Bug#4075: Possible security problem with lrzsz
Package: lrzsz
Version: 0.12a-5
I've not heard from the author wrt my query below. I'm filing this
bug to make sure that this issue gets checked rather than forgotten.
Thank you for your attention.
Ian.
Mike Neuffer writes ("Re: lrzsz ? Re: forwarded message from CERT Bulletin"):
...
> I'm not the author, but lrzsz is almost identical to the "normal" rzsz
> implemetation the only difference is that it gives more status information.
> The diff itself is rather small.
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In-Reply-To: <m0uldAg-0004PkC@chiark.chu.cam.ac.uk>
References: <m0uldAg-0004PkC@chiark.chu.cam.ac.uk>
To: Debian private list <debian-private@lists.debian.org>
Subject: lrzsz ? Re: forwarded message from CERT Bulletin
CERT write:
> Topic: "Trojan Horse" vulnerability via rz program
...
> 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.
The version we distribute is called `lrzsz', and I suspect it is a
different version.
Nevertheless I'd appreciate it if the author could confirm that the
command execution feature is disabled.
Ian.
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Article: 124 of comp.security.announce
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Newsgroups: comp.security.announce
Organization: CERT(sm) Coordination Center - +1 412-268-7090
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Keywords: security CERT
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Originator: cert-advisory@why.cert.org
From: CERT Bulletin <cert-advisory@cert.org>
Subject: CERT Vendor-Initiated Bulletin VB-96.12 - FreeBSD
Date: 30 Jul 1996 19:22:18 GMT
- -----BEGIN PGP SIGNED MESSAGE-----
=============================================================================
CERT(sm) Vendor-Initiated Bulletin VB-96.12
July 30, 1996
Topic: "Trojan Horse" vulnerability via rz program
Source: FreeBSD, Inc.
To aid in the wide distribution of essential security information, the CERT
Coordination Center is forwarding the following information from FreeBSD, Inc.
FreeBSD, Inc. urges you to act on this information as soon as possible.
FreeBSD, Inc. contact information is included in the forwarded text below;
please contact them if you have any questions or need further information.
=======================FORWARDED TEXT STARTS HERE============================
=============================================================================
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.
=============================================================================
========================FORWARDED TEXT ENDS HERE=============================
If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident Response
and Security Teams (FIRST).
We strongly urge you to encrypt any sensitive information you send by email.
The CERT Coordination Center can support a shared DES key and PGP. Contact
the CERT staff for more information.
Location of CERT PGP key
ftp://info.cert.org/pub/CERT_PGP.key
CERT Contact Information
- - ------------------------
Email cert@cert.org
Phone +1 412-268-7090 (24-hour hotline)
CERT personnel answer 8:30-5:00 p.m. EST
(GMT-5)/EDT(GMT-4), and are on call for
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Fax +1 412-268-6989
Postal address
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
USA
CERT publications, information about FIRST representatives, and other
security-related information are available from[-1z
http://www.cert.org/
ftp://info.cert.org/pub/
CERT advisories and bulletins are also posted on the USENET newsgroup
comp.security.announce
To be added to our mailing list for CERT advisories and bulletins, send your
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cert-advisory-request@cert.org
CERT is a service mark of Carnegie Mellon University.
This file: ftp://info.cert.org/pub/cert_bulletins/VB-96.12.freebsd
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