TLS Callback

Created the Monday 18 March 2019. Updated 6 months, 2 weeks ago.

TLS (Thread Local Storage) callbacks are a mechanism in Windows that allows a program to define a function that will be called when a thread is created. These callbacks can be used to perform various tasks, such as initializing thread-specific data or modifying the behavior of the thread.

As an anti-debugging technique, a program can use a TLS callback to execute code before the main entry point of the program, which is defined in the PE (Portable Executable) header. This allows the program to run secretly in a debugger, as the debugger will typically start at the main entry point and may not be aware of the TLS callback.

The program can use the TLS callback to detect whether it is being debugged, and if it is, it can terminate the process or take other actions to evade debugging. This technique can be used to make it more difficult for a debugger to attach to the process and to hinder reverse engineering efforts.

Technique Identifier

U0124

Technique Tags

TLS callbacks anti-debugging technique thread main entry point debugger

Featured Windows API's

Below, you will find a list of the most commonly used Windows API's that are currently utilized by malware authors for current evasion technique. This list is meant to provide an overview of the API's that are commonly used for this purpose. If there are any API's that you feel should be included on this list, please do not hesitate to contact us. We will be happy to update the list and provide any additional information or documentation that may be helpful.

ExitProcess

Code Snippets

#include "windows.h"
#include <stdio.h>

void NTAPI __stdcall TLSCallbacks(PVOID DllHandle, DWORD dwReason, PVOID Reserved);

#ifdef _M_IX86
#pragma comment (linker, "/INCLUDE:__tls_used")
#pragma comment (linker, "/INCLUDE:__tls_callback")
#else
#pragma comment (linker, "/INCLUDE:_tls_used")
#pragma comment (linker, "/INCLUDE:_tls_callback")
#endif
EXTERN_C
#ifdef _M_X64
#pragma const_seg (".CRT$XLB")
const
#else
#pragma data_seg (".CRT$XLB")
#endif

PIMAGE_TLS_CALLBACK _tls_callback = TLSCallbacks;
#pragma data_seg ()
#pragma const_seg ()

void NTAPI __stdcall TLSCallbacks(PVOID DllHandle, DWORD dwReason, PVOID Reserved)
{
	MessageBox(nullptr, "TLS Callback", "", 0);
	ExitProcess(0);
}

int main(int argc, char* argv[])
{
	printf("Main function!");
}

Detection Rules

                                    rule detect_tlscallback {
    meta:
        description = "Simple rule to detect tls callback as anti-debug."
        author = "Thomas Roccia | @fr0gger_"
    strings:
        $str1 = "TLS_CALLBACK" nocase
        $str2 = "TLScallback" nocase
    condition:
        uint32(uint32(0x3C)) == 0x4550 and any of them
}

Additional Resources

External Links

The resources provided below are associated links that will give you even more detailed information and research on current evasion technique. It is important to note that, while these resources may be helpful, it is important to exercise caution when following external links. As always, be careful when clicking on links from unknown sources, as they may lead to malicious content.