Products & Services

More Multiple Vulnerabilities in CBOS

Advisory ID: cisco-sa-20010522-cbos

Revision 1.1

For Public Release 2001 May 22 15:00  UTC (GMT)


Multiple vulnerabilities have been identified and fixed in CBOS, an operating system for the Cisco 600 family of routers.

  • Cisco CBOS Software contains a flaw that permits the successful prediction of TCP Initial Sequence Numbers. It only affects the security of TCP connections that originate or terminate on the affected Cisco device itself; it does not apply to TCP traffic forwarded through the affected device in transit between two other hosts.
    This vulnerability is documented as Cisco bug ID CSCds16078.
  • A Cisco 600 router may stop passing the traffic and responding to the console when an ECHO REQUEST packet with the record route option is routed through it.
    This vulnerability is documented as Cisco bug ID CSCds30150.
  • Passwords, exec and enable, are stored in the cleartext in the NVRAM.
    This vulnerability is documented as Cisco bug ID CSCdt04882.
  • When multiple, large ECHO REPLY packets are routed through an affected Cisco 600 router, it will enter the ROMMON mode and stop passing any further traffic.
    This vulnerability is documented as Cisco bug ID CSCds74567.

The following releases of CBOS are containing all of mentioned vulnerabilities: 2.0.1, 2.1.0, 2.1.0a, 2.2.0, 2.2.1, 2.2.1a, 2.3, 2.3.2, 2.3.5, 2.3.7 and 2.3.8.

These vulnerabilities are fixed in the following CBOS releases: 2.3.9, 2.4.1 and 2.4.2. Customers are urged to upgrade to releases that are not vulnerable as shown in detail in the section Software Versions and Fixes below.

This advisory is available at the

Affected Products

This section provides details on affected products.

Vulnerable Products

The affected models are: 627, 633, 673, 675, 675E, 677, 677i and 678.

These models are vulnerable if they run any of the following, or earlier, CBOS releases: 2.0.1, 2.1.0, 2.1.0a, 2.2.0, 2.2.1, 2.2.1a, 2.3, 2.3.2, 2.3.5, 2.3.7 and 2.3.8.

These vulnerabilities are fixed in the following CBOS releases: 2.3.9, 2.4.1 and 2.4.2.

Products Confirmed Not Vulnerable

No other releases of CBOS software are affected by these vulnerabilities. No other Cisco products are currently known to be affected by these vulnerabilities.



See also

TCP sequence numbers are 32-bit integers in the circular range of 0 to 4,294,967,295. The host devices at both ends of a TCP connection exchange an Initial Sequence Number (ISN) selected at random from that range as part of the setup of a new TCP connection.

This method provides reasonably good protection against accidental receipt of unintended data. However, to guard against malicious use, it should not be possible for an attacker to infer a particular number in the sequence. If the initial sequence number is not chosen randomly or if it is incremented in a non-random manner between the initialization of subsequent TCP sessions, then it is possible, with varying degrees of success, to forge one half of a TCP connection with another host in order to gain access to that host, or hijack an existing connection between two hosts in order to compromise the contents of the TCP connection. To guard against such compromises, ISNs should be generated as randomly as possible.


By sending ICMP ECHO REQUEST packets (ping) with the IP Record Route option set it is possible to freeze a Cisco 600 router. This can be done either by sending the specially crafted packet or by specifying the "-r" option on the most ping programs. The packet should not be destined to a router itself.


The exec and enable passwords are stored in the cleartext in NVRAM. Similarly, they are also stored in the cleartext in the configuration file if one is stored on a computer. Anyone who is in a position to see a router's configuration, either directly from the device or in the file on a computer, can learn the passwords.

This vulnerability is corrected by storing only an MD5 hash of the password in both NVRAM and in the configuration file, and the plaintext password itself is never retained.


When multiple ICMP ECHO REPLY packets, non standard size, are passed through the affected device the device will stop passing any further traffic. Packets must be larger than the usual size (64 bytes) but that can be easily accomplished either by crafting packets or by adjusting the response size, either via command line or by modifying the program source.

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 .

Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at .


CSCds16078 Forged packets can be injected into a network from a location outside its boundary so that they are trusted as authentic by the receiving host, thus resulting in a failure of integrity. Such packets could be crafted to gain access or make some other modification to the receiving system in order to attain some goal, such as gaining unauthorized interactive access to a system or compromising stored data. >From a position within the network where it is possible to receive the return traffic (but not necessarily in a position that is directly in the traffic path), a greater range of violations is possible. For example, the contents of a message could be diverted, modified, and then returned to the traffic flow again, causing a failure of integrity and a possible failure of confidentiality. NOTE: Any compromise using this vulnerability is only possible for TCP sessions that originate or terminate on the affected Cisco device itself. It does not apply to TCP traffic that is merely forwarded through the device.


It is possible to cause the Denial-of-Service.


Anyone who is in a position to see a router's configuration, either directly from the device or in the file on a computer, can learn the exec and enable passwords. Armed with that knowledge, an attacker can log into the device and change the router's configuration.

This vulnerability can be even more dangerous if the ISP is using the same passwords for all of the devices which it manages. Such practice, using the same passwords for multiple devices, is strongly discouraged.


It is possible to cause the Denial-of-Service to many affected devices.

Software Versions and Fixes

The following table summarizes the CBOS software releases affected by the vulnerabilities described in this notice and scheduled dates on which the earliest corresponding fixed releases will be available.

Major Release

Description or Platform

Availability of Repaired Releases


General Availability (GA)

All releases

All platforms

2.3.9 2001-MAR-19

All releases

All platforms

2.4.1 2000-DEC-11

All releases

All platforms

2.4.2 2001-MAY-14



There is no workaround.


There is no workaround.


There is no workaround.


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, or as otherwise set forth at Downloads at

Do not contact either "" or "" 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

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 in obtaining fixed software through their point of sale should obtain software patches and bug fixes 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:

Have your product serial number available and provide the URL of this notice as evidence of entitlement to a software patch or bug fix. Customers without service contracts should request a software patch or bug fix through the TAC.

Refer to 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

Vulnerabilitiy CSCds30150 has been made public on VULN-DEV list.

Altough we have not seen public discussion of vulnerability CSCdt04882 we understand that it is commonly known among users.

Vulnerability CSCds74567 has been reported to us by a customer.

Status of this Notice: Final


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 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 1.1


Changed Obtaining Fixed Software section to include the TAC contacts within the text paragraph instead of separated out in a list. Removed line in that same section about free software upgrades.

Revision 1.0


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 Web site at This includes instructions for press inquiries regarding Cisco security notices.