NOP Sled

Created the Saturday 23 March 2019. Updated 1 year, 1 month ago.

In computer security, a NOP slide is a sequence of NOP (no-operation) instructions that is inserted at the beginning of a code section. When a program branches to the start of this code section, it will "slide" through the NOP instructions until it reaches the desired, final destination. This technique is commonly used in software exploits to direct program execution when the target of a branch instruction is not known precisely.

For example, suppose an attacker has found a buffer overflow vulnerability in a program and wants to exploit it to execute arbitrary code. They might insert a NOP slide at the start of the buffer, followed by the code they want to execute. When the program overflows the buffer and branches to the start of the buffer, it will slide through the NOP instructions until it reaches the attacker's code, which will then be executed.

The NOP slide technique can be used in conjunction with shellcode to create more effective exploits. Shellcode is a small piece of code that is used as the payload in a software exploit. It typically contains the instructions needed to perform a specific action, such as spawning a shell, creating a network connection, or injecting code into another process.

To use a NOP slide with shellcode, the attacker would first insert a NOP sequence at the start of the code section they are targeting. They would then insert their shellcode after the NOP sequence, so that it is located at the final, desired destination. When the program branches to the start of the code section, it will slide through the NOP instructions until it reaches the shellcode, which will then be executed.

Using a NOP slide with shellcode can help to make exploits more reliable, as it provides a "buffer" of NOP instructions that can help to align the program's execution flow with the shellcode. This can help to prevent the exploit from failing due to small variations in the branch target address. It can also make it more difficult for defenses to detect and block the exploit, as the NOP instructions can help to obscure the shellcode.

NOP slides are a simple but effective technique for directing program execution in situations where the exact branch target is not known. They can also be used in conjunction with other exploit techniques, such as return-oriented programming, to create more complex and powerful exploits.

Technique Identifier

U0206

Technique Tags

NOP sled NOP ramp NOP padding NOP sequence NOP-based execution Instruction padding Code slide Buffer overflow Return-oriented programming Exploit mitigation Shellcode execution

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.

VirtualAlloc

Code Snippets

Description

This code creates a NOP slide with eight NOP instructions and then inserts the shellcode at the end of the slide. When the program branches to the start of the code section, it will slide through the NOP instructions until it reaches the shellcode, which will then be executed. This demonstrates how a NOP slide can be used to direct program execution to a specific location when the exact branch target is not known.

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

// Shellcode to spawn a cmd.exe process
unsigned char shellcode[] = "\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
                            "\x68\x63\x6d\x64\x00\x8b\xc4\x6a\x01\x50\x6a\x01\x6a\x02\x6a\x10"
                            "\x89\xe1\xb2\x0c\xcd\x80\x59\x6a\x3f\x58\xcd\x80\x49\x79\xf8\x68"
                            "\x2f\x63\x61\x6c\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\x99"
                            "\xb0\x0b\xcd\x80";

int main() {
    // Insert a NOP slide at the start of the code section
    __asm__("nop\n"
            "nop\n"
            "nop\n"
            "nop\n"
            "nop\n"
            "nop\n"
            "nop\n"
            "nop\n");

    // Insert the shellcode at the end of the NOP slide
    __asm__("jmp shellcode");

    // Allocate memory for the shellcode and copy it into place
    void *shellcode_mem = VirtualAlloc(0, sizeof(shellcode), MEM_COMMIT, PAGE_EXECUTE_READWRITE);
    memcpy(shellcode_mem, shellcode, sizeof(shellcode));

    return 0;
}

About Author Open Snippet

Author: Unprotect / Target Platform: Windows

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.

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