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Author Topic: Detect debugger with TLS callback  (Read 411 times)


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Detect debugger with TLS callback
« on: March 29, 2016, 11:54:27 pm »
TLS callback is a function that called before the process entry point executes. If you run the executable with a debugger, the TLS callback will be executed before the debugger breaks. This means you can perform anti-debugging checks before the debugger can do anything. Therefore, TLS callback is a very powerful anti-debugging technique.

To add a TLS callback to your program, you need to create a section called .CRT$XLB in the executable image, and then put the TLS callback function address into this section. You also need to add the __tls_used symbol to the executable image.

The following stack trace shows how the TLS callback called (from Process Hacker)

Code: [Select]
0, ntoskrnl.exe!KiDeliverApc+0x1c7
1, ntoskrnl.exe!KiCommitThreadWait+0x3dd
2, ntoskrnl.exe!KeWaitForSingleObject+0x19f
3, win32k.sys!xxxRealSleepThread+0x257
4, win32k.sys!xxxSleepThread+0x59
5, win32k.sys!NtUserWaitMessage+0x46
6, ntoskrnl.exe!KiSystemServiceCopyEnd+0x13
7, wow64cpu.dll!CpupSyscallStub+0x9
8, wow64cpu.dll!Thunk0Arg+0x5
9, wow64.dll!RunCpuSimulation+0xa
10, wow64.dll!Wow64LdrpInitialize+0x42a
11, ntdll.dll!LdrpInitializeProcess+0x17e3
12, ntdll.dll! ?? ::FNODOBFM::`string'+0x28ff0
13, ntdll.dll!LdrInitializeThunk+0xe
14, user32.dll!NtUserWaitMessage+0x15
15, user32.dll!DialogBox2+0x222
16, user32.dll!InternalDialogBox+0xe5
17, user32.dll!SoftModalMessageBox+0x757
18, user32.dll!MessageBoxWorker+0x269
19, user32.dll!MessageBoxTimeoutW+0x52
20, user32.dll!MessageBoxTimeoutA+0x76
21, user32.dll!MessageBoxExA+0x1b
22, user32.dll!MessageBoxA+0x18
23, tls.exe!TlsCallback+0x3c
24, ntdll.dll!LdrpCallInitRoutine+0x14
25, ntdll.dll!LdrpCallTlsInitializers+0x9e
26, ntdll.dll!LdrpRunInitializeRoutines+0x3ab
27, ntdll.dll!LdrpInitializeProcess+0x1400
28, ntdll.dll!_LdrpInitialize+0x78
29, ntdll.dll!LdrInitializeThunk+0x10

You can see the TLS callback is called by the loader during process startup.


Here is example code.

Code: [Select]
#include <stdio.h>
#include <Windows.h>

#pragma comment(lib,"ntdll.lib")

#pragma comment(linker,"/include:__tls_used") // This will cause the linker to create the TLS directory
#pragma section(".CRT$XLB",read) // Create a new section

extern "C" NTSTATUS NTAPI NtQueryInformationProcess(HANDLE hProcess,ULONG InfoClass,PVOID Buffer,ULONG Length,PULONG ReturnLength);

#define NtCurrentProcess() (HANDLE)-1

// The TLS callback is called before the process entry point executes, and is executed before the debugger breaks
// This allows you to perform anti-debugging checks before the debugger can do anything
// Therefore, TLS callback is a very powerful anti-debugging technique

void WINAPI TlsCallback(PVOID Module,DWORD Reason,PVOID Context)
PBOOLEAN BeingDebugged=(PBOOLEAN)__readfsdword(0x30)+2;

if(*BeingDebugged) // Read the PEB
MessageBox(NULL,"Debugger detected!","TLS callback",MB_ICONSTOP);

MessageBox(NULL,"No debugger detected","TLS callback",MB_ICONINFORMATION);

// Another check

7, // ProcessDebugPort
&DebugPort, // If debugger is present, it will be set to -1 | Otherwise, it is set to NULL
MessageBox(NULL,"Debugger detected!","TLS callback",MB_ICONSTOP);

MessageBox(NULL,"No debugger detected","TLS callback",MB_ICONINFORMATION);

__declspec(allocate(".CRT$XLB")) PIMAGE_TLS_CALLBACK CallbackAddress[]={TlsCallback,NULL}; // Put the TLS callback address into a null terminated array of the .CRT$XLB section

// The entry point is executed after the TLS callback

int main()
printf("Hello world");

return 0;

Process Hacker download:


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