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Home : Advisories : Cisco IOS Command History Release at Login Prompt

Title: Cisco IOS Command History Release at Login Prompt
Released by: CISCO
Date: 14th October 1998
Printable version: Click here
Field Notice:

Cisco IOS Command History Release at Login Prompt


Revision 1.0

For release 08:00 AM US/Pacific, Wednesday, October 14, 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 obtain fragments of text entered

by prior interactive users of the device.  This text may contain sensitive

information, possibly including passwords. This vulnerability exposes only

text entered at prompts issued by  the IOS device itself; the contents of

data packets forwarded by IOS devices are not exposed, nor are data entered

as part of outgoing interactive connections, such as TELNET connections,

from the IOS device to other network nodes.

This applies only to devices running classic Cisco IOS software, including

most, but not all, Cisco router products. The easiest way to determine

whether your device is running classic Cisco IOS software is to use the show

version command as detailed under "Who Is Affected " below.

Although the conditions under which it can be exploited are similar, this

vulnerability is  not related to the remote crash vulnerability announced in

August, 1998.

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. Note that all of the repaired versions are

quite recent as of the date of this notice, and that it is unlikely that

most Cisco users have installed them. The vulnerability affects the vast

majority of systems running Cisco IOS software as of this date.

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

devices.  It is not necessary to be able to actually log in to exploit this

vulnerability; simply establishing a terminal connection is sufficient.

Affected Devices


It is impossible to list all Cisco products in this notice; the lists below

included only the most commonly used or most asked-about products.

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". Other Cisco devices either will not have the show version

command, or will give different  output.

Cisco devices that run classic Cisco IOS software include:

   * Cisco routers in the AGS/MGS/CGS/AGS+, IGS, RSM, 8xx, 1xxx, 25xx, 26xx,

     30xx, 36xx, 40xx, 45xx, 47xx, AS52xx, AS53xx, 70xx, 72xx (including the

     ubr72xx), 75xx, and 12xxx series

   * Most recent versions of the LS1010 ATM switch

   * Some versions of the Catalyst 2900XL LAN switch.

   * The Cisco DistributedDirector

If you are not running classic Cisco IOS software, then you are not affected

by this vulnerability. Cisco devices which do not run classic Cisco IOS

software, and are not affected by this vulnerability, include the following:

   * 7xx dialup routers (750, 760, and 770 series) are not affected.

   * Catalyst 19xx, 28xx, 29xx, 3xxx, and 5xxx LAN switches are not

     affected, except for some versions of the Catalyst 2900XL. However,

     optional router modules running Cisco IOS software in switch

     backplanes, such as the RSM module for the Catalyst 5000 and 5500, are


   * WAN switching products in the IGX and BPX lines are not affected.

   * The MGX (formerly known as the AXIS shelf) is not affected.

   * No host-based software is affected.

   * The Cisco PIX Firewall is not affected.

   * The Cisco LocalDirector is not affected.

   * The Cisco Cache Engine is not affected.



If attackers know the details of the Cisco IOS software error, they will be

able to obtain fragments of the last few lines of text entered in response

to IOS prompts on the physical or virtual TTYs to which they are connected.

The exact amount of recoverable text varies, and will be split among

fragments of various lines. Nearly complete lines, and  fragments tens of

characters long, can sometimes be obtained.

If the previous session was brief, the available information may include

part or all of the password that a previous user used to log into the router

or to enable privileged mode. If a previous user changed a system password,

such as the enable password, and logged out shortly thereafter, it may be

possible to recover the new password by reading the configuration command

used to make the change.

This vulnerability does not expose anything entered as part of an outgoing

session from the IOS device to another node. For example, if a user logs

into an IOS router, and then makes a TELNET connection to a remote host,

none of the data in the TELNET connection itself can be recovered.



This vulnerability has been assigned Cisco bug ID CSCdk43920.

Affected and Repaired Software Versions

- -------------------------------------

This vulnerability affects all releases of Classic Cisco IOS software,

including special, interim, and beta software,  from 9.1 up to, but not

including, the following corrected releases:

 Earliest Regular Releases                 Earliest Interim Releases

 -------------------------                 -------------------------

 11.0(22)                                  11.0(21.2)

 11.1(22), 11.1(22)CA, 11.1(21)CC1,        11.1(22), 11.1(21.2)CA,

 11.1(22)CE                                11.1(21)CC1, 11.1(21.1)CE

 11.2(16), 11.2(16)P,                      11.2(15.4), 11.2(15.4)P,

 11.2(16)BC,11.2(8)SA4                     11.2(15.4)BC, 11.2(8)SA4

 11.3(6), 11.3(6)T, 11.3(6)AA, 11.3(1)MA6, 11.3(5.6), 11.3(5.6)T,

 11.3(6)NA, 11.3(9)WA4                     11.3(5.6)AA, 11.3(1)MA54,


 12.0(1), 12.0(1)T, 12.0(1)S, other 12.0   Will be integrated in initial

                                           12.0(1)x releases

It is not necessary to run the specific versions listed above; the fix is

present in all subsequent versions of the same releases as well. For

example, 11.2(16)P is fixed, so 11.2(17)P will also be fixed.

