gzip TOCTOU file-permissions vulnerability

From: Imran Ghory (imranghory_at_gmail.com)
Date: 04/04/05

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To: bugtraq@securityfocus.com

    [This bug is caused by essentially the same coding issue as the bzip2
    issue (bugtraq id 12954) - the file is extracted and file descriptor
    closed before the file is chmod'ed]

    ================================
    gzip TOCTOU file-permissions vulnerability
    ================================

    Software: gzip
    Version: 1.2.4, 1.3.3
    Software URL: <http://www.gzip.org>
    Platform: Unix, Linux.
    Vulnerability type: Time-of-Check-Time-Of-Use
    Severity: Low, local user, badly set permissions.

    Vulnerable software
    ====================

    gzip 1.2.4 and 1.3.3 and previous versions running on unix.

    Vulnerability
    ==============

    If a malicious local user has write access to a directory in which a
    target user is using gzip to extract or compress a file to then a
    TOCTOU bug can be exploited to change the permission of any file
    belonging to that user.

    On decompressing gzip copies the permissions from the compressed
    gzip file to the uncompressed file. However there is a gap between the
    uncompressed file being written (and it's file handler being close)
    and the permissions of the file being changed.

    During this gap a malicious user can remove the decompressed file and
    replace it with a hard-link to another file belonging to the user.
    gzip will then change the permissions on the hard-linked file to be
    the same as that of the gzip file.

    Fix
    ====

    Ensure that any directory which is being used by gzip to
    compress/decompress files is only writeable by the user or
    alternatively set the sticky bit on the directory's permissions.

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