Operating System:


12 August 1998

Protect yourself against future threats.


              AUSCERT External Security Bulletin Redistribution
              ESB-98.133 -- Cisco IOS software security notice
                       Cisco IOS Remote Router Crash
                               13 August 1998


Cisco Systems, Inc. has released the following advisory concerning a
vulnerability in Cisco IOS software which may cause the router to crash
and reload without having to log in to the router.  This vulnerability may
be exploited using direct console or asynchronous serial connections
(including dialup connections), TELNET connections, UNIX "r" command
connections, LAT connections, MOP connections, X.29 connections, V.120
connections, and possibly others.

- ---------------------------BEGIN INCLUDED TEXT---------------------------


Field Notice:
Cisco IOS Remote Router Crash
Revision 1.2
For release 08:00 AM US/Pacific, Wednesday, August 12, 1998
Cisco internal use only until release date

An error in Cisco IOS software makes it possible for untrusted,
unauthenticated users who can gain access to the login prompt of a router or
other Cisco IOS device, via any means, to cause that device to crash and

This applies only to devices running classic Cisco IOS software. This
includes most Cisco routers with model numbers greater than or equal to
1000, but does not include the 7xx series, the Catalyst LAN switches, WAN
switching products in the IGX or BPX lines, the AXIS shelf, early models of
the LS1010 or LS2020 ATM switches, or any host-based software.

Who Is Affected
All users of classic Cisco IOS software versions 9.1 and later, but earlier
than the repaired versions listed in the "Details" section of this notice,
whose devices can be connected to interactively by untrusted users, are
affected by this vulnerability. It is not necessary to be able to actually
log in to exploit this vulnerability; simply establishing a terminal
connection is sufficient.

Note that some of the repaired software has been in the field for some time;
you may already have installed it. Please check your software version number
before assuming that you are affected.

The vulnerability can be exploited using direct console or asynchronous
serial connections (including dialup connections), TELNET connections, UNIX
"r" command connections, LAT connections, MOP connections, X.29 connections,
V.120 connections, and possibly others. Except in extraordinary security
environments, administrators are strongly encouraged to assume that hostile
users can find ways to make interactive connections to their Cisco IOS

If you are not running classic Cisco IOS software, then you are not affected
by this vulnerability. If you are unsure whether your device is running
classic Cisco IOS software, log into the device and issue the command show
version. Classic Cisco IOS software will identify itself simply as "IOS" or
"Internetwork Operating System Software", and affected software will have a
version number greater than or equal to 9.1. Other Cisco devices either will
not have the show version command, or will give different output.

If attackers know the details of the Cisco IOS software error they will be
able to cause the router to crash and reload without having to log in to the
router. Because this problem involves damage to an internal data struture,
it is possible that other, more subtle or targeted effects on system
operation could also be induced by proper exploitation. Such exploitation,
if it is possible at all, would require significant engineering skill and a
thorough knowledge of the internal operation of Cisco IOS software,
including Cisco trade secret information.

The Cisco IOS software error has been assigned Cisco bug ID CSCdj43337.

Affected and Repaired Software Versions
- - -------------------------------------
This vulnerability affects all releases of Classic Cisco IOS software from
9.1 up to, but not including, the following corrected releases (including
interim and beta software):

   * 11.3(1),  11.3(1)ED, 11.3(1)T
   * 11.2(10), 11.2(9)P, 11.2(9)XA, 11.2(10)BC
   * 11.1(15)CA, 11.1(16), 11.1(16)IA, 11.1(16)AA, 11.1(17)CC, 11.1(17)CT
   * 11.0(20.3)

Releases of Cisco IOS software up to and including 10.3 have reached end of
support, and no fixes are currently or planned to be available for those
releases. All releases after 9.1 do, however, contain the problem.

All planned fixes to Cisco IOS software have been completed and tested.
Integration into regular released software is complete for all versions
except 11.0. If you are running a version of software earlier than the ones
listed above, please contact the Cisco TAC for assistance.

As of the date of this notice, the fix for this problem is available for the
11.0 release only in the 11.0(20.3) version. This  is an interim release,
and has not been subjected to the same degree of testing as a regular
IOS release. The first regular 11.0 release containing the fix will be
11.0(21). Release of 11.0(21) is tentatively scheduled for mid-September,
1998; this schedule is subject to change. Because of the relative maturity
of the 11.0 Cisco IOS software, Cisco believes that installation of
11.0(20.3) carries less risk than would installation of an interim release
for a newer Cisco IOS version, but customers are advised to use caution in
installing 11.0(20.3), or any other interim release, in any critical device.

