- 1. FindWindow()
- 2. Parent Process Check
- 3. Selectors
- 4. DbgPrint()
- 5. DbgSetDebugFilterState()
- 6. NtYieldExecution() / SwitchToThread()
This technique includes the simple enumeration of window classes in the system and comparing them with known windows classes of debuggers.
The following functions can be used:
Normally, a user-mode process is executed by double-clicking on a file icon. If the process is executed this way, its parent process will be the shell process (“explorer.exe”).
The main idea of the two following methods is to compare the PID of the parent process with the PID of “explorer.exe”.
This method includes obtaining the shell process window handle using user32!GetShellWindow() and obtaining its process ID by calling user32!GetWindowThreadProcessId().
Then, the parent process ID can be obtained from the PROCESS_BASIC_INFORMATION structure by calling ntdll!NtQueryInformationProcess() with the ProcessBasicInformation class.
The parent process ID and the parent process name can be obtained using the kernel32!CreateToolhelp32Snapshot() and kernel32!Process32Next() functions.
Selector values might appear to be stable, but they are actually volatile in certain circumstances, and also depending on the version of Windows. For example, a selector value can be set within a thread, but it might not hold that value for very long. Certain events might cause the selector value to be changed back to its default value. One such event is an exception. In the context of a debugger, the single-step exception is still an exception, which can cause some unexpected behavior.
On the 64-bit versions of Windows, single-stepping through this code will cause an access violation exception at l1 because the DS selector will be restored to its default value even before l1 is reached. On the 32-bit versions of Windows, the DS selector will not have its value restored, unless a non-debugging exception occurs. The version-specific difference in behaviors expands even further if the SS selector is used. On the 64-bit versions of Windows, the SS selector will be restored to its default value, as in the DS selector case. However, on the 32-bit versions of Windows, the SS selector value will not be restored, even if an exception occurs.
then when the “int 3” instruction is reached at l1 and the breakpoint exception occurs, the exception handler at l2 is not called as expected. Instead, the process is simply terminated.
A variation of this technique detects the single-step event by simply checking if the assignment was successful.
The FS and GS selectors are special cases. For certain values, they will be affected by the single-step event, even on the 32-bit versions of Windows. However, in the case of the FS selector (and, technically, the GS selector), it will be not restored to its default value on the 32-bit versions of Windows, if it was set to a value from zero to three. Instead, it will be set to zero (the GS selector is affected in the same way, but the default value for the GS selector is zero). On the 64-bit versions of Windows, it (they) will be restored to its (their) default value.
This code is also vulnerable to a race condition caused by a thread-switch event. When a thread-switch event occurs, it behaves like an exception, and will cause the selector values to be altered, which, in the case of the FS selector, means that it will be set to zero.
A variation of this technique solves that problem by waiting intentionally for a thread-switch event to occur.
However, this code is vulnerable to the problem that it was trying to detect in the first place, because it does not check if the original assignment was successful. Of course, the two code snippets can be combined to produce the desired effect, by waiting until the thread-switch event occurs, and then performing the assignment within the window of time that should exist until the next one occurs. [Ferrie]
The debug functions such as ntdll!DbgPrint() and kernel32!OutputDebugStringW() cause the exception DBG_PRINTEXCEPTION_C (0x40010006). If a program is executed with an attached debugger, then the debugger will handle this exception. But if no debugger is present, and an exception handler is registered, this exception will be caught by the exception handler.
The functions ntdll!DbgSetDebugFilterState() and ntdll!NtSetDebugFilterState() only set a flag which will be checked be a kernel-mode debugger if it is present. Therefore, if a kernel debugger is attached to the system, these functions will succeed. However, the functions can also succeed because of side-effects caused by some user-mode debuggers. These functions require administrator privileges.
This method is not really reliable because it only shows if there a high priority thread in the current process. However, it could be used as an anti-tracing technique.
When an application is traced in a debugger and a single-step is executed, the context can’t be switched to other thread. This means that ntdll!NtYieldExecution() returns STATUS_NO_YIELD_PERFORMED (0x40000024), which leads to kernel32!SwitchToThread() returning zero.
The strategy of using this technique is that there is a loop which modifies some counter if kernel32!SwitchToThread() returns zero, or ntdll!NtYieldExecution() returns STATUS_NO_YIELD_PERFORMED. This can be a loop which decrypts strings or some other loop which is supposed to be analyzed manually in a debugger. If the counter has the expected value (expected i.e. the value if all kernel32!SwitchToThread() returned zero) after leaving the loop, we consider that the debugger is present.
In the example below, we define a one-byte counter (initialized with 0) which shifts one bit to the left if kernel32!SwitchToThread returns zero. If it shifts 8 times, then the value of the counter will become 0 and the debugger is considered to be present.
During debugging: Fill anti-debug pr anti-traced checks with NOPs.
For anti-anti-debug tool development:
For FindWindow(): Hook user32!NtUserFindWindowEx(). In the hook, call the original user32!NtUserFindWindowEx() function. If it is called from the debugged process and the parent process looks suspicious, then return unsuccessfully from the hook.
- For Parent Process Checks: Hook ntdll!NtQuerySystemInformation(). If SystemInformationClass is one of the following values:
and the process name looks suspicious, then the hook must modify the process name.
For Selectors: No mitigations.
For DbgPrint: you have to implement a plugin for a specific debugger and change the behavior of event handler which is triggered after the DBG_PRINTEXCEPTION_C exception has arrived.
For DbgSetDebugFilterState(): Hook ntdll!NtSetDebugFilterState(). If the process is running with debug privileges, return unsuccessfully from the hook.
- For SwitchToThread: Hook ntdll!NtYieldExecution() and return an unsuccessful status from the hook.