It is important to clarify upfront:
The server processed SSI directives inside the included file, exposing system files. The attacker found this entry point by searching inurl:view index.shtml on Bing (which still supported it at the time). inurl view index shtml 14 2021
If you are concerned about your site's security, you might want to consider: Checking your file. It is important to clarify upfront: The server