Advisory ID: cisco-sa-20020523-cbos-dos
For Public Release 2002 May 23 16:00 UTC (GMT)
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://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20020523-cbos-dos.
This section provides details on 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: 626, 627, 633, 673, 675, 675e, 676, 677, 677i and 678.
No other Cisco products are currently known to be affected by these vulnerabilities.
This section details the vulnerabilities described in this document.
- 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.
Cisco has provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in this Security Advisory is done in accordance with CVSS version 2.0.
CVSS is a standards-based scoring method that conveys vulnerability severity and helps determine urgency and priority of response.
Cisco has provided a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.
Cisco has provided an FAQ to answer additional questions regarding CVSS at
Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at
By repeatedly exploiting these vulnerabilities an attacker can cause a DOS for an indeterminate period of time.
All vulnerabilities are fixed in CBOS version 2.4.5 or later.
This section describes workarounds for the vulnerabilities described in this document.
- CSCdw90020 - The workaround is to filter DHCP requests. This task must be executed while in enable mode.
To filter DHCP packets use this procedure:
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 184.108.40.206. 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 you LAN side (behind the eth0 interface).
cbos# set filter 0 on allow incoming eth0 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0 protocol
udp srcport 68-68 destport 67-67
cbos# set filter 1 on allow outgoing eth0 220.127.116.11 255.255.255.255 0.0.0.0 0.0.0.0
protocol udp srcport 67-67 destport 68-68
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/en/US/products/hw/modems/ps296/products_installation_guide_book09186a008007dd7e.html.
- 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
- CSCdx36121 - There is no workaround.
Cisco has made free software available to address this vulnerability for affected customers. Prior to deploying software, customers should consult their maintenance provider or check the software for feature set compatibility and known issues specific to their environment.
Customers may only install and expect support for the feature sets they have purchased. By installing, downloading, accessing or otherwise using such software upgrades, customers agree to be bound by the terms of Cisco's software license terms found at http://www.cisco.com/public/sw-license-agreement.html, or as otherwise set forth at Cisco.com Downloads at http://www.cisco.com/public/sw-center/sw-usingswc.shtml.
Do not contact either "email@example.com" or "firstname.lastname@example.org" for software upgrades.
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 website 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 guidance and assistance with the appropriate course of action in regards to this advisory.
The effectiveness of any workaround or fix is dependent on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. Due to the variety of affected products and releases, customers should consult with their service provider or support organization to ensure any applied workaround or fix is the most appropriate for use in the intended network before it is deployed.
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: email@example.com
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.
Refer to http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.
These vulnerabilities were reported by Knud Erik HÃ¸jgaard from Cybercity, Denmark. The exploit code for CSCdv50135 was made public by a third party unrelated to Knud HÃ¸jgaard in any way. This vulnerability was also publicly discussed.
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A stand-alone copy or Paraphrase of the text of this document 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://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20020523-cbos-dos. 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:
- firstname.lastname@example.org (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.
Updated Software Versions and Fixes section
Updated Affected Products section
Initial public release