Over the years, internet archivists have used this method to find lost video game soundtracks, obscure 1980s programming manuals, out-of-print textbooks, and massive caches of public domain literature. Entire communities exist on Reddit and specialized forums dedicated to sharing high-quality open directories they have discovered, treating the web like a vast, unexplored cave system.
An attacker goes into blog/assets/ . The server has no index.html there. It shows an "Index of /blog/assets" page. That page contains a link: index of parent directory
To understand the "Index of parent directory," one must understand how web servers handle URLs. Over the years, internet archivists have used this