A NOP sled (also called a NOP slide or NOP ramp) is a classic technique used in software exploitation, especially in buffer overflow attacks, to increase the chances of successfully executing malicious code.

🧠 What Is a NOP Sled?

• NOP stands for No Operation—an instruction that does nothing but advance the CPU to the next instruction.
• A NOP sled is a long sequence of NOP instructions placed in memory before the actual payload (e.g., shellcode).
• If an attacker can redirect the program’s execution to any point within the sled, the CPU will “slide” through the NOPs until it reaches the payload.

Think of it like a snowy hill: no matter where you land on the slope, you’ll slide down to the bottom—where the malicious code waits.

🧪 Why Use a NOP Sled?

• In buffer overflow attacks, it’s hard to predict the exact memory address of the payload.
• A NOP sled widens the target area, making it easier to land somewhere that leads to successful code execution.
• It’s especially useful when the attacker can only guess or approximate the return address.

🔐 Defensive Implications

• Because NOP sleds are common in exploits, intrusion detection systems often scan for long sequences of NOPs.
• Modern attackers use non-canonical NOPs (e.g., mov eax, eax) or harmless instructions to evade detection.

📘 Learn More

• Wikipedia’s NOP Slide article offers a technical overview.
• ReasonLabs Cyberpedia explains how NOP sleds are used in malware evasion.

Here’s how you can write a NOP sled in assembly, especially for use in shellcode or buffer overflow experiments:

🧪 Example: NOP Sled in x86-64 Assembly

.global _start
.intel_syntax noprefix

_start:
  .rept 1024        # Repeat the NOP instruction 1024 times
    nop             # NOP = No Operation (opcode 0x90)
  .endr

  mov rax, 0x3b     # syscall: execve
  lea rdi, [rip+filename]
  xor rsi, rsi      # argv = NULL
  xor rdx, rdx      # envp = NULL
  syscall

filename:
  .string "/bin/sh"

This code creates a NOP sled of 1024 bytes, followed by shellcode that spawns a shell using the execve syscall.

🛠️ How to Compile and Test

1. Save the code as shellcode.s.
2. Compile:

   gcc -nostdlib -static shellcode.s -o shellcode
   

3. Disassemble to verify the NOP sled:

   objdump -d shellcode
   

You’ll see a long sequence of 0x90 bytes (NOPs) followed by the actual shellcode.

📘 Source

This technique is discussed in detail on Stack Overflow and Dhaval Kapil’s Shellcode Injection blog.

Let’s break down what 0x90 0x90 0x90 means in the context of computer opcodes:

🧠 What Is an Opcode?

An opcode (short for operation code) is a single byte in machine language that tells the CPU what operation to perform. Every instruction in assembly language corresponds to one or more opcodes in machine code.

🔍 What Does 0x90 Mean?

• 0x90 is the hexadecimal representation of the NOP instruction in x86 architecture.
• NOP stands for No Operation.
• When the CPU encounters 0x90, it simply does nothing for one clock cycle and moves to the next instruction.

So, when you see:

0x90 0x90 0x90

You’re looking at three consecutive NOP instructions. The CPU will execute each one, doing nothing three times in a row.

🧊 Why Use NOPs?

NOPs are used in several scenarios:

• NOP Sleds in Exploits: Attackers use long sequences of NOPs to increase the chance that the CPU “slides” into their shellcode during a buffer overflow.
• Timing Adjustments: Developers might use NOPs to fine-tune timing in low-level code.
• Instruction Alignment: NOPs can help align instructions to certain memory boundaries for performance or compatibility.

🧪 Example in Assembly

nop     ; 0x90
nop     ; 0x90
nop     ; 0x90

This is exactly what 0x90 0x90 0x90 looks like in assembly.