Fusil is a fuzzing framework designed to expose bugs in software by changing random bits of its input. . It helps to start process with a prepared environment (limit memory, environment variables, redirect stdout, etc.), start network client or server, and create mangled files. Fusil has many probes to detect program crash: watch process exit code, watch process stdout and syslog for text patterns (eg. "segmentation fault"), watch session duration, watch cpu usage (process and system load), etc. . Fusil is based on a modular architecture. It computes a session score used to guess fuzzing parameters like number of injected errors to input files.