SKILL: Anti-Debugging Techniques — Detection & Bypass Playbook

AI LOAD INSTRUCTION: Expert anti-debug techniques across Linux and Windows. Covers ptrace, PEB flags, NtQueryInformationProcess, timing attacks, signal-based detection, TLS callbacks, VEH tricks, and all corresponding bypass methods. Base models often miss the distinction between user-mode and kernel-mode detection and the correct patching strategy for each.

0. RELATED ROUTING

• code-obfuscation-deobfuscation when the binary also uses control flow flattening, VM protection, or string encryption
• vm-and-bytecode-reverse when the anti-debug sits inside a custom VM dispatcher
• symbolic-execution-tools when you want to symbolically skip anti-debug checks entirely

Advanced Reference

Also load ANTI_DEBUG_MATRIX.md when you need:

• Complete cross-reference matrix of technique × OS × detection method × bypass method
• Per-technique reliability ratings and false-positive notes
• Tool compatibility chart (GDB, x64dbg, WinDbg, Frida, ScyllaHide)

Quick bypass picks

1. LINUX ANTI-DEBUG TECHNIQUES

1.1 ptrace(PTRACE_TRACEME)

The classic self-attach: a process calls ptrace(PTRACE_TRACEME, 0, 0, 0). If a debugger is already attached, the call fails (returns -1).

if (ptrace(PTRACE_TRACEME, 0, 0, 0) == -1) {
    exit(1); // debugger detected
}

Bypass methods:

1.2 /proc/self/status — TracerPid

FILE *f = fopen("/proc/self/status", "r");
// parse TracerPid: if non-zero → debugger attached

Bypass: Mount a FUSE filesystem over /proc/self, or LD_PRELOAD hook fopen/fread to filter TracerPid to 0.

1.3 Timing Checks (rdtsc / clock_gettime)

Measures elapsed time between two points; debugger single-stepping causes noticeable delay.

rdtsc
mov ebx, eax       ; save low 32 bits
; ... protected code ...
rdtsc
sub eax, ebx
cmp eax, 0x1000    ; threshold
ja  debugger_detected

Bypass: Set hardware breakpoint after second rdtsc, modify eax to pass the comparison. Or use Frida to replace the timing function.

1.4 Signal-Based Detection (SIGTRAP)

volatile int caught = 0;
void handler(int sig) { caught = 1; }
signal(SIGTRAP, handler);
raise(SIGTRAP);
if (!caught) exit(1); // debugger swallowed the signal

When a debugger is attached, SIGTRAP is consumed by the debugger rather than delivered to the handler. Bypass: In GDB, use handle SIGTRAP nostop pass to forward the signal.

1.5 /proc/self/maps & LD_PRELOAD Detection

Checks for injected libraries or memory regions characteristic of debuggers/instrumentation.

FILE *f = fopen("/proc/self/maps", "r");
while (fgets(buf, sizeof(buf), f)) {
    if (strstr(buf, "frida") || strstr(buf, "LD_PRELOAD"))
        exit(1);
}

Bypass: Hook fopen("/proc/self/maps") to return a filtered version, or rename Frida's agent library.

1.6 Environment Variable Checks

Some protections check for LD_PRELOAD, LINES, COLUMNS (set by GDB's terminal), or debugger-specific env vars.

Bypass: Unset suspicious env vars before launch, or hook getenv().

2. WINDOWS ANTI-DEBUG TECHNIQUES

2.1 IsDebuggerPresent / CheckRemoteDebuggerPresent

if (IsDebuggerPresent()) ExitProcess(1);

BOOL debugged = FALSE;
CheckRemoteDebuggerPresent(GetCurrentProcess(), &debugged);
if (debugged) ExitProcess(1);

Bypass: Hook kernel32!IsDebuggerPresent to return 0, or patch PEB directly.

