Vulnerability Note VU#149953

ISC "dhcrelay" fails to limit hop count when malicious bootp packet is received

Overview

A vulnerability in the Internet Software Consortium's "dhcrelay" makes it possible for a remote attacker to use dhcrelay to launch a denial-of-service attack against a victim dhcp server.

I. Description

The Internet Software Consortium (ISC) produces a "freely redistributable reference implementation of all aspects of the DHCP protocol, through a suite of tools." One of these tools is a dhcp relay agent (dhcrelay). From the dhcrelay man page:

The Internet Software Consortium DHCP Relay Agent, dhcrelay, provides a means for relaying DHCP and BOOTP requests from a subnet to which no DHCP server is directly connected to one or more DHCP servers on other subnets. The DHCP Relay Agent listens for DHCP and BOOTP queries and responses. When a query is received from a client, dhcrelay forwards it to the list of DHCP servers specified on the command line. When a reply is received from a server, it is broadcast or unicast (according to the relay agent's ability or the client's request) on the network from which the original request came.

A vulnerability exists in the way dhcrelay processes incoming bootp requests. This vulnerability can allow a remote attacker to launch a denial-of-service attack against DHCP servers configured to communicate with the dhcrelay host. Debian Security Advisory DSA 245-1 succinctly summarizes the problem:

When the dhcp-relay receives a BOOTP request it forwards the request to the DHCP server using the broadcast MAC address ff:ff:ff:ff:ff:ff which causes the network interface to reflect the packet back into the socket. To prevent loops the dhcrelay checks whether the relay-address is its own, in which case the packet would be dropped. In combination with a missing upper boundary for the hop counter an attacker can force the dhcp-relay to send a continuing packet storm towards the configured dhcp server(s).

II. Impact

A remote attacker can use dhcrelay to launch a denial-of-service attack against DHCP servers configured to communicate with the dhcrelay host.

III. Solution

Apply a patch from your vendor.

Systems Affected

Vendor	Status	Date Notified
Apple Computer Inc.	Not Vulnerable	5-Feb-2003
Cisco Systems Inc.	Not Vulnerable	13-Feb-2003
Conectiva Linux	Vulnerable	7-Apr-2003
Debian	Vulnerable	4-Feb-2003
Hewlett-Packard Company	Not Vulnerable	5-Feb-2003
Ingrian Networks	Not Vulnerable	5-Feb-2003
Internet Software Consortium	Vulnerable	5-Feb-2003
Lotus Software	Not Vulnerable	5-Feb-2003
Network Appliance	Not Vulnerable	4-Feb-2003
OpenPKG	Vulnerable	26-Feb-2003
Red Hat Inc.	Vulnerable	1-Apr-2003
Xerox Corporation	Not Vulnerable	30-May-2003

References

http://online.securityfocus.com/bid/6628
http://online.securityfocus.com/archive/1/307451
http://linuxcommand.org/man_pages/dhcrelay8.html
http://www.uniras.gov.uk/l1/l2/l3/brief2003/Brief%20-%205803.txt
http://www.cis.ohio-state.edu/cgi-bin/rfc/rfc1531.html

Credit

This vulnerability was discovered by Florian Lohoff and reported to the BugTraq mailing list on January 15, 2003.

This document was written by Ian A Finlay.

Other Information

Date Public:	2003-01-15
Date First Published:	2003-02-04
Date Last Updated:	2003-05-30
CERT Advisory:
CVE-ID(s):	CAN-2003-0039
NVD-ID(s):	CAN-2003-0039
US-CERT Technical Alerts:
Metric:	8.10
Document Revision:	17

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