Stack Spoofing Dev

Stack Spoofing — Windows x64

Produce a spoofed call stack that survives unwinder-based inspection (ETW-TI, EDR stack walkers, StackWalk64). Each frame must have a legitimate .pdata entry, an unwind description that matches the planted frame size, and a return address that points inside a known module's .text.

This skill assumes you already understand .pdata / UNWIND_INFO at the level described in windows-internals/references/exception-unwind.md. It focuses on implementing the spoofer, not on teaching the format.

When to activate

• Implementing or reviewing Draugr / SilentMoonwalk / NtContinue / YouMayPasser / VulcanRaven / Unwinder spoofers in C, C++, Rust, Go, or Plan9 ASM
• Choosing between spoof strategies for a specific Windows build or thread context (main, TP worker, alertable, console-attached)
• Debugging spoof_init: FAIL jmp_rbx or unwinder-reported frame-size mismatches
• Adjusting MinJmpRbxFrameSize / MinAddRspX thresholds after .pdata inventory changes across Windows builds
• Integrating a spoofer with an indirect syscall dispatcher (RecycleGate / Hell's / FreshyCalls)
• Hardening a pre-existing spoofer against modern EDR correlation (Eclipse, SAVE_NONVOL safety, backed-vs-unbacked caller)
• Porting a spoofer between languages without breaking the ASM/context-struct contract

If the question is "what does UNWIND_INFO look like" → wrong skill, read windows-internals/references/exception-unwind.md. If the question is "how do I make NtWriteVirtualMemory appear to come from RtlUserThreadStart" → right skill.

The three strategies, side by side

Default choice: Draugr if you control the thread (main thread of an EXE, or explicit CreateThread with known root). SilentMoonwalk if you run on thread pool workers or need .pdata-coherent frames all the way down.

Notable implementations and variants

The strategies above are conceptual; below are the public PoCs/implementations you will encounter in the wild. Each is a concrete realization (or precursor) of one of the three strategies, with its own quirks.

YouMayPasser (Waldo-irc) — return-address-only minimalist baseline

64-bit weaponization of Gargoyle that extends Namaszo's original Return Address Spoofing PoC. Targets Cobalt Strike beacons. Spoofs only the immediate return address of the calling function — not a full multi-frame chain — so it is the cheapest stack-masquerading primitive.

• Strategy mapping: precursor / strict subset of Draugr (1 frame, 1 gadget).
• When to pick: BOF or short-lived primitive where one syscall's caller-frame must be hidden and a full Draugr chain is overkill.
• Caveat: hardcoded gadget offsets per Windows build — you must re-tune per build, exactly as Win11 22H2+ measurements above warn for any JMP [RBX] consumer. Walks the same gadget cliffs as Draugr.

VulcanRaven — template-based stack mimicry with VEH cleanup

Spoofs the call stack by mirroring a real captured stack from telemetry (SysMon ProcessAccess on lsass), shipping with three example profiles selected via --wmi, --rpc, --svchost. Each profile is a captured frame chain of a legitimate Windows service path; the spoofer reproduces it byte-for-byte before issuing NtOpenProcess.

• Strategy mapping: orthogonal to Draugr/SilentMoonwalk — instead of computing a generic plausible chain, it copies a specific real one. Fewer correlation surface marks because the chain came from real telemetry.
• VEH twist: registers a vectored exception handler before resuming the spoofed thread; on access violation it redirects to RtlExitUserThread so the thread terminates cleanly rather than crashing the host. Adopt this pattern any time you mutate CONTEXT.Rsp and cannot guarantee the planted chain unwinds correctly.
• When to pick: targeted credential-access flows where you want the call chain to match a known-good svchost/RPC/WMI invocation rather than merely look generic.
• Limit: the captured chain ages — re-collect SysMon templates after major Windows feature updates or you start mimicking a chain that no longer exists in production.

Unwinder (Kudaes) — Rust weaponization of SilentMoonwalk

Rust crate (unwinder on crates.io) implementing full SilentMoonwalk DESYNC with stable, idiomatic Rust ergonomics. Supports calling arbitrary functions or indirect syscalls with up to 11 parameters, retrieves return values, and the spoof can be chained any number of times without growing the call stack (frames are recycled per call).

