A filesystem event API is an operating system interface that tells software when a file or directory changes. Programs such as editors, sync clients, antivirus tools, and file managers use these notifications to stay responsive without constantly scanning the disk.
In security, the same event stream can become a side channel. Even if an attacker cannot read protected files, watching when and where change events occur may reveal user activity, application behavior, or the presence of security tools. Defenders should treat watcher permissions and event visibility as sensitive, apply least privilege, and limit broad observers where possible. Careful design of file-notification features helps reduce metadata leakage while preserving legitimate automation.