Cisco Security Advisory
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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 https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19971216-pw-buffer.
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This section provides details on affected products.
Vulnerable 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.
Products Confirmed Not Vulnerable
No other Cisco products are currently known to be affected by these vulnerabilities.
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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.
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The vulnerability may be avoided by controlling access to the system console port, and by restricting access to the TELNET facility to trusted hosts.
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.
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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.
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Cisco has had no known reports of malicious exploitation of this vulnerability.
This vulnerability has been discussed on the "bugtraq@netspace.org" 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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To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
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Revision 2.0
1998-June-16
Updated to reflect software fix availability.
Revision 1.0
1997-December-16
Initial version.
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