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Home » Hacking News » CERT Advisory CA-2002-12 Format String Vulnerability in ISC DHCPD

CERT Advisory CA-2002-12 Format String Vulnerability in ISC DHCPD

by Nikola Strahija on May 9th, 2002 The Internet Software Consortium (ISC) provides a Dynamic Host Configuration Protocol Daemon (DHCPD), which is a server that is used to allocate network addresses and assign configuration parameters to hosts. A format string vulnerability may permit a remote attacker to execute code with the privileges of the DHCPD (typically root). We have not seen active scanning or exploitation of this vulnerability.

I. Description

ISC's DHCPD listens for requests from client machines connecting to
the network. Versions 3 to 3.0.1rc8 (inclusive) of DHCPD contains an
option (NSUPDATE) that is enabled by default. NSUPDATE allows the DHCP
server to send information about the host to the DNS server after
processing a DHCP request. The DNS server responds by sending an
acknowledgement message back to the DHCP server that may contain
user-supplied data (like a host name). When the DHCP server receives
the acknowledgement message from the DNS server, it logs the

A format string vulnerability exists in ISC's DHCPD code that logs the
transaction. This vulnerability may permit a remote attacker to
execute code with the privileges of the DHCP daemon.

II. Impact

A remote attacker may be able to execute code with the privileges of
the DHCPD (typically root).

III. Solution

Note that some of the mitigation steps recommended below may have
significant impact on your normal network operations. Ensure that any
changes made based on the following recommendations will not
unacceptably affect any of your operations.

Apply a patch from your vendor

Appendix A contains information provided by vendors for this advisory.

Disable the DHCP service

As a general rule, the CERT/CC recommends disabling any service or
capability that is not explicitly required. Depending on your network
configuration, you may not need to use DHCP.

Ingress filtering

As a temporary measure, it may be possible to limit the scope of this
vulnerability by blocking access to DHCP services at the network

Ingress filtering manages the flow of traffic as it enters a network
under your administrative control. In the network usage policy of many
sites, there are few reasons for external hosts to initiate inbound
traffic to machines that provide no public services. Thus, ingress
filtering should be performed at the border to prohibit externally
initiated inbound traffic to non-authorized services. For DHCP,
ingress filtering of the following ports can prevent attackers outside
of your network from reaching vulnerable devices in the local network
that are not explicitly authorized to provide public DHCP services.

bootps 67/tcp # Bootstrap Protocol Server
bootps 67/udp # Bootstrap Protocol Server
bootpc 68/tcp # Bootstrap Protocol Client
bootpc 68/udp # Bootstrap Protocol Client

Appendix A. - Vendor Information

This appendix contains information provided by vendors for this
advisory. As vendors report new information to the CERT/CC, we will
update this section and note the changes in our revision history. If a
particular vendor is not listed below, please check the Vulnerability
Note (VU#854315) or contact your vendor directly.


The security of our customers' networks is of highest priority for
Alcatel. Alcatel is aware of this security issue in the DHCP
implementation of ISC and has put measures in place to assess which
of its products might be affected and to apply the necessary fixes
where required. An update will be shortly published to provide more
details on any affected products.


Conectiva Linux 8 ships dhcp-3.0 and is vulnerable to this problem.
Updates will be available at our ftp site and an announcement will
be sent to our mailing lists as soon as CERT publishes its

F5 Networks, Inc.

F5 Networks' products do not include any affected version of ISC's
DHCPD, and are therefore not vulnerable.


The FreeBSD base system does not ship with the ISC dhcpd server by
default and is not affected by this vulnerability. The ISC dhcpd
server is available in the FreeBSD Ports Collection; updates to the
ISC dhcp port (ports/net/isc-dhcp3) are in progress and corrected
packages will be available in the near future.


IBM's AIX operating system, all versions, is not vulnerable.

Internet Software Consortium

A patch is included below, and we have a patched version of 3.0
available (3.0pl1) and a new release candidate for the next bug-fix
release (3.0.1RC9). Both of these new releases are not vulnerable.

--- common/print.c Tue Apr 9 13:41:17 2002
+++ common/print.c.patched Tue Apr 9 13:41:56 2002
@@ -1366,8 +1366,8 @@
*s++ = '.';
*s++ = 0;
if (errorp)
- log_error (obuf);
+ log_error ("%s",obuf);
- log_info (obuf);
+ log_info ("%s",obuf);
#endif /* NSUPDATE */

Lotus Development Corporation

This issue does not affect Lotus products.

Microsoft Corporation

Microsoft does not ship the ISC DHCPD program.


NetBSD fixed this during a format string sweep performed on
11-Oct-2000. No released version of NetBSD is vulnerable to this

Silicon Graphics, Inc.

SGI is not vulnerable.

The CERT Coordination Center acknowledges Next Generation Security
Technologies as the discoverer of this vulnerability and thanks them
and the Internet Software Consortium (ISC) for their cooperation,
reporting, and analysis of this vulnerability.

Feedback can be directed to the author: Ian A. Finlay

This document is available from:

CERT/CC Contact Information

Email: [email protected]
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
Postal address:
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890

CERT/CC personnel answer the hotline 08:00-17:00 EST(GMT-5) /
EDT(GMT-4) Monday through Friday; they are on call for emergencies
during other hours, on U.S. holidays, and on weekends.

Using encryption

We strongly urge you to encrypt sensitive information sent by email.
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If you prefer to use DES, please call the CERT hotline for more

Getting security information

CERT publications and other security information are available from
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send email to [email protected] Please include in the body of your

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* "CERT" and "CERT Coordination Center" are registered in the U.S.
Patent and Trademark Office.

Any material furnished by Carnegie Mellon University and the Software
Engineering Institute is furnished on an "as is" basis. Carnegie
Mellon University makes no warranties of any kind, either expressed or
implied as to any matter including, but not limited to, warranty of
fitness for a particular purpose or merchantability, exclusivity or
results obtained from use of the material. Carnegie Mellon University
does not make any warranty of any kind with respect to freedom from
patent, trademark, or copyright infringement.

Conditions for use, disclaimers, and sponsorship information

Copyright 2002 Carnegie Mellon University.

Revision History
May 8, 2002: Initial release

Version: PGP 6.5.8


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