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Cisco Catalyst Memory Leak Vulnerability
Published: 2000-12-06 00:00:00
Updated: 2000-12-06 00:00:00

Cisco Catalyst Memory Leak Vulnerability

Revision 1.0
For Release 2000 December 6 08:00 AM US/Pacific (UTC+0700)



A series of failed telnet authentication attempts to the switch can cause
the Catalyst Switch to fail to pass traffic or accept management
connections until the system is rebooted or a power cycle is performed. All
types of telnet authentication are affected, including Kerberized telnet,
and AAA authentication.

This vulnerability has been assigned Cisco bug ID CSCds66191.

The complete advisory can be viewed at

Affected Products

Catalyst 4000 and 5000 images running version  4.5(2) up to 5.5(4) and
Catalyst 6000 images running version 5.3(1)CSX, up to and including 5.5(4),

No other releases of Cisco Catalyst software are affected by this
No other Cisco products are affected by this vulnerability.


The telnet process fails to release resources upon a failed authentication,
or a successful login of extremely short duration such as a telnet from
within an automated script. This memory leak eventually results in the
failure of the switch to perform any other processes, such as forwarding
traffic or management; a power cycle or reboot is required for recovery.

The command "show process memory" will indicate increased "Holding" memory
after failed telnet authentication attempts. This value will not decrease
over time except when a reboot, reload, or power cycle occurs. This bug may
be triggered over a period of time in the course of normal operation by
legitimate users that occasionally fail authentication.

lt-6509-e> (enable) sh proc mem

Memory Used:   3974544
       Free:  15265168
      Total:  19239712

PID        TTY        Allocated  Freed      Holding    Process
- ---------- ---------- ---------- ---------- ---------- ---------------
1          -2         1707632    3488       1704144    Kernel and Idle

24         -2         16         0          16         telnetd


This vulnerability enables a Denial of Service attack on the Catalyst

Software Versions and Fixes

Cisco has made the following fixed software available to customers:

Catalyst Release 4.5(10) for Catalyst 4000 and 5000.
Catalyst Release 5.5(4b) for Catalyst 4000, 5000 and 6000.

The fix will be carried forward into all future releases.

Getting Fixed Software

Cisco is offering free software upgrades to remedy 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

Customers without contracts 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:

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 configuration workaround to eliminate the problem.  However, if
you are unable to upgrade to an unaffected version, you may use other
devices to strictly control or prohibit telnet access to the switch,
permitting only connections from your local network.

Access control lists on the switch can limit the remote exploitation of the
vulnerability.  To limit access to known hosts use the following commands:

    set ip permit enable telnet
    set ip permit <addr> [mask]

Remote management of the switch can also be disabled.

The above workarounds are provided as an option; however, the
recommendation is to upgrade to fixed code as soon as possible.

Exploitation and Public Announcements

Cisco Systems knows of no public discussion nor active exploits involving
this vulnerability, which was reported by a customer who noted the memory

Status of This Notice: FINAL

This is a final field 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 1.006-DEC-2000 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.


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, including
all date and version information.



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