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Introduction:
Classic direct syscall techniques such as Hell’s Gate and Halo’s Gate allow malware to invoke kernel system calls without touching user‑mode API hooks, but their static stub patterns (e.g., 4C 8B D1 B8 ?? ?? ?? ?? 0F 05) are highly predictable and easily flagged by YARA rules. YetAnotherGate solves this fatal flaw by introducing a polymorphic syscall engine that takes clean assembly and intentionally “de‑optimizes” it – making the stub longer, messier, and signature‑free while preserving the exact same CPU behavior, effectively evading modern EDRs.
Learning Objectives:
- Understand the structural weaknesses of static syscall stubs and how YARA rules detect them.
- Learn how assembly de‑optimization and polymorphism break signature‑based detection.
- Implement and customize YetAnotherGate to generate unique, runtime‑variant syscall stubs for red teaming.
You Should Know:
- The Anatomy of a Static Syscall Stub and Its YARA Signature
Classic direct syscall stubs follow a predictable pattern: move the syscall number into `eax` (or rax), set up arguments in other registers, and execute `syscall` (or int 2e). This uniformity allows defenders to write YARA rules that match the byte sequence. For example, a typical Hell’s Gate stub might look like this in assembly:
mov r10, rcx mov eax, <syscall_number> syscall ret
The corresponding YARA rule to catch such stubs is straightforward:
rule HellGateStub {
strings:
$sig = { 4C 8B D1 B8 [bash] 0F 05 }
condition:
$sig
}
YetAnotherGate’s polymorphism breaks this by inserting junk instructions, reordering operations where safe, and using different register allocations each time. Instead of a fixed pattern, the engine generates code like:
push rbx mov r10, rcx xor rbx, rbx mov eax, 0x12345678 add eax, 0x11111111 ; syscall number computed at runtime shl rbx, 2 syscall pop rbx ret
Step‑by‑step guide to identifying a static stub:
- Compile a simple program using inline assembly with a fixed syscall number.
- Dump the binary with `objdump -d` (Linux) or `dumpbin /disasm` (Windows).
- Extract the byte sequence around the `syscall` instruction.
- Use `yara64.exe -s rule.yara binary.exe` to see if it matches.
2. Deconstructing YetAnotherGate’s Polymorphic Engine
YetAnotherGate operates by taking a “clean” reference stub and passing it through an assembly de‑optimizer. The engine randomly applies transformations: inserting `nop` or `xchg` instructions, replacing immediate moves with arithmetic sequences, splitting register loads, and adding conditional jumps that always take the same branch. The result is a functionally identical stub with a completely different byte signature.
Step‑by‑step workflow of the engine:
- Input: A standard syscall stub (e.g., Hell’s Gate template).
- Parsing: The engine tokenizes the assembly instructions (using a lightweight disassembler like Zydis or BeaEngine).
- Transformation: For each instruction, the engine randomly decides to:
– Insert 1–3 garbage instructions (e.g., push rax; pop rax).
– Replace `mov eax, imm32` with xor eax, eax; add eax, imm32.
– Interleave unrelated register operations.
4. Linking: The transformed instructions are assembled back into machine code.
5. Output: A unique stub is generated per call.
To see this in action, clone the repository:
git clone https://github.com/Oorth/YetAnotherGate cd YetAnotherGate
Build the PoC using Visual Studio (open the `.sln` file) and run the executable. Each execution prints a differently encoded syscall stub.
- Building Your Own Polymorphic Syscall Stub (Windows + Linux Examples)
While YetAnotherGate is Windows‑focused (syscalls on x64), the concept applies equally to Linux `int 0x80` or `syscall` invocations. Below is a manual approach to generating a simple polymorphic syscall on Linux that prints “hello” using write.
Linux example (assembly de‑optimization):
Original `write` stub:
mov rax, 1 ; sys_write mov rdi, 1 ; stdout mov rsi, msg mov rdx, len syscall
De‑optimized version (randomized each run via a script):
push rbx mov rax, 0xFFFFFFFF xor rax, 0xFFFFFFFE ; rax = 1 mov rdi, 5 sub rdi, 4 ; rdi = 1 lea rsi, [rip + msg] mov rdx, 6 nop syscall pop rbx
Windows command to compile a custom stub (using MASM or cl.exe):
cl /c /Fa stub_asm.asm link stub_asm.obj /out:stub.exe
To automate polymorphism, write a Python script that uses `keystone-engine` to assemble random variants:
from keystone import
import random
ks = Ks(KS_ARCH_X86, KS_MODE_64)
syscall_num = 0x100
variants = [
f"mov eax, {syscall_num}; syscall",
f"xor eax, eax; add eax, {syscall_num}; syscall",
f"push rbx; mov eax, {syscall_num}; pop rbx; syscall"
]
code, _ = ks.asm(random.choice(variants))
with open("stub.bin", "wb") as f: f.write(bytes(code))
4. Evading EDRs: Testing YetAnotherGate Against Common Hooks
Most EDRs place hooks at the beginning of ntdll.dll functions (e.g., NtCreateFile). Direct syscalls bypass those hooks, but static stubs are still caught by memory scanning (YARA, Sigma). YetAnotherGate defeats this because the stub changes on every execution – even within the same process.
