The vulnerability that the attacker was trying to exploit is SQL injection, which is a type of attack that exploits a vulnerability in a web application that allows an attacker to execute malicious SQL statements on a database server. SQL injection can allow an attacker to perform various actions on the database, such as reading, modifying, deleting, or creating data, or executing commands on the underlying OS. The log shows that the attacker was sending thousands of requests to the same URL with different parameters, such as id=1' OR 1=1;-, id=1' AND 1=2;-, or id=1' UNION SELECT * FROM users;-. These parameters are examples of SQL injection payloads, which are crafted SQL statements that are designed to manipulate or bypass the intended SQL query. For example, id=1' OR 1=1;-- is a payload that terminates the original query with a single quote and a semicolon, appends an OR condition that is always true (1=1), and comments out the rest of the query with two dashes (-). This payload can cause the web application to return all records from the database table instead of just one record with id=1. The other options are not vulnerabilities that match the log entries. Session hijacking is a type of attack that exploits a vulnerability in a web application that allows an attacker to take over an active session of another user by stealing or guessing their session identifier or cookie. URL manipulation is a type of attack that exploits a vulnerability in a web application that allows an attacker to modify parameters or values in the URL to access unauthorized resources or functions. Insecure direct object reference is a type of attack that exploits a vulnerability in a web application that allows an attacker to access objects or resources directly by modifying their identifiers or references in the URL or request.