Advisory ID: cisco-sa-19980810-ios-login
For Public Release 1998 August 10 16:00 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 cause that device to crash and
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.
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. It is not necessary to be able to actually log in to exploit
this vulnerability; simply establishing a terminal connection is sufficient.
The vulnerability can be exploited from any interactive prompt issued by the
router, including but not limited to, the login prompt.
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 devices.
It is impossible to list all Cisco products in this notice. 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 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 include the following:
7xx dialup routers (750, 760, and 770 series) are
Catalyst LAN switches (except for the Catalyst 2900XL) are
WAN switching products in the IGX or BPX lines are not
The AXIS shelf is not
The LS1010 or LS2020 ATM switches are not
affected. Earlier versions of this notice said that some LS1010 switches were
affected. This was an error; the 11.2WAx and 11.3WAx versions of Cisco IOS
software used in LS1010 switches are based on repaired variants.
Any host-based software is not
The Cisco PIX Firewall is not
The Cisco LocalDirector is not
The Cisco Cache Engine is not
No other Cisco products are currently known to be affected by these
The Cisco IOS software error has been assigned Cisco bug ID
Note: If you are a
user and you have logged in, you can view bug details.
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 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.
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
It is not necessary to run the specific releases listed above; the fix
is present in all subsequent versions of the same releases as well. For
example, 11.2(9)P is fixed, so 11.2(10)P is also fixed.
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
Cisco 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.
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.
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 "email@example.com" or "firstname.lastname@example.org"
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 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.
THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.
A stand-alone copy or Paraphrase of the text of this document that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.
This notice will be posted on Cisco's Worldwide Web site at
In addition to Worldwide Web posting, the initial version of this notice is
being sent to the following e-mail and Usenet news recipients:
email@example.com (includes CERT/CC)
Various internal Cisco mailing lists
Future updates of this advisory, 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
above URL for any updates.
Error with respect to impact on LS1010 switches corrected.
11.2(8)SA3 added to repaired version list.
Various common misunderstandings corrected. More information
about which Cisco IOS software versions are vulnerable and about which products
run classic Cisco IOS software.
Initial public release.
Please report security issues with Cisco products, and/or sensitive
security intrusion emergencies involving Cisco products, to
Reports may be encrypted using PGP; public RSA and DSS keys for
"firstname.lastname@example.org" are on the public PGP keyservers.
The alias "email@example.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
Please do not use "firstname.lastname@example.org" 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 follows:
+1 800 553 2447 (toll-free from within North America)
+1 408 526 7209 (toll call from anywhere in the world)
All formal public security notices generated by Cisco are sent to the
public mailing list "email@example.com". For information on
subscribing to this mailing list, send a message containing the single line
"info cust-security-announce" to "firstname.lastname@example.org". An analogous list,
"email@example.com" is available for public discussion of the
notices and of other Cisco security issues.