Cisco is offering free software upgrades to all vulnerable customers,
regardless of contract status. Customers with service contracts may upgrade
to any Cisco IOS software version. Customers without contracts may upgrade
to the latest versions of the images that they are already running (for
example, from 11.2(2) to 11.2(11), but not from 11.2(2) to 11.3(3)).
Customers without contracts who are running 10.3 or older software will
receive free upgrades to fixed 11.0 versions, but should be careful to make
sure that their hardware can support the new software before upgrading.

Customers with contracts should obtain upgraded software through their
regular update channels (generally via Cisco's Worldwide Web site).
Customers without contracts should contact the Cisco TAC as explained in the
"Cisco Security Procedures" section of this document, and should refer to
the URL of this document as evidence of their entitlement.

As with any software upgrade, you should check to make sure that your
hardware can support the new software before upgrading.  The most common
problem is inadequate RAM. Assistance is available on Cisco's Worldwide Web
site at http://www.cisco.com.

- - ---------
It is possible to work around this problem by preventing interactive access
to the Cisco IOS device. If only IP-based interactive access is of concern,
this can be done by using the ip access-class line configuration to apply an
access list to all virtual terminals in the system. However, it is important
to remember that non-IP-based means of making interactive connections to
Cisco IOS devices do exist, and to eliminate those means as possible routes
of attack. Interactive access can be prevented completely by applying the
configuration command no exec to any asynchronous line, or the command
transport input none to any virtual terminal line, that may be accessible to
untrusted users.

Exploitation and Public Announcements
Cisco has had no actual reports of malicious exploitation of this
vulnerability.  However, there have been sporadic reports of unexplained
crashes that have been consistent with the crashes caused by this
vulnerability; the vulnerability was initially identified because of such a
report. It is possible that the reported crashes could have been caused by
random events, but it is also possible that they could have been deliberate.
Cisco has essentially no information that would be useful in determining
which is the case.  None of the customers reporting the crashes indicated
any suspicion of a deliberate attack.

Cisco knows of no public announcements of this vulnerability before the date
of this notice.

Status of This Notice
This is a final field notice. Although Cisco cannot guarantee the accuracy
of all statements in this notice, all 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
http://www.cisco.com/warp/public/770/ioslogin-pub.shtml. In addition to
Worldwide Web posting, the initial version of this notice is being sent to
the following e-mail and Usenet news recipients:

   * cust-security-announce@cisco.com
   * bugtraq@netspace.org
   * first-teams@first.org (includes CERT/CC)
   * cisco@spot.colorado.edu
   * comp.dcom.sys.cisco
   * first-info@first.org
   * 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, 9:00   Initial released version
 AM US/Pacific,    

Cisco Security Procedures
Please report security issues with Cisco products, and/or sensitive security
intrusion emergencies involving Cisco products, to security-alert@cisco.com.
Reports may be encrypted using PGP; public RSA and DSS keys for
"security-alert@cisco.com" are on the public PGP keyservers.

The alias "security-alert@cisco.com" is used only for reports incoming to
Cisco. Mail sent to the list goes only to a very small group of users within
Cisco. Neither outside users nor unauthorized Cisco employees may subscribe
to "security-alert@cisco.com".

Please do not use "security-alert@cisco.com" for configuration questions,
for security intrusions that you do not consider to be sensitive
emergencies, or for general, non-security-related support requests. We do
not have the capacity to handle such requests through this channel, and will
refer them to the TAC, delaying response to your questions. We advise
contacting the TAC directly with these requests. TAC contact numbers 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: tac@cisco.com

All formal public security notices generated by Cisco are sent to the public
mailing list "cust-security-announce@cisco.com". For information on
subscribing to this mailing list, send a message containing the single line
"info cust-security-announce" to "majordomo@cisco.com". An analogous list,
"cust-security-discuss@cisco.com" is available for public discussion of the
notices and of other Cisco security issues.

This notice is copyright 1998 by Cisco Systems, Inc. This notice may be
redistributed freely after the release date given at the top of the notice,
provided that redistributed copies are complete and unmodified, including
all date and version information.

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