2.2 PEB Flags

mov rax, gs:[0x60]    ; PEB
movzx eax, byte [rax+0x02]  ; BeingDebugged
test eax, eax
jnz debugger_detected

Bypass: Zero all four fields. ScyllaHide does this automatically.

2.3 NtQueryInformationProcess

Bypass: Hook ntdll!NtQueryInformationProcess to return clean values per info class.

2.4 Hardware Breakpoint Detection

CONTEXT ctx;
ctx.ContextFlags = CONTEXT_DEBUG_REGISTERS;
GetThreadContext(GetCurrentThread(), &ctx);
if (ctx.Dr0 || ctx.Dr1 || ctx.Dr2 || ctx.Dr3)
    ExitProcess(1);

Bypass: Hook GetThreadContext to zero DR0–DR3, or use NtSetInformationThread(ThreadHideFromDebugger) preemptively (ironically, the anti-debug technique itself).

2.5 INT 2D / INT 3 / UD2 Exception Tricks

INT 2D is the kernel debug service interrupt. Without a debugger, it raises STATUS_BREAKPOINT; with a debugger, behavior differs (byte skipping).

xor eax, eax
int 2dh
nop          ; debugger may skip this byte
; ... divergent execution path ...

Bypass: Handle in VEH or patch the interrupt instruction.

2.6 TLS Callbacks

TLS callbacks execute before main() / WinMain(). Anti-debug checks placed here run before the debugger's initial break.

Bypass: In x64dbg, set "Break on TLS Callbacks" option. In WinDbg, use sxe ld to break on module load.

2.7 NtSetInformationThread(ThreadHideFromDebugger)

NtSetInformationThread(GetCurrentThread(), ThreadHideFromDebugger, NULL, 0);

After this call, the thread becomes invisible to the debugger — breakpoints and single-stepping stop working silently.

Bypass: Hook NtSetInformationThread to NOP when ThreadInfoClass == 0x11.

2.8 VEH-Based Detection

Registers a Vectored Exception Handler that checks EXCEPTION_RECORD for debugger-specific behavior (single-step flag, guard page violations with debugger semantics).

Bypass: Understand the VEH logic and ensure the exception chain behaves identically to non-debugged execution.

3. ADVANCED MULTI-LAYER TECHNIQUES

3.1 Self-Debugging (fork + ptrace)

The process forks a child that attaches to the parent via ptrace. If an external debugger is already attached, the child's ptrace fails.

pid_t child = fork();
if (child == 0) {
    if (ptrace(PTRACE_ATTACH, getppid(), 0, 0) == -1)
        kill(getppid(), SIGKILL);
    else
        ptrace(PTRACE_DETACH, getppid(), 0, 0);
    _exit(0);
}
wait(NULL);

Bypass: Patch the fork() return or kill/detach the watchdog child.

3.2 Multi-Process Debugging Detection

Parent and child cooperatively check each other's debug state, creating a mutual-watch pattern.

Bypass: Attach to both processes (GDB follow-fork-mode, or two debugger instances).

3.3 Timing-Based with Multiple Checkpoints

Distributes timing checks across multiple functions, comparing cumulative drift. Single patches fail because the total still exceeds threshold.

Bypass: Frida Interceptor.replace all timing sources (rdtsc, clock_gettime, QueryPerformanceCounter) to return controlled values.

3.4 Nanomite / INT3 Patching

Original conditional jumps are replaced with INT3 (0xCC). A parent debugger process handles each INT3, evaluates the condition, and sets the child's EIP accordingly.

Bypass: Reconstruct the original jump table by tracing all INT3 handlers, then patch the binary.

