FreeBSD Security Advisory: FreeBSD-SA-00:34.dhclient

FreeBSD Security Advisories security-advisories at
Mon Aug 14 15:48:59 PDT 2000


FreeBSD-SA-00:34                                           Security Advisory
                                                                FreeBSD, Inc.

Topic:		dhclient vulnerable to malicious dhcp server

Category:	core, ports
Module:		dhclient, isc-dhcp2 (ports), isc-dhcp3 (ports)
Announced:	2000-08-14
Affects:        All releases of FreeBSD after FreeBSD 3.2-RELEASE and
                prior to the correction date (including FreeBSD 4.0
                and 3.5, but not 4.1)
                Ports collection prior to the correction date.
Credits:	OpenBSD
Vendor status:	Updated version released
Corrected:	2000-07-20 [FreeBSD 4.0 base system]
                2000-08-01 [isc-dhcp2 port]
                2000-07-21 [isc-dhcp3 port]
FreeBSD only:	NO

I.   Background

ISC-DHCP is an implementation of the DHCP protocol containing client
and server. FreeBSD 3.2 and above includes the version 2 client by
default in the base system, and the version 2 and version 3 clients
and servers in the Ports Collection.

II.  Problem Description

The dhclient utility (DHCP client), versions 2.0pl2 and before (for
the version 2.x series), and versions 3.0b1pl16 and before (for the
version 3.x series) does not correctly validate input from the server,
allowing a malicious DHCP server to execute arbitrary commands as root
on the client. DHCP may be enabled if your system was initially
configured from a DHCP server at install-time, or if you have
specifically enabled it after installation.

FreeBSD 4.1 is not affected by this problem since it contains the
2.0pl3 client.

III. Impact

An attacker who has or gains control of a DHCP server may gain
additional root access to DHCP clients running vulnerable versions of

If you are not using dhclient to configure client machines via DHCP,
or your DHCP server is "trusted" according to your local security
policy, then this vulnerability does not apply to you.

IV.  Workaround

Disable the use of DHCP for configuring client machines: remove the
case-insensitive string "dhcp" from the "ifconfig_<foo>" directives in
/etc/rc.conf and replace it with appropriate static interface
configuration according to the rc.conf(5) manpage.

An example of a DHCP-enabled interface is the following line in


V.   Solution

NOTE: At this time the FreeBSD 3.x branch has not yet been patched,
due to logistical difficulties. Users running a vulnerable 3.x system
are advised to either upgrade to FreeBSD 4.1, disable the use of
DHCP as described above, or use the dhclient binary from the isc-dhcp2
port dated after the correction date.

1) Upgrade your vulnerable FreeBSD 4.0 system to a version dated after the
correction date. See

for instructions on how to upgrade and recompile your FreeBSD system
from source, or perform a binary upgrade, e.g. to FreeBSD 4.1-RELEASE,
described here:

2) (If using the isc-dhcp2 or isc-dhcp3 ports) One of the following:

2a) Upgrade your entire ports collection and rebuild the isc-dhcp2 or isc-dhcp3 port.

2b) Deinstall the old package and install a new package dated after the
correction date, obtained from:


NOTE: The isc-dhcp2 port is not available as a package.

2c) download a new port skeleton for the isc-dhcp2 or isc-dhcp3 port from:

and use it to rebuild the port.

2d) Use the portcheckout utility to automate option (3) above. The
portcheckout port is available in /usr/ports/devel/portcheckout or the
package can be obtained from:

Version: 2.6.2


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