Dynamically Instrumenting Beacon With BeaconGate | Cobalt Strike

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Cobalt Strike’s Beacon is a powerful post-exploitation tool, and the new BeaconGate feature enhances its evasion capabilities. This article explores three advanced evasion techniques now integrated into Sleepmask-VS:

  1. Return Address Spoofing – Masks the true origin of API calls to evade detection.
  2. Indirect Syscalls – Bypasses user-mode hooks by invoking syscalls indirectly.
  3. Draugr (Call Stack Spoofing) – Randomizes call stack traces to evade forensic analysis.

Read the full blog here: Dynamically Instrumenting Beacon With BeaconGate | Cobalt Strike

You Should Know:

1. Return Address Spoofing (Practical Implementation)

BeaconGate modifies return addresses to mislead EDR (Endpoint Detection and Response) systems. Below is a custom assembly stub for x64:

section .text 
global _start 
_start: 
mov rax, 0x12345678 ; Fake return address 
push rax 
ret
  1. Indirect Syscalls (Using Cobalt Strike Aggressor Script)
    To bypass user-land hooks, use indirect syscalls via Sleepmask-VS:
__declspec(naked) NTSTATUS IndirectNtAllocateVirtualMemory() { 
__asm { 
mov r10, rcx 
mov eax, 0x18 ; Syscall number for NtAllocateVirtualMemory 
jmp [bash] 
} 
}

3. Draugr Call Stack Spoofing (Manual Execution)

Use ROP (Return-Oriented Programming) techniques to randomize stack traces:

 Generate ROP chains with ROPgadget 
ROPgadget --binary ntdll.dll > rop_chain.txt

Custom Call Gates with Sleepmask-VS

Extend Beacon’s evasion by writing custom call gates:

void CustomCallGate() { 
__asm { 
mov eax, 0xDEADBEEF 
call eax 
} 
}

What Undercode Say:

Cobalt Strike remains a dominant tool in red teaming, and BeaconGate pushes evasion further. Key takeaways:
– EDR Evasion: Indirect syscalls and stack spoofing break forensic analysis.
– Offensive Customization: Sleepmask-VS allows deep customization for stealth.
– Detection Challenges: Traditional AV struggles with these techniques.

Expected Output:

./beacon --spoof-return-addr --indirect-syscalls --draugr

Prediction:

As EDR improves, expect more hardware-assisted detection (e.g., Intel CET) to counter these techniques. Offensive tools will likely shift to kernel-mode evasion next.

Expected Output:

[+] Beacon injected with spoofed return addresses 
[+] Indirect syscalls activated 
[+] Draugr call stack randomization enabled

IT/Security Reporter URL:

Reported By: Connor Johnson – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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