How does a NOP sled work?
I can't find a good source that answers this question. I know that a nop sled is a technique used to circumvent stack randomization in a buffer overflow attack, but I can't get my head around how it works.
What's a simple example that illustrates this method?
What do terms like 128-byte nop sled mean?
Answer
Some attacks consist of making the program jump to a specific address and continue running from there. The injected code has to be loaded previously somehow in that exact location.
Stack randomization and other runtime differences may make the address where the program will jump impossible to predict, so the attacker places a NOP sled in a big range of memory. If the program jumps to anywhere into the sled, it will run all the remaining NOPs, doing nothing, and then will run the payload code, just next to the sled.
The reason the attacker uses the NOP sled is to make the target address bigger: the code can jump anywhere in the sled, instead of exactly at the beginning of the injected code.
A 128-byte NOP sled is just a group of NOP intructions 128 bytes wide.
NOTE #1: NOP (No-OPeration) is an instruction available in most (all?) architectures that does nothing, other than occupying memory and some runtime.
NOTE #2: in architectures with variable length instructions, a NOP instruction is usually just one byte in length, so it can be used as a convenient instruction padding. Unfortunately, that also makes it easy to do a NOP sled.