Step‑by‑step test using API Monitor and custom YARA:
- Generate a static stub (from Hell’s Gate) and a polymorphic stub (from YetAnotherGate).
- Write a YARA rule that matches the static stub’s signature.
- Scan both binaries with `yara64.exe rule.yara static.exe` – the static one triggers, the polymorphic one does not.
- Run each binary under API Monitor (filter on `syscall` events) – both show the same system call invocations, proving functional equivalence.
- Deploy an EDR simulator like `EDR_Check` – the polymorphic stub remains undetected while the static stub is flagged.
Windows command to scan a directory with YARA:
for %f in (.exe) do yara64.exe myrule.yara %f
5. Mitigations for Blue Teams: Detecting Polymorphic Syscalls
Polymorphism defeats static signatures, but advanced detection is still possible. Blue teams can focus on behavioral and heuristic indicators rather than byte patterns.
Techniques to detect polymorphic syscall engines:
- Call stack analysis – A direct syscall from non‑ntdll memory (e.g., from a private executable region) is suspicious.
- Syscall frequency & arguments – Unusual combinations or high rates of sensitive syscalls (e.g.,
NtOpenProcess,NtAllocateVirtualMemory). - Instruction entropy – Polymorphic stubs often contain high entropy due to junk instructions. Measure Shannon entropy of code sections.
- Hardware breakpoints – Monitor the `syscall` instruction address via `Dr0-Dr3` registers.
Example PowerShell command to check for syscalls originating outside ntdll (basic):
Get-Process | ForEach-Object {
$modules = $_ | Get-Process -Module
$ntdllBase = ($modules | Where-Object {$_.ModuleName -eq "ntdll.dll"}).BaseAddress
This is a simplified check – full implementation requires a kernel driver
}
For robust detection, deploy a minifilter driver that inspects the return address on every `syscall` and validates it belongs to ntdll.
6. Integrating YetAnotherGate into Red Team Tooling
To use YetAnotherGate in your own red team payloads, treat the engine as a code generator. At compile time, invoke the engine to produce a unique stub, then embed that stub into your malware. For runtime polymorphism, generate a fresh stub every few minutes and hot‑patch the running code.
Step‑by‑step integration:
- Compile YetAnotherGate as a static library or use its header‑only generator.
- In your C++ code, call `GeneratePolymorphicStub(syscall_number)` before invoking the syscall.
- Store the stub in a `std::vector
` and execute via a function pointer (ensure `PAGE_EXECUTE_READWRITE` memory). - Optional – on‑disk evasion: Use the stub immediately and wipe it from memory after use.
Example snippet:
include "YetAnotherGate.h" auto stub = YetAnotherGate::GenerateStub(0x3A); // NtCreateFile auto pStub = VirtualAlloc(NULL, stub.size(), MEM_COMMIT, PAGE_EXECUTE_READWRITE); memcpy(pStub, stub.data(), stub.size()); ((void()())pStub)(); // invoke VirtualFree(pStub, 0, MEM_RELEASE);
7. Future of Syscall Evasion: AI‑Generated Stubs
The next frontier is using generative models (small LLMs or reinforcement learning) to produce syscall stubs that not only bypass static signatures but also mimic legitimate call patterns to evade behavioral analysis. AI could learn to insert instructions that resemble common compiler output, lowering entropy and avoiding heuristic flags. YetAnotherGate already provides a foundation – the next step is to replace random junk with semantically neutral but plausible instructions.
Proof‑of‑concept idea: Fine‑tune a small GPT model on x64 assembly from ntdll and system libraries. Prompt it with “generate a syscall stub for NtReadFile that looks like normal compiler output but still works as a direct syscall.” The model would output unique, realistic stubs on demand.
What Undercode Say:
- Static signatures are dead – YetAnotherGate proves that any fixed byte pattern can be obfuscated while retaining full functionality. Red teams should adopt polymorphic syscall engines as a baseline.
- Behavioral detection is the new battleground – Blue teams must shift from signature matching to context‑aware monitoring: call stack origin, syscall frequency, and argument anomalies. Simple YARA rules are no longer sufficient against even moderately skilled adversaries.
- Open‑source innovation drives both offense and defense – The release of YetAnotherGate on GitHub allows defenders to study and build countermeasures, accelerating the cat‑and‑mouse cycle. Expect EDR vendors to quickly add heuristics for high‑entropy code regions and non‑ntdll syscall origins.
Prediction:
Within 12 months, most major EDRs will deploy machine learning models that detect “artificial” instruction sequences – such as the de‑optimized stubs generated by YetAnotherGate – by comparing them to natural compiler output. In response, the next generation of syscall engines will use generative AI to produce stubs that are indistinguishable from legitimate ntdll code. This will shift the arms race entirely into the behavioral realm, where syscall frequency, timing, and argument semantics become the primary detection signals, and kernel‑callbacks will replace user‑mode hooks as the last line of defense.
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IT/Security Reporter URL:
Reported By: Arth Maurya – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
