How does a SQL injection attack compromise a web application?

A SQL injection attack compromises a web application primarily by manipulating the SQL queries sent to the database. When an application does not properly validate or sanitize user input, an attacker can insert or "inject" SQL commands into an entry field. For example, if an application uses user-submitted data directly in a SQL query without adequate checks, an attacker can craft their input to alter the behavior of that query.

This could enable the attacker to execute unauthorized actions, such as reading sensitive data (like user credentials), modifying database entries, or even deleting entire tables. The root of the vulnerability lies in the trust that the application places on user inputs, which, if not handled with care, can be exploited to run untrusted commands on the database. Thus, the process of hijacking the intended SQL command flow through injection is what enables attackers to compromise the web application's security.

This understanding highlights the necessity for web developers to implement strong input validation and to use prepared statements or parameterized queries to mitigate the risks posed by SQL injection.