Today's internet system has various vulnerabilities and threats due to excessive use of it. These threats are increasing day-by-day among which one is DDoS attack. It is very critical task for today's defenders to detect DDoS attacks against flash crowds. Both DDoS attack and flash crowd cause surges of access to a server, but flash crowds are unexpected and legitimate. Main tool behind DDoS attack is botnet and botmasters try to disable detection strategy of DDoS attack by mimicking the patterns of flash crowd. So it is the most challenging task today to detect DDoS attack against flash crowd.