• Strategy mapping: SilentMoonwalk DESYNC (4 frames, JMP[RBX] + ADD RSP,X), Rust-native.
• When to pick: Rust implants where you want SilentMoonwalk without rolling your own global_asm! trampoline. Treat it as the canonical Rust answer to the lang-c-rust-go reference's SilentMoonwalk slot.
• Caveat: still subject to all the Win11 22H2+ gadget-population limits. Cascade module ordering is internal to the crate — read its source before assuming wininet → user32 → kernelbase is wired the way you want.

Decision tree

Implant runs on...
│
├── Main thread of a dedicated loader EXE?
│   └── Draugr (simplest, fewest gadgets, zero Eclipse concerns)
│
├── Thread pool worker (TpWorkCallback, timer, TP_IO)?
│   └── SilentMoonwalk DESYNC — only strategy with .pdata-coherent frames
│                               beyond BaseThreadInitThunk/RtlUserThreadStart
│
├── Beacon in a module-stomped host (rundll32, legitimate PE)?
│   └── SilentMoonwalk DESYNC or NtContinue — Draugr's assumption
│       "this thread was started by RtlUserThreadStart" does not hold
│
├── Single one-shot syscall with console attached?
│   └── Indirect syscall only (skip spoofing) — NtContinue races console
│
└── Need template-based mimicry of a real process's stack (e.g. svchost/RPC/WMI)?
    └── VulcanRaven — synthetic stack mirroring a captured SysMon profile, VEH-based cleanup

Frame math — the numbers you actually need

These are the non-negotiable sizing rules. Full derivation in references/frame-math.md.

Minimum frame sizes for JMP [RBX] gadget

The trampoline frame must hold the shadow area (0x20) plus all stack args of the syscall you are spoofing. For NT syscalls:

Classical Draugr literature uses 0xD8 as a "safe for everything" floor. This is wrong on Windows 11 22H2+: kernelbase.dll has had its FF 23 gadget population drastically reduced and often exposes no gadget with frame ≥ 0xD8. Use the real minimum for your specific syscall.

Rule: compute shadow (0x20) + args_on_stack * 8 + padding (0x08) and use that as your min_frame. For the common NtCreateThreadEx(11) path, 0x60 is correct.

Windows 11 22H2+ field measurements (kernelbase.dll)

Implication: hardcoded 0xD8 breaks. Eclipse from kernelbase alone is infeasible. Cascade wininet → user32 → kernelbase is the correct strategy; or accept the lower threshold and drop Eclipse.

Minimum ADD RSP,X;RET (SilentMoonwalk only)

X must be larger than the JMP [RBX] trampoline's frame size, so arg slots placed at [SP+0x28..SP+0x90] within the AddRspX frame never collide with the JmpRbxGadget word written at [SP + 8 + X].

Rule: min_x = max(jmp_rbx_frame_size, MIN_FLOOR) where MIN_FLOOR = 0x60 on Win11 22H2+ (was 0xB0 on Win10).

UNWIND_INFO safety filters

Reject any candidate where calc_frame_size returns 0. Causes:

• No .pdata entry (leaf function)
• UWOP_SAVE_NONVOL / UWOP_SAVE_NONVOL_FAR with save_offset >= total_alloc → writes past frame → stack corruption when used as spoof frame
• UWOP_SAVE_XMM128 present — spoof does not preserve XMM regs; executing the real unwinder on this function causes a #UD when unwinding saved XMM

See references/frame-math.md for the full calc_frame_size algorithm including chained unwind info (UNW_FLAG_CHAININFO) handling.

Gadget scanner — non-negotiable rules

1. Scan .text of the target module only. Never scan .rdata; byte sequence FF 23 occurs in data.
2. Match byte[i] == 0xFF && byte[i+1] == 0x23 for JMP [RBX]. This is a 2-byte opcode with no REX prefix.
3. For each hit, compute frame_size via .pdata binary search. Reject if 0.
4. If Eclipse required: check byte[gadget - 5] == 0xE8 (CALL rel32). Do not check 0x41 FF D_ or other CALL variants — callsite validation in Eclipse papers specifically relies on the 5-byte E8 displacement CALL.
5. Deterministic selection: pick the largest frame_size that passes filters. Random selection makes failure modes unreproducible.
6. Emit diagnostic counters on failure (FF23_total, fs_zero, below_min, eclipse_fail, best_belowmin_fs/addr). Without these, kernelbase-has-no-gadgets failures look identical to bad-threshold failures.

