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Software Fault Tree Analysis (SFTA) and Software Failure Modes and Effects Analysis (SFMEA) are the techniques used to improve the reliability of software systems. SFTA is a top down deductive approach while SFMEA is a bottom up inductive approach to do risk analysis. In this paper, we present a formal approach to analyze and validate the functional requirements using SFTA and to identify the vulnerable failure modes with SFMEAs. The approach comprises of three steps. First, the functional requirement specified as a set of use cases are mapped to success tree and fault tree. Second, fault tree and success tree are synthesized to obtain minimum cut sets. These cut sets are then used to identify missing or incorrectly documented requirements. Third, we propose a formalized method to find vulnerable failure modes of software systems with the combination of SFMEA and SFTA techniques. Further, we suggest compensating provisions to avoid the occurrence of these critical failures. We demonstrate the applicability of the proposed approach using an illustrative example.