Why can users delete each others files in this case I have two users : bob and sally, who resepctively own two files : bob100 and sally100: > ls -altrh /path/to/dir : -rw-r--r--. 1 bob test 0 Jan 7 12:59 bob100 -rw-r--r--. 1 sally test 0 Jan 7 13:06 sally100 drwxrwxrwx. 8 root root 4.1K Jan 7 13:06 . However, BOB can delete files in SALLY's account: > runuser -l bob -c 'rm -f /path/to/dir/sally100' Why is it that bob is allowed to delete a file which clearly only sally has write permissions on?

Because group/others have write permissions on a parent directory. File deletion is actually unlinking it from the directory. The directory is being modified, not the file.