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Cisco Security Advisory

CBOS - Improving Resilience to Denial-of-Service Attacks

Advisory ID: cisco-sa-20020523-cbos-dos

http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20020523-cbos-dos

Revision 1.2

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


Summary

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.

Affected Products

This section provides details on affected products.

Vulnerable 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.

Products Confirmed Not Vulnerable

No other Cisco products are currently known to be affected by these vulnerabilities.

Details

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.

Vulnerability Scoring Details

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
http://www.cisco.com/web/about/security/intelligence/cvss-qandas.html .

Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at
http://intellishield.cisco.com/security/alertmanager/cvss .


Impact

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 or later.


Workarounds

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:
    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 1.2.3.4 255.255.255.255 0.0.0.0 0.0.0.0
    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 1.2.3.4. 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).

    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
    cbos# write
  • CSCdx36121 - There is no workaround.

Obtaining Fixed Software

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

Customers with Service Contracts

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 Using Third-Party Support Organizations

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 Without Service Contracts

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

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.

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 discussed.

Status of This Notice: Final

THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.

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.


Distribution

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:

  • 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 1.2

2002-June-17

Updated Software Versions and Fixes section

Revision 1.1

2002-May-31

Updated Affected Products section

Revision 1.0

2002-May-23

Initial public 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/web/about/security/psirt/security_vulnerability_policy.html. This includes instructions for press inquiries regarding Cisco security notices. All Cisco Security Advisories are available at http://www.cisco.com/go/psirt.


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