According to wikipedia fuzz testing or fuzzing is a software testing technique, often automated or semi-automated, that involves providing invalid, unexpected, or random data to the inputs of a computer program. The program is then monitored for exceptions such as crashes, or failing built-in code assertions or for finding potential memory leaks. Fuzzing is commonly used to test for security problems in software or computer systems. The field of fuzzing originates with Barton Miller at the University of Wisconsin in 1988. “The original work was inspired by being logged on to a modem during a storm with lots of line noise. And the line noise was generating junk characters that seemingly were causing programs to crash. The noise suggested the term ‘fuzz’”-Barton Miller. In this thesis we will introduce a file fuzzing tool called “Windows Phone File Fuzzer” for Windows Phone 8.1. This tool is a “black box”/mutilation-based fuzzing tool for ascii and binary file formats based in the concept of FileFuzz presented by Michael Sutton, Adam Greene and Pedram Amini in Fuzzing: Brute Force Vulnerability Discovery.