Obscuring Control Flow

Created the Monday 18 March 2019. Updated 6 months, 3 weeks ago.

Obscuring control flow is an anti-disassembling technique that involves using methods of flow control that are difficult or impossible for disassemblers and debuggers to follow. This can make it more difficult for analysts to understand the program's behavior and can also make it more difficult for other tools, such as debuggers, to accurately interpret the program.

One example of this technique is the use of the Structure Exception Handling (SEH) mechanism in the Microsoft Windows operating system. SEH provides a way to handle exceptions, which are runtime errors that occur during the execution of a program. It allows the programmer to define a set of exception handlers that are called whenever an exception occurs. These exception handlers can be used to handle the exception and continue the program's execution, or to log the error and terminate the program.

SEH provides a method of flow control that is difficult for disassemblers and debuggers to follow, because the exception handlers are not part of the program's normal control flow. Disassemblers and debuggers are not able to accurately reconstruct the program's original instructions when SEH is used, because they cannot determine the intended behavior of the program without actually executing it.

Technique Identifier

U0212

Technique Tags

Obscuring control flow Anti-disassembling Exception handling Structure Exception Handling (SEH) Control flow Exception handler

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.

RaiseException

Code Snippets

Description

This code includes an exception handler that is called whenever an exception occurs. This exception handler calls the my_function function, which simply prints "Hello, world!" to the console. The obfuscated instructions use the SEH mechanism to obscure the control flow of the program, by raising an exception and handling it with the exception handler.

#include <Windows.h>
#include <stdio.h>

void my_function() {
    printf("Hello, world!\n");
}

LONG WINAPI exception_handler(EXCEPTION_POINTERS *exception) {
    my_function();
    return EXCEPTION_EXECUTE_HANDLER;
}

int main() {
    int eax = 0x12345678;

    // Obfuscated instructions using SEH
    __try {
        if (eax == 0) {
            my_function();
        }
        RaiseException(0x12345678, 0, 0, NULL);
    } __except (exception_handler(GetExceptionInformation())) {
    }

    return 0;
}

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.

https://www.malwinator.com/2015/11/27/anti-disassembly-techniques-used-by-malware-a-primer-part-2/