Input validation is an essential and a very important feature in any software system that has intensive interaction with its users. In this paper, we introduce some invariant properties with regards to input validation through analyzing the control and data dependency among inputs accessed and effects raised in a program. We then propose a method for the automated recovery of input validation from program source code. Based on the information recovered, we present the techniques to aid the understanding and maintenance of the feature using program slicing. A prototype tool has been implemented to validate the approach, and the empirical results show that the proposed approach can be very useful and effective for both experienced and inexperienced programmers.