Glossary
NOP Sled
A sequence of NOP (no-operation) instructions prepended to shellcode so that imprecise jumps still land in executable payload territory.
A NOP sled (also called a NOP slide) is a long sequence of NOP instructions (0x90 on x86) placed before shellcode in an exploit payload. Because NOP does nothing but advance the instruction pointer, any jump that lands anywhere within the sled will harmlessly "slide" down until it reaches the actual payload. NOP sleds compensate for uncertainty in the exact address of injected code — for example, when stack addresses vary slightly due to environment differences. Modern mitigations like ASLR have made classic NOP sleds less effective, but the concept appears in many exploit proof-of-concepts and is important to recognize when analyzing shellcode.