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

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

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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 http://www.cisco.com/warp/public/707/ 

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 

No other Cisco products are affected. 

    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. 

    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. 

    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 http://www.cisco.com. 

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: tac@cisco.com 

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 "psirt@cisco.com" or 
"security-alert@cisco.com" for software upgrades. 

    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: http://www.cisco.com/ 
    xtocid365615 . 

    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 
    There is no workaround. 

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

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:// 
www.cisco.com/warp/public/707/CBOS-DoS.shtml. 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: 

  * cust-security-announce@cisco.com 
  * bugtraq@securityfocus.com 
  * first-teams@first.org (includes CERT/CC) 
  * cisco@spot.colorado.edu 
  * comp.dcom.sys.cisco 
  * firewalls@lists.gnac.com 
  * 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 http://www.cisco.com/warp/public/707/sec_incident_response.shtml 
This includes instructions for press inquiries regarding Cisco security 

All Cisco Security Advisories are available at 

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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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Version: PGP 6.5.3 



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Several Cisco ASA products - appliances, firewalls, switches, routers, and security modules - have been found sporting a flaw that can ultimately lead to remote code execution by attackers.

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