MDI Course Reflective Injection Explained – Sektor 7 MDI Course ReflectiveLoader source review – Sektor 7 MDI Course TitanLdr AceLdr KaynLdr
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
@smelly__vx & @am0nsec ( Creators/Publishers of the Hells Gate technique ) Could not have made my implementation of HellsGate without them 🙂 Awesome work on this method, really enjoyed working through it myself. Thank you! https://github.com/am0nsec/HellsGate Link to the Hell’s Gate paper: https://vxug.fakedoma.in/papers/VXUG/Exclusive/HellsGate.pdf
Aggressor Scripting
sinusoid @the_bit_diddler : returnIntegerFromArray
Cobalt Strike User Defined Reflective Loader
https://www.cobaltstrike.com/help-user-defined-reflective-loader
Great Resource for learning Intel ASM
ETW and AMSI Bypass
@anthemtotheego inline-ExecuteAssembly @mariuszbit – for awesome idea to implement bypasses in reflective loader! @XPN Hiding Your .NET – ETW ajpc500/BOFs Offensive Security OSEP
Implementing ASM in C Code with GCC
https://outflank.nl/blog/2020/12/26/direct-syscalls-in-beacon-object-files/ https://www.cs.uaf.edu/2011/fall/cs301/lecture/10_12_asm_c.html http://gcc.gnu.org/onlinedocs/gcc-4.0.2/gcc/Extended-Asm.html#Extended-Asm
Cobalt Strike C2 Profiles
Tylous’s epic SourcePoint project threatexpress/malleable-c2/jquery-c2.4.7.profile
