But... traditionally, Unix systems do allow creating hardlinks of other users' files. And yes, this misfeature is a source of great number of security holes. A

But... traditionally, Unix systems do allow creating hardlinks of other users' files. And yes, this misfeature is a source of great number of security holes. An option to disable this behavior (/proc/sys/fs/protected_hardlinks) was addded only in Linux 3.6, and then it's still disabled by default.