Control Flow Graph Flattening

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

Control flow flattening is a technique used to obfuscate the control flow of a program, in order to make it more difficult for a disassembler to accurately interpret the program's behavior. This technique involves breaking up the nesting of loops and if-statements in a program, and then hiding each of them in a case of a large switch statement. This switch statement is then wrapped inside the body of a loop, which makes it difficult for the disassembler to follow the control flow and generate correct disassembly output.

Control flow flattening is often used by malware authors to make it more difficult for analysts to reverse engineer the malware and understand its behavior. It can also be used to protect legitimate software from being reverse engineered by competitors or hackers.

Technique Identifier

U0216

Technique Tags

Control flow flattening Anti-disassembling Obfuscation Switch statement Loop statement Control flow Nesting Unstructured code Complex control flow

Code Snippets

Description

This code contains the original instructions, which declare and initialize the variables x and y, and use an if statement to compare their values. The obfuscated instructions use control flow flattling to make it more difficult for the disassembler to accurately interpret the instructions and generate correct disassembly output.

The while loop in the obfuscated instructions contains a large switch statement, which has multiple cases that hide the original if statements. This creates a complex and unstructured control flow that is difficult for the disassembler to follow.

#include <iostream>

int main() {
    // Original instructions
    int x = 0x12345678;
    int y = 0x87654321;
    if (x == y) {
        x = 0x11111111;
        y = 0x22222222;
    } else {
        x = 0x33333333;
        y = 0x44444444;
    }

    // Obfuscated instructions using control flow flattening
    int i = 0;
    while (true) {
        switch (i) {
            case 0:
                if (x == y) {
                    x = 0x55555555;
                    y = 0x66666666;
                }
                break;
            case 1:
                if (x != y) {
                    x = 0x77777777;
                    y = 0x88888888;
                }
                break;
            // ...
            default:
                break;
        }
        i++;
        if (i > 10) {
            break;
        }
    }

    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.

http://ac.inf.elte.hu/Vol_030_2009/003.pdf