Even today, a single poorly written SQL query can allow an attacker to bypass authentication or expose sensitive data. And the scary part? It often comes down to just one line of code. In the previous articles, we saw how small implementation decisions can introduce serious vulnerabilities. SQL Injection is one of the clearest examples of this—simple to understand, yet still widely exploited. SQL Injection occurs when untrusted user input is included directly in a SQL query. Instead of being treated as data, the input is interpreted as part of the SQL command itself. This allows attackers to manipulate queries and control how the database behaves. Consider a typical login query: SELECT * FROM users The application expects input to be normal user data. But what if an attacker provides this instead? ' OR 1=1 -- OR 1=1 → always true -- → comments out the rest of the query 👉 The database ends up executing a modified query that ignores authentication checks. Let’s look at how this vulnerability often appears in real code. String username = request.getParameter("username"); String password = request.getParameter("password"); String query = "SELECT * FROM users WHERE username = '" + username + "' AND password = '" + password + "'"; Statement stmt = connection.createStatement(); ResultSet rs = stmt.executeQuery(query); User input is directly concatenated into the SQL query No separation between code and data The database cannot distinguish between intended query logic and attacker input username: admin 💣 Resulting Query SELECT * FROM users 👉 The condition OR 1=1 is always true, so the query returns results regardless of the password. Result: Authentication is bypassed. SQL Injection is not just theoretical—it can lead to serious consequences: Unauthorized login (authentication bypass) Exposure of sensitive data Modification or deletion of database records Full database compromise Preventing SQL Injection is straightforward—but only if done correctly. String query = "SELECT * FROM users WHERE username = ? AND password = ?"; PreparedStatement pstmt = connection.prepareStatement(query); pstmt.setString(1, username); pstmt.setString(2, password); ResultSet rs = pstmt.executeQuery(); 👉 Why this works: Query structure is fixed User input is treated strictly as data, not executable SQL User user = userRepository.findByUsernameAndPassword(username, password); 👉 ORMs generate parameterized queries internally, which helps reduce the risk of SQL injection when used correctly. Limit input length ⚠️ Important: Input validation alone is not sufficient to prevent SQL Injection. Database users should have only the permissions they need Relying only on input validation Manually escaping strings instead of using parameterized queries Trusting frontend validation Logging raw queries with sensitive data SQL Injection isn’t a complex attack—it’s usually the result of a simple coding mistake. But its impact can be severe. As a developer, the takeaway is clear: Small decisions in how you write queries can determine whether your application is secure—or completely exposed.