Full scanner pseudocode + instrumentation patterns in references/frame-math.md.

Trampoline contract (all languages)

Every spoofer expresses the same contract between a high-level caller and a small ASM trampoline:

Caller (C / Rust / Go):
  1. Resolve: module bases, function retaddrs, gadget(s), frame sizes
  2. Populate a fixed-layout SpoofContext struct
  3. Pre-allocate a spoofing buffer (heap-safe; see below)
  4. Call ASM trampoline: (ssn, syscall_ret_addr, &ctx, args...)

ASM trampoline:
  1. Save callee-saved (RBX, RBP, R12–R15, XMM6–15 if used)
  2. Anchor the real RSP in a non-volatile reg (R12 is canonical)
  3. Switch SP to the pre-allocated buffer (top-aligned to 16)
  4. Plant synthetic frames bottom-up (sentinel 0 → outermost → innermost)
  5. Load SSN into EAX, set MOV R10, RCX (syscall ABI)
  6. JMP/CALL into syscall;ret gadget (never embed bare `syscall` — leaves your .text as source)
  7. After return: restore SP from R12, pop callee-saved, RET

Buffer rule: never allocate the fake stack in a local variable of the ASM trampoline's frame. You are about to rewrite RSP; any local temporaries die. Pre-allocate a heap buffer (or a stable static) in the high-level caller, pass in bufPin + fakeStackTop, and use R12 to anchor the real RSP for fixup.

Why the buffer matters in Go

Go's runtime grows goroutine stacks dynamically. A large SUB SP, imm inside the trampoline can overflow stack.lo, or worse, produce a valid stack that the GC scanner then tries to walk — finding planted return addresses, treating them as Go frames, and crashing with "runtime: unreachable". The pre-allocated heap buffer sidesteps both issues:

// Pre-allocate once at Init; pin through GC via unsafe.Pointer arg
total := 8 + f2 + f1 + trampoline + 256
total = (total + 15) &^ 15
buf := make([]byte, total)
bufPin := unsafe.Pointer(&buf[0])
fakeStackTop := (uintptr(bufPin) + uintptr(len(buf))) &^ 15

Pass bufPin as an explicit arg so the GC keeps it alive for the syscall duration.

Why the buffer matters in Rust / C

• Rust: #[naked] / global_asm! with local sub rsp, imm blows through canaries and -Z stack-check instrumentation. Use a Box<[u8; N]> allocated in the caller and passed via rdi/rsi.
• C (mingw-w64): __attribute__((naked)) + inline AT&T asm; use a file-scope static __thread uint8_t buf[N] (TLS-backed) or a heap buffer allocated once in spoof_init. alloca is unsafe here — it uses _chkstk which generates CFG indirect calls.

The five implementation rules

These apply across C, C++, Rust, Go, and raw ASM.

R1. Resolve frame sizes at runtime. Hardcoding BaseThreadInitThunk+0x14 and RtlUserThreadStart+0x21 is fine (those offsets are stable since Win10 1809); hardcoding Frame1Size = 0x30 is not (it changed between 20H1 and 22H2). Always parse .pdata.

R2. Cascade gadget search across modules. Never commit to a single module. Order: wininet → user32 → kernelbase → ntdll (for SM); kernelbase → ntdll (for Draugr). Emit a log line on each fallback so you know which module won at runtime.

R3. Instrument the scanner in debug builds. Zero-match failures are ambiguous without counters. See the debug pattern in references/frame-math.md §Scanner Instrumentation.

R4. Invalidate the spoof context on init failure. Do not leave partial state; downstream callers must be able to check a single SPOOF_READY flag and fall back to unspoofed dispatch. Never "partially succeed".

R5. Strip the spoofer from release builds when you do not need it. A 500-line SilentMoonwalk with 4 frames and cascade logic is a strong detection target by itself — string constants, control-flow patterns, and .pdata scans are all observable. If the binary can run backed-on-disk in a legitimate PE, skip the spoof. See the minimalism principle in edr-evasion.

