A directory traversal attack is a type of web application attack that exploits insufficient input validation or filtering to access files or directories that are outside of the web root folder. A directory traversal attack can allow an attacker to read, modify, or execute files on the target server that are not intended to be accessible via web requests. The URL in the alert contains an example of a directory traversal attack, as indicated by the use of ".../" sequences in the query string. These sequences are used to navigate up one level in the directory hierarchy, potentially reaching sensitive files or folders on the server. In this case, the attacker is trying to access /etc/passwd file, which contains user account information on Linux systems.