Low Level Exploits - Packet Storm

Return to Heap (Heap Spraying)
Heap spraying is an unreliable method of increasing the
chances of returning to executable instructions in a buffer
overflow attack.
Attackers create thousands of data structures in memory which
contain mostly null operations (the 0x90 instruction) with the
executable machine instructions at the end of the structure.
Statistically the chances of returning to a valid location on a
"NOP sled" are increased by increasing the size of the data
structures. Sometimes the chance can be up to 50%.