III Exploit General Explanation - kkli08/Buffer-Overflow GitHub Wiki

The improvement in this new version of the exploit (exploit3) is the use of a NOP sled (also known as a NOP slide). Let's break down the changes and how they contribute to a more effective exploit:

Previous Version:

  • Precision Required: The earlier exploit version required accurately guessing the exact offset to overwrite the return address with the address where the shellcode begins.
  • Difficulty: If the guessed address was off by even a single byte, the exploit would likely fail, leading to a segmentation fault or an illegal instruction error.

Improved Version with NOP Sled:

  1. Use of NOP Sled:

    • A NOP (No Operation) sled is a sequence of NOP instructions (0x90 in x86 assembly). These instructions do nothing but pass control to the next instruction.
    • In exploit3, the first half of the buffer is filled with NOP instructions.

  2. Shellcode Placement:

    • The shellcode is placed in the middle of the buffer, after the NOP sled.
    • This placement ensures that even if the exact start of the shellcode is not hit, as long as execution hits anywhere within the NOP sled, it will slide down to the shellcode.

  3. Address Guessing:

    • Although an address still needs to be guessed to overwrite the return address, the NOP sled provides a much larger target.
    • The return address doesn't need to point exactly to the start of the shellcode. If it points anywhere within the NOP sled, the execution will slide down to the shellcode, making the exploit more likely to succeed.

How It Works:

  • Executing the NOP Sled: When the vulnerable program overflows its buffer and the return address is overwritten with an address within the NOP sled, the CPU starts executing the NOP instructions sequentially.

  • Reaching the Shellcode: Eventually, after executing the NOPs, the CPU reaches the shellcode. This is where the actual intended exploit (spawning a shell) happens.

About the Return Address:

  • Estimation Still Required: You still need to estimate the return address to overwrite, but you don't need to be as precise. As long as it lands somewhere within the NOP sled, the exploit will work.
  • Address Calculation: The address used in the overwrite is still calculated based on the current stack pointer (ESP) and the provided offset, similar to the previous exploit.

Conclusion:

The key improvement in this version of the exploit is the addition of the NOP sled, which significantly increases the likelihood of the exploit's success by providing a larger "landing zone" for the overwritten return address. The exploit no longer requires pinpoint precision, as hitting anywhere within the NOP sled will eventually lead to the execution of the shellcode. This technique is a common method to make buffer overflow exploits more reliable.