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

testv October 5, 2024

0 0 4 minutes read

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

Contributors:

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

Project Origins

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

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