Security in Mobile Ad-Hoc NETwork (MANET) is the most important concern for the basic functionality of network. In this paper, the authors focus on analyzing and improving the security against blackhole attack for one of the most popular hybrid ad hoc routing protocol in MANET viz. zone routing protocol. They have proposed an algorithm Secure Detection Technique (SDT) for ZRP protocol which can be used to prevent black hole attack in MANETs. This is a novel technique for detecting and blocking of black hole node in zone based network. It is less complex and inexpensive.