Fault attacks recover secret keys by exploiting faults injected during the execution of a block cipher. However, not all faults are exploitable and every exploitable fault is associated with an offline complexity to determine the key. The ideal fault attack would recover maximum key bits with minimum offline effort. Finding the ideal fault attack for a block cipher is a laborious manual task, which can take several months to years before such an attack is discovered. In this paper, we present a framework that would analyze block ciphers for their vulnerabilities to faults and automatically predict whether a differential fault attack would be successful. The framework, which we call XFC, uses colors to analyze the fault propagation and exploitability in the cipher. XFC would be able to (a) predict the key bits that can be derived by the fault attack and (b) estimate the offline complexity. It can thus be used to identify the ideal fault attack for a block cipher. As a proof of concept, we have applied XFC to the block ciphers AES, CLEFIA and SMS4 and were able to automatically derive fault attacks that correspond to the best known till date in the single fault model. © 2017 ACM.