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TUCoPS :: Cisco :: ciacl086.txt

Cisco Multiple Vulnerabilities in CBOS


                       The U.S. Department of Energy
                     Computer Incident Advisory Center
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                          /       |     /_\   /
                          \___  __|__  /   \  \___

                             INFORMATION BULLETIN

                     Cisco Multiple Vulnerabilities in CBOS

May 23, 2001 01:00 GMT                                            Number L-086
PROBLEM:       TCP Sequence Prediction, echo request denial of service 
               vulnerabilities, NVRAM password stored cleartext. 
PLATFORM:      Cisco 600 series routers (CBOS Software) 
DAMAGE:        Ranges from denial of service to unauthorized access and 
               disclosure, depending on vulnerability exploited. 
SOLUTION:      Upgrade to releases not vulnerable, as described below. 
VULNERABILITY  MEDIUM. There are multiple vulnerabilities described, ranging 
ASSESSMENT:    from temporary denial of service, to compromise of the router. 

[******  Start Cisco Advisory ******]

        Cisco Security Advisory: More multiple vulnerabilities in CBOS
Revision 1.0

  For public release 2001 May 22 08:00 (GMT -0800)

   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
       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
       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

   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.
   No other releases of CBOS software are affected by these
   vulnerabilities. No other Cisco products are affected by these
   These vulnerabilities are fixed in the following CBOS releases: 2.3.9,
   2.4.1 and 2.4.2.

          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
          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.

          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
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.
   |           |                |                                     |
   |  Release  | Description or |  Availability of Repaired Releases  |
   |           |   Platform     |=====================================+
   |           |                |      General Availability (GA)      |
   |    All    | All platforms  |      2.3.9                          |
   | releases  |                |      2001-Mart-19                   |
   |    All    | All platforms  |      2.4.1                          |
   | releases  |                |      2000-December-11               |
   |    All    | All platforms  |      2.4.2                          |
   | releases  |                |      2001-May-14                    |

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 via the point-of-sale or, if they posses a Cisco
   Connection Online account, they can download it from the Software
   Center on Cisco's Worldwide Web site at
   Customers without contracts should get their upgrades by contacting
   the Cisco Technical Assistance Center (TAC). TAC contacts are as
     * +1 800 553 2447 (toll-free from within North America)
     * +1 408 526 7209 (toll call from anywhere in the world)
     * e-mail:
   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.

          There is no workaround.
          There is no workaround.
          There is no workaround.
          There is no workaround.
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

   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.

   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 1.0 2001-May-22 08:00 GMT-0800 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.
   This notice is Copyright 2000 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.

[******  End Cisco Advisory ******]


CIAC wishes to acknowledge the contributions of Cisco for the 
information contained in this bulletin.

CIAC, the Computer Incident Advisory Center, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
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