How SQL Injection Works

To understand SQL injection, consider a simple login form that checks usernames and passwords. A vulnerable application might construct a query like: SELECT * FROM users WHERE username = 'john' AND password = 'secret123'. If the application directly inserts user input into this query without protection, an attacker could enter admin' -- as the username. The resulting query becomes: SELECT * FROM users WHERE username = 'admin' --' AND password = 'anything'. The -- comments out the password check, potentially allowing access to the admin account without knowing the password.

More sophisticated attacks can extract entire databases, modify data, or even execute operating system commands on vulnerable servers. Attackers use techniques like UNION queries to combine results from multiple tables, blind SQL injection to extract data bit by bit when results aren't directly visible, and time-based attacks that infer information based on response delays. Modern automated tools can exploit SQL injection vulnerabilities to dump entire databases in minutes, making this a favorite technique for cybercriminals seeking large-scale data theft.