4. COUNTERMEASURE TOOLS

5. SYSTEMATIC BYPASS METHODOLOGY

Step 1: Static analysis — identify anti-debug calls
  └─ Search for: ptrace, IsDebuggerPresent, NtQuery, rdtsc,
     GetTickCount, SIGTRAP, INT 2D, TLS directory entries

Step 2: Classify each check
  ├─ API-based → hook or patch the call
  ├─ Flag-based → patch PEB/proc fields
  ├─ Timing-based → spoof time source
  ├─ Exception-based → forward/handle exception correctly
  └─ Multi-process → handle both processes

Step 3: Apply bypass (order matters)
  1. Load ScyllaHide / set LD_PRELOAD (covers 80% of checks)
  2. Handle TLS callbacks (break before main)
  3. Patch remaining custom checks (Frida or binary patch)
  4. Verify: run with breakpoints, confirm no premature exit

Step 4: Validate bypass completeness
  └─ Set BP on ExitProcess/exit/_exit — if hit unexpectedly,
     a check was missed → trace back from exit call

6. DECISION TREE

Binary exits/crashes under debugger?
│
├─ Crashes immediately before main?
│  └─ TLS callback anti-debug
│     └─ Enable TLS callback breaking in debugger
│
├─ Crashes at startup?
│  ├─ Linux: check for ptrace(TRACEME)
│  │  └─ LD_PRELOAD hook or NOP patch
│  └─ Windows: check IsDebuggerPresent / PEB
│     └─ ScyllaHide or manual PEB patch
│
├─ Crashes after some execution?
│  ├─ Consistent crash point → API-based check
│  │  ├─ NtQueryInformationProcess → hook return values
│  │  ├─ /proc/self/status → filter TracerPid
│  │  └─ Hardware BP detection → hook GetThreadContext
│  │
│  ├─ Variable crash point → timing-based check
│  │  └─ Hook rdtsc / QueryPerformanceCounter
│  │
│  └─ Crash on breakpoint hit → exception-based check
│     ├─ INT 2D / INT 3 trick → handle in VEH
│     └─ SIGTRAP handler → GDB: handle SIGTRAP pass
│
├─ Debugger loses control silently?
│  └─ ThreadHideFromDebugger
│     └─ Hook NtSetInformationThread
│
├─ Child process detects and kills parent?
│  └─ Self-debugging (fork+ptrace)
│     └─ Patch fork() or handle both processes
│
└─ All basic bypasses applied but still detected?
   └─ Multi-layer / custom checks
      ├─ Use Frida for comprehensive API hooking
      ├─ Full emulation with Qiling
      └─ Trace all calls to exit/abort to find remaining checks

7. CTF & REAL-WORLD PATTERNS

Common CTF Anti-Debug Patterns

Real-World Protections

8. QUICK REFERENCE — BYPASS CHEAT SHEET

Linux One-Liners

# LD_PRELOAD anti-ptrace
echo 'long ptrace(int r, ...) { return 0; }' > /tmp/ap.c
gcc -shared -o /tmp/ap.so /tmp/ap.c
LD_PRELOAD=/tmp/ap.so ./target

# GDB: catch and bypass ptrace
(gdb) catch syscall ptrace
(gdb) commands
> set $rax = 0
> continue
> end

Frida Anti-Debug Bypass (Cross-Platform)

// Hook IsDebuggerPresent (Windows)
Interceptor.replace(
  Module.getExportByName('kernel32.dll', 'IsDebuggerPresent'),
  new NativeCallback(() => 0, 'int', [])
);

// Hook ptrace (Linux)
Interceptor.replace(
  Module.getExportByName(null, 'ptrace'),
  new NativeCallback(() => 0, 'long', ['int', 'int', 'pointer', 'pointer'])
);

// Timing spoof
Interceptor.attach(Module.getExportByName(null, 'clock_gettime'), {
  onLeave(retval) {
    // manipulate timespec to hide debugger delay
  }
});

x64dbg ScyllaHide Quick Setup

1. Plugins → ScyllaHide → Options
2. Check: PEB BeingDebugged, NtGlobalFlag, HeapFlags
3. Check: NtQueryInformationProcess (all classes)
4. Check: NtSetInformationThread (HideFromDebugger)
5. Check: GetTickCount, QueryPerformanceCounter
6. Apply → restart debugging session