Languages — what changes

C / C++ (mingw-w64 or MSVC)

• __attribute__((naked)) function with AT&T inline asm (GNU) or .code block (MASM with MSVC)
• Context struct: #pragma pack(push, 8) → fixed field order; offsets referenced in asm as 0(%rdi), 8(%rdi), …
• Prefer mingw-w64 over MSVC for spoofers: no _chkstk injection on large stack frames, predictable codegen
• Link with -nostdlib -fno-ident -fno-asynchronous-unwind-tables so your own .pdata does not confuse investigators reversing your loader

See references/lang-c-rust-go.md for a ready-to-compile Draugr trampoline in mingw-w64 AT&T syntax.

Rust

• #[naked] (stable as of Rust 1.88) or global_asm! for the trampoline
• #[repr(C)] on the context struct — never #[repr(Rust)]
• no_std + no_main for implant builds; link with -C link-args=/NODEFAULTLIB
• Caveat: LLVM aggressively allocates RBX across inline asm blocks. Always list rbx in clobbers, or use options(noreturn) + a tail call to the next phase.

See references/lang-c-rust-go.md.

Go

• Plan 9 ASM syntax (.s files), one per architecture. See draugr_spoof_x64.s template in the reference file.
• Frame size $0-N — always $0 (no local frame). N = size of args passed from Go (sum of typed-arg sizes rounded to 8).
• BYTE $0x90 NOP scattered between instructions: not decorative. Plan 9 ASM's go assembler reorders "optimizable" sequences; the NOPs are padding to keep the assembler from merging or eliminating instructions that look redundant to it but are necessary for the spoof.
• Never touch g (GS:0x30 on Windows) in the trampoline. The Go runtime's thread-local lookup needs it intact for goroutine scheduling on return.

See references/lang-c-rust-go.md.

Integration with indirect syscall dispatchers

A stack spoofer does not resolve SSNs or find syscall;ret gadgets — that is the indirect-syscall skill's job. The integration point is a small interface:

spoof_trampoline(ssn: u16, syscall_ret_addr: *const u8, ctx: *const SpoofContext, args...) -> NTSTATUS

Where the caller resolves (ssn, syscall_ret_addr) via RecycleGate / Hell's / FreshyCalls, and the spoof trampoline dispatches the actual syscall;ret through the spoofed stack. Loading one skill does not require the other, but production loaders combine both. The layering is:

high-level wrapper
  └─ indirect_syscall.execute(ssn, gadget_addr, args…)
       └─ if (spoof_ctx != 0 && spoof_dispatch != NULL):
              spoof_dispatch(ssn, gadget_addr, spoof_ctx, args…)   ← spoof trampoline
          else:
              direct_indirect_syscall(ssn, gadget_addr, args…)     ← plain trampoline

See indirect-syscall/SKILL.md for the SSN side of this interface.

Diagnostic workflow (when init fails)

The failure diagnosis sequence, from most common to least:

1. FF23_total == 0 → target module has been stripped of gadgets (Win11 24H2 kernel32.dll). Add another module to the cascade.
2. fs_zero dominates → SAVE_NONVOL filter is rejecting the scanner's inventory. Verify UWOP_SAVE_NONVOL handling: max_save_offset >= total_alloc is the rejection criterion; off-by-one here eats half the population.
3. below_min dominates, best_belowmin_fs == 0x70 → threshold too high. Compute actual required frame for your syscall's arg count; lower MIN_JMP_RBX accordingly.
4. eclipse_fail == FF23_total → no E8 byte at gadget - 5. On Win11 22H2+ this is expected for kernelbase. Cascade through wininet / user32 first, then drop Eclipse for kernelbase last-resort.
5. Init succeeds but runtime crash → buffer too small. Recompute: 8 + frame2 + frame1 + trampoline_frame + args*8 + 0x100 padding, align to 16.
6. Unwinder sees "broken" stack → frame sizes mismatch between your plant and the real UNWIND_INFO. Re-read .pdata for the retaddr, not the function entry.

Full diagnostic script + instrumentation pattern in references/frame-math.md §Diagnosing Init Failures.

Resources

• references/frame-math.md — calc_frame_size algorithm, SAVE_NONVOL safety filter, gadget scanner with instrumentation, Win11 22H2+ empirical inventory, diagnosing init failures
• references/lang-c-rust-go.md — Per-language trampoline patterns (mingw-w64 AT&T asm, Rust global_asm!, Go Plan 9), context-struct layout rules, buffer-management patterns, interop caveats
• Start with references/frame-math.md; bad unwind math invalidates every language-specific trampoline.