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                 NetBSD Security Advisory 2000-008
                 =================================

Topic:    dhclient vulnerability
Version:  All releases before 2000/07/10
Severity:  possible remote root access from rogue dhcp server.


Abstract
========

The DHCP client program, dhclient(8), did not correctly handle DHCP
options it receives in DHCP response messages, possibly permitting a
rogue dhcp server to send maliciously formed options which resulted in
a remote root compromise.

Technical Details
=================

dhclient uses a shell script, /etc/dhclient-script, to make changes to
system configurations based on configuration information received from
a DHCP server.  Unfortunately, dhcp options values were passed from
dhclient to the script without sufficient quoting.

Maliciously formed messages could therefore exploit shell features to
trick the script into executing arbitrary shell commands.  Since
dhclient-script needs to run as root to reconfigure system interfaces,
these arbitrary commands would also be executed as root.


Solutions and Workarounds
=========================

Systems running formal releases of NetBSD-1.4.2 and prior may be vulnerable.

Systems running versions of NetBSD prior to 1.4 should be upgraded to
NetBSD 1.4.2 before applying the fixes described here.

If your system does not and will never run the "/sbin/dhclient" daemon
to dynamically obtain an IP address, your system is not vulnerable to
this problem.

The command:

  % ps auxww | grep dhclient

will show if dhclient is running on a system.

If you are running any NetBSD 1.4.x release, you should download the
patch listed below, and apply it to src/usr.sbin/dhcp/client/options.c
using the patch(1) command.  If you are running NetBSD-current or
NetBSD-release, you should update your source tree (with either sup or
anonymous CVS) to a version containing the fix.  The problem was
corrected on the NetBSD-current mainline on 2000/06/24, on the
netbsd-1-4 release branch on 2000/06/29, and on the netbsd-1-5 release
branch on 2000/07/10.

In all cases you should then rebuild and reinstall DHCP:

  % cd src/usr.sbin/dhcp
  % make all
  # make install

You should then kill off and restart any existing dhclient processes.

Patch for all releases of 1.4.x:

    ftp://ftp.NetBSD.ORG/pub/NetBSD/misc/security/patches/20000708-dhclient

Thanks To
=========

Todd Fries and others of OpenBSD (for discovery of the problem),
Jun-ichiro Hagino <itojun@netbsd.org>, and Ted Lemon of ISC.

Revision History
================

  2000/06/28 - Initial version.
  2000/07/08 - Released
  2000/07/10 - Clarified correction dates for 1.4 vs 1.5 vs -current.

More Information
================

Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.ORG/ and http://www.NetBSD.ORG/Security/.


Copyright 2000, The NetBSD Foundation, Inc.  All Rights Reserved.

$NetBSD: NetBSD-SA2000-008.txt,v 1.3 2000/07/10 23:11:06 sommerfeld Exp $

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