Vulnerability Note VU#149953

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

Original Release date: 04 Feb 2003 | Last revised: 30 May 2003


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.


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).


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


Apply a patch from your vendor.

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
Conectiva LinuxAffected-07 Apr 2003
DebianAffected-04 Feb 2003
Internet Software ConsortiumAffected04 Feb 200305 Feb 2003
OpenPKGAffected-26 Feb 2003
Red Hat Inc.Affected-01 Apr 2003
Apple Computer Inc.Not Affected-05 Feb 2003
Cisco Systems Inc.Not Affected-13 Feb 2003
Hewlett-Packard CompanyNot Affected04 Feb 200305 Feb 2003
Ingrian NetworksNot Affected04 Feb 200305 Feb 2003
Lotus SoftwareNot Affected-05 Feb 2003
Network ApplianceNot Affected-04 Feb 2003
Xerox CorporationNot Affected-30 May 2003
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base N/A N/A
Temporal N/A N/A
Environmental N/A N/A



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

  • CVE IDs: CAN-2003-0039
  • Date Public: 15 Jan 2003
  • Date First Published: 04 Feb 2003
  • Date Last Updated: 30 May 2003
  • Severity Metric: 8.10
  • Document Revision: 17


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