The fix is available in all regular releases as of the date of this notice.

However, the fix has not yet been released for all "two-letter" early

deployment software. Integration is under way for the unreleased

"two-letter" versions.

Some releases of Cisco IOS software have been obsoleted or have reached end

of maintenance. The upgrade paths for the users of these releases are as


 Obsolete Release                    Upgrade To

 ----------------                    ----------

 1.x - 8.x, 9.1, 9.14, 9.17, 9.21,   11.0 (be especially careful to check

 10.1, 10.2, 10.3 (all variants)     hardware compatibility)

 11.0BT                              11.1

 11.1AA                              11.2(16)P

 11.2(4)XA, 11.2(9)XA                11.2(16)P

 11.3(2)XA                           11.3(3)

 11.2F                               11.3(6)

Getting Fixed Software

- --------------------

Cisco is offering free software upgrades to remedy this vulnerability for

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 releases and

feature sets that they are already running (for example, from 11.2(2) to

11.2(16), but not from 11.2(2) to 11.3(6)).

Customers without contracts who are running obsolete software will receive

free upgrades to the fixed versions listed in the table above. Furthermore,

if there is no immediately available fix for software being run by a

customer without a contract, then that customer will immediately be given

the most appropriate available fixed software, even if a release or feature

upgrade is involved.

Customers with contracts should obtain upgraded software through their

regular update channels (generally via Cisco's Worldwide Web site).

Customers without contracts should get their upgrades by contacting the

Cisco 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

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 "psirt@cisco.com" or

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

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.


- ---------

There are two major workarounds for this vulnerability:

  1. Prevent untrusted users from having  interactive access to the Cisco

     IOS device. If only IP-based interactive access is of concern, access

     can be restricted by using the ip access-class line configuration

     command 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. The transport input command

     is particularly useful in controlling the protocols that can be used to

     get interactive access. 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.

  2. Overwrite any potentially sensitive information before logging out of

     any interactive session on an IOS device. This can be done by entering

     repeated spaces at an IOS command prompt until the command interpreter

     will accept no more input on the line, then pressing the "return" key.

     Follow this by entering a printing character, such as "q", repeatedly

     until no more input is accepted, then pressing control-A, followed by

     control-K, then "return" again. This procedure vastly reduces the

     probability of information leakage, but has not been verified to

     completely eliminate the possibility in all affected versions of Cisco

     IOS software.

Cisco recommends installing upgraded software in preference to using either

of these workarounds. The first workaround should be part of normal security

configuration in any Cisco IOS device, but cannot usually be used to

eliminate all possible risk, since some interactive access must be available

for system management. The second workaround is prone to human error, and,

although it greatly reduces the probability of an attacker's finding

anything sensitive, it does not completely eliminate that possibility.

Exploitation and Public Announcements


Cisco knows of no public announcements or discussion of the details of this

vulnerability prior to the date of this notice. An inadvertant

preannouncement was made to certain Cisco customers during the week of

October 5, but the only information given to those customers consisted of

the bug ID and the bug headline, which was "Security Problem". In other

words, they were told that a security problem existed in a version of Cisco

IOS software, but were given absolutely no details. A later message to those

same customers informed them that the vulnerability had been found by a

trusted customer, that Cisco knew of no exploitation of the vulnerability,

and that a formal notice would be forthcoming. Extreme care was taken to

avoid giving information that could be used to localize the vulnerability to

any particular part of the Cisco IOS software, or other information that

might be useful in finding the details.

Cisco knows of no malicious exploitation of this vulnerability. This

vulnerability was found by a Cisco customer during  laboratory testing.

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/ioshist-pub.html . 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

   * fib-beta@external.cisco.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 1.0, 22:30 US/Pacific,    Initial released version


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 text,

provided that redistributed copies are complete and unmodified, including

all date and version information.


(C) 1999-2000 All rights reserved.