Advisory ID: cisco-sa-19981014-ios-hist
For Public Release 1998 October 14 15:00 UTC (GMT)
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.
This advisory is posted at
This section provides details on affected products.
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, local-area transport (LAT) connections, Maintenance
Operation Protocol (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.
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
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,
WAN switching products in the IGX and BPX lines are
The MGX (formerly known as the AXIS shelf) is not
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.
No other Cisco products are currently known to be affected by these
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
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 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
11.1(22), 11.1(22)CA, 11.1(21)CC1, 11.1(22)CE
11.1(22), 11.1(21.2)CA, 11.1(21)CC1, 11.1(21.1)CE
11.2(16), 11.2(16)P, 11.2(16)BC,11.2(8)SA4
11.2(15.4), 11.2(15.4)P, 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(6)NA,
11.3(5.6), 11.3(5.6)T, 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"
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
1.x - 8.x, 9.1, 9.14, 9.17, 9.21, 10.1, 10.2, 10.3 (all
11.0 (be especially careful to check hardware
There are two major workarounds for this vulnerability:
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.
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.
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 Cisco.com Downloads at
Do not contact either "firstname.lastname@example.org" or "email@example.com"
for software upgrades.
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 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 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)
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.
for additional TAC contact information, including special localized telephone
numbers and instructions and e-mail addresses for use in various
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.
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In addition to Worldwide Web posting, the initial version of this notice is
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Initial public release
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