SQL injection is a type of attack that takes advantage of vulnerabilities in a web application's database query software, allowing an attacker to send malicious SQL commands through the application to the database. These commands can manipulate the database and can lead to unauthorized data access or manipulation.

SQL injection: In the scenario described, the malicious user was able to export an entire website's user database by entering specific commands into a field on the company's website, which is a classic example of an SQL injection attack. This type of attack exploits vulnerabilities in the database layer of an application to execute unauthorized SQL commands.

Cross-site scripting (A) involves injecting malicious scripts into content from otherwise trusted websites. A brute-force attack (C) is an attempt to gain access to a system by systematically checking all possible keys or passwords until the correct one is found. A DDoS attack (D) is an attempt to make a machine or network resource unavailable to its intended users by overwhelming it with a flood of internet traffic.