Advisory ID: cisco-sa-19971216-pw-buffer
For Public Release 1997 December 16 01:00 UTC (GMT)
Some Cisco 7xx routers can be crashed by connecting with TELNET and
typing very long password strings. There exists a small possibility that this
bug could be exploited to launch other attacks against the router, other than
simply crashing it.
This advisory is posted at
This section provides details on affected products.
All Cisco 7xx routers running IOS/700 software version 4.1(1), 4.1(2),
or 4.1 interim releases earlier than 4.1(2.1) are affected. Systems running
releases earlier than 4.1 are not affected. In order to
exploit the vulnerability, an attacker must have access to the password prompt.
This means that the attacker must be able to TELNET to the target router, or to
gain access to its console port.
This vulnerability affects systems running IOS/700 version 4.1
releases, including 4.1(1), 4.1(2), and 4.1 interim releases earlier than
4.1(2.1). IOS/700 releases other than 4.1 are not
affected. 4.2 and later releases are not affected.
No other Cisco products are currently known to be affected by these
This vulnerability has been assigned bug ID CSCdj66458.
Insufficient bounds checking on the data buffer used for password input
allows the incoming password to exceed the buffer size, overwriting the
contents of memory beyond the end of the buffer. When the system tries to use
the now-incorrect data in that memory, unpredictable results occur. If the data
are randomly chosen, this unpredictable behavior can be expected to result in
the detection of errors, such as accesses to illegal addresses, which result in
system crashes. It might be possible to craft a data string that, instead of
creating detectable errors, causes particular system behavior desired by the
attacker. However, Cisco development engineers have been unable to construct
such a string.
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
This vulnerability allows attackers to force 7xx routers to reboot,
denying service to legitimate users during the reboot period, and possibly
causing excessive "call flapping" as routers shut down and restart.
It is possible that including the right data at the right place in the
too-long password string could enable an attacker to take complete control of
the router, or to cause it to hang indefinitely. Engineering analysis of the
data structures surrounding the affected buffer has not revealed any viable way
of doing this. A person who succeeds in such an attack would be able to
reconfigure the router or modify its functionality, theoretically in any way at
A software fix was integrated in IOS/700 version 4.1(2.1). The first
regular production release containing this fix was 4.2(1). Cisco will be making
the fixed software available to all IOS/700 customers who are presently running
4.1 software, regardless of contract status. Customers under contract may
obtain the software through their regular upgrade channels. Customers not under
contract should contact the Cisco TAC and reference the URL of this document.
The vulnerability may be avoided by controlling access to the system
console port, and by restricting access to the TELNET facility to trusted
TELNET access may be restricted either by using filters on firewalls or
surrounding routers, or by using filters on the 7xx router itself. To restrict
access to the TELNET service on a 7xx router running 4.1(x) software to a
single trusted management host, use the command
set ip filter tcp in source = not trusted-ip-address destination = 7xx-address:23 block
The command should be applied in every profile that may be active when
the router is connected to a potentially hostile network.
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 at obtaining fixed software through their point of sale 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)
Have your product serial number available and 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.
for additional TAC contact information, including special localized telephone
numbers and instructions and e-mail addresses for use in various
Cisco has had no known reports of malicious exploitation of this
This vulnerability has been discussed on the "email@example.com"
mailing list, and is therefore certain to be widely known in the cracker
community. The first public announcement of this vulnerability of which Cisco
is aware was on December 11, 1997.
The vulnerability can be exploited to crash systems with no special
tools or knowledge; no exploitation program as such is required.
Assuming that it is possible to exploit the vulnerability to take total
control of the system, an exploitation program would be needed in order to do
so. A person seeking to develop such an exploitation program would need to be a
competent assembly language programmer. She would also need detailed knowledge
of the internal workings of the IOS/700 software and/or the 7xx router
hardware. Such knowledge has not been made public by Cisco, but could be
obtained by reverse engineering or by theft of trade secrets from Cisco.
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Future versions of this notice will be posted on Cisco's Web site, but
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about this problem are encouraged to check the Web site for updates.
This notice will be posted in the "Field
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"Technical Tips" in the "Software and Support" section. The URL is
The copy on the Worldwide Web will be updated as appropriate.
Updated to reflect software fix availability.
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