RE: gzip TOCTOU file-permissions vulnerability

From: Mark Senior (Mark.Senior_at_gov.ab.ca)
Date: 04/14/05

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    Date: Thu, 14 Apr 2005 09:27:11 -0600
To: "Derek Martin" <code@pizzashack.org>, <bugtraq@securityfocus.com>

     

    > From: Derek Martin [mailto:code@pizzashack.org]
    > Sent: April 13, 2005 08:50
    >
    >
    > The open() call is at fault here. If instead of being called
    > with a mode of RW_USER, it is called with the final intended
    > access mode, there is no need to later call chmod(), and the
    > problem is averted.

    One wrinkle - if the file is not intended to have user write permission
    on it, and gzip (unzip/cpio/pax...) initially created it with the
    intended permissions, there would be no way to then write the file.

    The problem, to my understanding, is that the program opens the file by
    name, then closes it, and then chmod's it, again referring to it by
    name. During which time, as you pointed out, we could be dealing with a
    different inode.

    If the program kept the file open, and used fchmod to change its
    permissions, referring to it by file descriptor, you could be more sure
    that it was the same inode it had just been writing to.

    Cheers
    Mark

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