The Anatomy of SQL Injection Vulnerabilities

SQL injection vulnerabilities arise from a fundamental flaw in how applications construct database queries. When user input is directly concatenated into SQL statements without proper sanitization or parameterization, attackers can manipulate the query structure. Consider a simple login query: SELECT * FROM users WHERE username = 'admin' AND password = 'password123'. If the application builds this query by directly inserting user input, an attacker entering admin' -- as the username transforms the query into SELECT * FROM users WHERE username = 'admin' --' AND password = 'password123', effectively commenting out the password check.

The vulnerability extends beyond simple authentication bypasses. Attackers use techniques like UNION-based injection to combine results from multiple tables, Boolean-based blind injection to extract data bit by bit, time-based blind injection when no direct output is available, and even out-of-band injection using database features to exfiltrate data through DNS or HTTP requests. Modern automated tools can exploit these vulnerabilities to dump entire databases in minutes, making manual protection crucial for every developer.