Hello, on Linux it is possible for any user to create a hard link to a file belonging to another user. This hard link continues to exist even if the original file is removed by the owner. However, as the link still belongs to the original owner, it is still counted to his quota. If a malicious user creates hard links for every temp file created by another user, this can make the victim run out of quota (or even fill up the hard disk). This makes a local DoS attack possible. Furthermore, users can even create links to a setuid binary. If there is a security whole like a buffer overflow in any setuid binary, a cracker can create a hard link to this file in his home directory. This link still exists when the administrator has fixed the security whole by removing or replacing the insecure program. This makes it possible for a cracker to keep a security whole open until an exploit is available. It is even possible to create links to every setuid program on the system. This doesn't create new security wholes but makes it more likely that they are exploited. To solve the problem, the kernel shouldn't allow users to create hard links to files belonging to someone else. I could reproduce the problem on linux 2.2.19 and 2.4.21 (and found nothing about it in the changelogs to 2.4.23-rc3). If you can check whether this problem also exists on other unix-like operating systems, please post the results. Regards Jakob
