Security Advisory: CBOS - Improving Resilience to DoS Attacks

Type securityvulns
Reporter Securityvulns
Modified 2002-05-24T00:00:00



Cisco Security Advisory: CBOS - Improving Resilience to Denial-of-Service Attacks ========================================================================= Revision 1.0

For Public Release 2002 May 23 16:00 (UTC+0000)


Three new vulnerabilities are identified in Cisco Broadband Operating System (CBOS), an operating system for the Cisco 600 family of routers. Each vulnerability can cause a Denial of Service (DoS) by freezing the customer premises equipment (CPE). All three vulnerabilities can be exploited remotely.

No other Cisco product is vulnerable.

Workarounds are provided for two of the three vulnerabilities. Note that the workarounds provided may not be applicable in all cases. See the Workarounds section for further details.

This advisory is available at CBOS-DoS.shtml.

Affected Products

All Cisco DSL CPE devices from the 600 family running CBOS software up to and including 2.4.4 release are vulnerable. The complete list of vulnerable hardware models is: 605, 626, 627, 633, 673, 675, 675e, 676, 677, 677i and 678.

No other Cisco products are affected.


CSCdw90020 By sending a large packet to the Dynamic Host Configuration Protocol (DHCP) port it is possible to freeze the CPE. DHCP service is enabled by default.

CSCdv50135 By sending a large packet to the Telnet port it is possible to freeze the CPE. It is not necessary to be logged in or to authenticate in any way. Telnet is enabled by default.

CSCdx36121 The TCP/IP stack will consume all memory while processing received packets. This will happen only if the CPE must process a high number of overly large packets. These packets must have the CPE as the destination. After the memory is exhausted the CPE will lock up and stop forwarding any further packets.


By repeatedly exploiting these vulnerabilities an attacker can cause a DoS for an indeterminate period of time.

Software Versions and Fixes

All vulnerabilities are fixed in CBOS version 2.4.5.

Obtaining Fixed Software

Cisco is offering free software upgrades to eliminate this vulnerability for all affected customers.

Customers with contracts should obtain upgraded software through their regular update channels. For most customers, this means that upgrades should be obtained through the Software Center on Cisco's Worldwide Web site at

Customers whose Cisco products are provided or maintained through prior or existing agreement with third-party support organizations such as Cisco Partners, authorized resellers, or service providers should contact that support organization for assistance with the upgrade, which should be free of charge.

Customers who purchase direct from Cisco but who do not hold a Cisco service contract and customers who purchase through third-party vendors but are unsuccessful at obtaining fixed software through their point of sale should get their upgrades by contacting the Cisco Technical Assistance Center (TAC). TAC contacts are as follows:

  • +1 800 553 2447 (toll-free from within North America)
  • +1 408 526 7209 (toll call from anywhere in the world)
  • e-mail:

Please have your product serial number available and give the URL of this notice as evidence of your entitlement to a free upgrade. Free upgrades for non-contract customers must be requested through the TAC.

Please do not contact either "" or "" for software upgrades.


CSCdw90020 The workaround is to filter DHCP requests. This task must be executed while in enable mode.

To filter DHCP packets use this procedure:

cbos# set filter 1 on allow incoming eth0 protocol
udp srcport 68-68 destport 67-67
cbos#set filter 2 on allow outgoing eth0
protocol udp srcport 67-67 destport 68-68

The filter "0" will allow all DHCP requests from your internal network
to the CPE. The filter "1" will allow all DHCP responses from the CPE.
In this example, the eth0 interface of the CPE has the IP address of You must substitute this address with the IP address of your
eth0 port. This configuration is not the complete workaround since you
are still exposed from your LAN side (behind the eth0 interface).

Note: There is an implicit "deny all" as the last filter so you must
include additional "permit" filters to allow a normal traffic flow. If
you already have filters configured, you should combine this example
with the configured filters and, probably, change the filter numbers to
suit your configuration. Also note that this workaround is not
applicable if you must have DHCP enabled on the WAN side.

For information regarding filters, refer to:
xtocid365615 .

CSCdv50135 The workaround is to disable Telnet. This task must be executed while in enable mode.

To disable Telnet use this procedure:

    cbos# set telnet disable
    cbos# write

CSCdx36121 There is no workaround.

Exploitation and Public Announcements

These vulnerabilities were reported by Knud Erik Hшjgaard from Cybercity, Denmark. The exploit code for CSCdv50135 was made public by a third party unrelated to Knud Hшjgaard in any way. This vulnerability was also publicly discussed.

Status of This Notice: FINAL

This is a final notice. Although Cisco cannot guarantee the accuracy of all statements in this notice, all of the facts have been checked to the best of our ability. Cisco does not anticipate issuing updated versions of this notice unless there is some material change in the facts. Should there be a significant change in the facts, Cisco may update this notice.

A standalone copy or paraphrase of the text of this security advisory that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.


This notice will be posted on Cisco's Worldwide Web site at http:// In addition to Worldwide Web posting, a text version of this notice is clear-signed with the Cisco PSIRT PGP key and is posted to the following e-mail and Usenet news recipients:

  • (includes CERT/CC)
  • Various internal Cisco mailing lists

Future updates of this notice, if any, will be placed on Cisco's Worldwide Web server, but may or may not be actively announced on mailing lists or newsgroups. Users concerned about this problem are encouraged to check the URL given above for any updates.

Revision History

+------------------------------------------------------+ |Revision |2002-May-23 16:00 |Initial public | |1.0 |UTC+0000 |release | +------------------------------------------------------+

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco products, obtaining assistance with security incidents, and registering to receive security information from Cisco, is available on Cisco's worldwide website at This includes instructions for press inquiries regarding Cisco security notices.

All Cisco Security Advisories are available at

This notice is Copyright 2002 by Cisco Systems, Inc. This notice may be redistributed freely after the release date given at the top of the text, provided that redistributed copies are complete and unmodified, and include all date and version information.

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