Day 20 — How I Built a File Integrity Monitor in Python to Detect File Tampering

What if one of your critical system files changed right now? Would you notice? Attackers rarely need to install complex malware. Often they simply modify existing files — a web server script, a cron job, or a configuration file. That tiny change can be the difference between a secure system and a persistent backdoor. This is exactly the problem File Integrity Monitoring solves. Introduction Every system administrator or security engineer has faced the same unsettling question at some point: has this file been tampered with? Whether it's a config file silently modified by malware, a binary swapped out during a supply-chain attack, or a web asset defaced by an intruder — unauthorized file changes are one of the most common indicators of compromise. FIM (File Integrity Monitor) is a lightweight, terminal-native Python tool that answers that question definitively. It uses cryptographic hashing to create a trusted snapshot of your files and then compares future states against that snapshot