[UNIX] Linux nfs-utils xlog() Off-by-One Bug

From: SecuriTeam (support_at_securiteam.com)
Date: 07/17/03

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Date: 17 Jul 2003 13:40:28 +0200

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      Linux nfs-utils xlog() Off-by-One Bug
    ------------------------------------------------------------------------

    SUMMARY

     <http://sourceforge.net/projects/nfs/> Linux NFS utils package contains
    remotely exploitable off-by-one bug. A local or remote attacker could
    exploit this vulnerability by sending a specially crafted request to
    rpc.mountd daemon.

    DETAILS

    Vulnerable systems:
     * nfs-utils version 1.0.3 and prior

    Immune systems:
     * nfs-utils version 1.0.4 and above

    An off-by-one bug exist in xlog() function which handles logging of
    requests. An overflow occurs when function is trying to add missing
    trailing new line character to logged string.

    Due to miscalculation, if a string passed to the functions is equal or
    longer than 1023 bytes, the '\0' byte will be written beyond the buffer:
    - ------8<------cut-here------8<------

            char buff[1024];
            ...
     
            va_start(args, fmt);
            vsnprintf(buff, sizeof (buff), fmt, args);
            va_end(args);
            buff[sizeof (buff) - 1] = 0;

            if ((n = strlen(buff)) > 0 && buff[n-1] != '\n') {
                    buff[n++] = '\n'; buff[n++] = '\0';
            }

    - ------8<------cut-here------8<------

    Impact:
    Local or remote attacker that is capable of sending an RPC request to a
    vulnerable mount daemon could execute arbitrary code or cause denial of
    service.

    Vendor status:
    Vendor has been notified on June 10, 2003. The fix is incorporated in
    recent 1.0.4 release of nfs-utils.

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:funkysh@isec.pl> Janusz
    Niewiadomski.

    ========================================

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