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    Subjecthard links create local DoS vulnerability and security problems
    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).


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