A SQL injection attack is a security vulnerability that occurs when an attacker inserts or "injects" malicious SQL code into an application's input fields. This can manipulate the database behind the application, enabling unauthorized access to sensitive data or altering the database contents. These attacks exploit weaknesses in input validation and can lead to severe consequences, including data breaches or loss. To safeguard against SQL injection, developers must implement strong input validation and utilize parameterized queries. Understanding this threat is crucial for maintaining database security and protecting user information.