To answer this question, the user needs to know about common types of attacks on web applications and their characteristics.

Option A: Session Fixation Attack, which is also known as a session hijacking attack, involves an attacker fixing the session identifier of a user to gain unauthorized access to the user's account. This option is incorrect because it does not match the given scenario.

Option B: Brute force attack is the correct answer. A brute force attack is a common type of attack where an automated process tries a large number of possible values to crack a password or find a key. This type of attack is often used to gain unauthorized access to a system or application. This option matches the given scenario.

Option C: Insecure Direct Object Reference is a type of attack where an attacker can access an unauthorized resource by manipulating an object reference. This option is incorrect because it does not match the given scenario.

Option D: Injection Flaws are a type of attack where an attacker sends malicious input to an application to exploit a vulnerability and gain unauthorized access to the system. This option is incorrect because it does not match the given scenario.

Therefore, the correct answer is:

The Answer is: B. Brute force attack.