FreeBSD Security Advisory 96:17 - rzsz

[security-officer () freebsd org (FreeBSD Security Officer)]

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=============================================================================
FreeBSD-SA-96:17                                            Security Advisory
Revised: Tue Jul 16 21:44:54 PDT 1996                           FreeBSD, Inc.

Topic:          "Trojan Horse" vulnerability via rz program

Category:       ports
Module:         rzsz
Announced:      1996-07-16
Affects:        All FreeBSD ports collections released before 2.1.5-RELEASE
Corrected:      ports collection as of 1996-07-06
Source:         rzsz shareware package
FreeBSD only:   no

Patches:        ftp://freebsd.org/pub/CERT/patches/SA-96:17/

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

I.   Background

     All existing versions of the rz program (a program for receiving
     files over serial lines using the Z-Modem protocol) are equipped
     with a feature that allows the sender of a file to request the
     execution of arbitrary commands on the receiver's side.  The user
     using rz does not have any control over this feature.

     The workaround is to have rz never execute any command, and
     always pretend a successful execution.

     All FreeBSD users are encouraged to use the workaround provided.
     Since the intent of the Z-Modem protocol is to provide a reliable
     connection between systems of a vastly different architecture,
     the execution of local commands at request of the sending side
     cannot even be considered a useful feature at all.


II.  Problem Description

     The Z-Modem protocol specifies a mechanism which allows the
     transmitter of a file to execute an arbitrary command string
     as part of the file transfer.  This is typically used to rename
     files or eliminate temporary files.  A malicious "trusted" sender
     could send down a command that could damage a user's environment.


III. Impact

     The rzsz package is an optional port that made be installed on
     some FreeBSD systems.  This program is not installed by default.
     Systems without this program are not vulnerable.

     rz allows "Trojan Horse" type attacks against unsuspecting users.
     Since the rz executable does not run with special privileges,
     the vulnerability is limited to changes in the operating environment
     that the user could willingly perform.

     This vulnerability is a fundamental flaw in the Z-Modem protocol.
     Other operating systems and other implementations of the Z-Modem
     protocol may also suffer similar vulnerabilities.

IV.  Workaround

     Disable the rz program.  If it has been installed, it would
     typically be found in /usr/local/bin.

        # chmod 000 /usr/local/bin/rz
        # ls -l /usr/local/bin/rz
        ----------  1 root  wheel  23203 Mar  4 23:12 /usr/local/bin/rz


V. Solution(s)

     This feature is a relatively unknown part of the Z-Modem protocol.
     It is not critical to file transfers in general.  The safest
     approach is to disable this feature in the receiving program.

     Any rzsz port that is obtained from the official ports collection
     after 1996-07-06 includes the following patch to disable this feature.
     This patch applies to rzsz v3.42, if you have an earlier version
     of the rzsz sources, please upgrade to the latest version first.

    *** rz.c.orig       Sat Jul  6 17:34:26 1996
    --- rz.c    Sat Jul  6 17:44:52 1996
    ***************
    *** 1020,1039 ****
    --- 1020,1045 ----
                    case ZCOMMAND:
                            cmdzack1flg = Rxhdr[ZF0];
                            if (zrdata(secbuf, 1024) == GOTCRCW) {
    + #ifdef BIG_SECURITY_HOLE
                                    void exec2();

                                    if (cmdzack1flg & ZCACK1)
                                            stohdr(0L);
                                    else
                                            stohdr((long)sys2(secbuf));
    + #else
    +                           stohdr(0L);
    + #endif
                                    purgeline();        /* dump impatient questions */
                                    do {
                                            zshhdr(4,ZCOMPL, Txhdr);
                                    }
                                    while (++errors<20 && zgethdr(Rxhdr) != ZFIN);
                                    ackbibi();
    + #ifdef BIG_SECURITY_HOLE
                                    if (cmdzack1flg & ZCACK1)
                                            exec2(secbuf);
    + #endif
                                    return ZCOMPL;
                            }
                            zshhdr(4,ZNAK, Txhdr); goto again;

=============================================================================
FreeBSD, Inc.

Web Site:                       http://www.freebsd.org/
Confidential contacts:          security-officer () freebsd org
PGP Key:                        ftp://freebsd.org/pub/CERT/public_key.asc
Security notifications:         security-notifications () freebsd org
Security public discussion:     security () freebsd org

Notice: Any patches in this document may not apply cleanly due to
        modifications caused by digital signature or mailer software.
        Please reference the URL listed at the top of this document
        for original copies of all patches if necessary.
=============================================================================

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