mailing list archives
Re: Three possible DoS attacks against some IOS versions.
From: Sharad Ahlawat <sahlawat () cisco com>
Date: Thu, 6 Jun 2002 15:51:55 -0700
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This email is in response to the BugTraq posting at
There are three issues in the original email, their responses are given
A Cisco 2621 router with 12.1(6a) could not be crashed by using the nmap
command and by mirroring the setup, used by Andrew. Other IOS releases
were tried and no crashes were observed. This currently seems to be a
setup specific issue.
Andrew was using an Interim release of IOS during his testing. Interim
releases are built at regular intervals between maintenance releases
and receive less testing. Interim releases should be selected only if
there is no other suitable release that addresses an issue, and interim
images should be upgraded to the next available maintenance release as
soon as possible. Interim releases are not available via manufacturing,
and usually are not available for customer download from CCO without
prior arrangement with the Cisco TAC.
I have published 12.1(6a) for Andrew to have him test in his network
setup. I will have a followup conversation with him on this.
UDP port 1985 found open when no HSRP configured. Cisco Bug ID
CSCdt64533 - has already been integrated in 12.2.
High CPU utilization is expected behavior when one directs a continuous
stream of data at any open port, as the data needs to be processed by
the router. Streaming UDP traffic at a Cisco 2621 router's port 1985
does not freeze the router though it has been observed on lower end
routers to cause high CPU utilization and unresponsiveness, but no
an excerpt form RFC 2281 - Cisco HSRP
7. Security Considerations
This protocol does not provide security. The authentication field
found within the message is useful for preventing misconfiguration.
The protocol is easily subverted by an active intruder on the LAN.
This can result in a packet black hole and a denial-of-service
attack. It is difficult to subvert the protocol from outside the
LAN as most routers will not forward packets addressed to the
all-routers multicast address (18.104.22.168).
Cisco is considering using MD5 to improve the protection of HSRP in
future releases of IOS.
However, there are some other factors that must be considered in
- - this vulnerability can be exploited only from the local segment
(not over the Internet).
- - the same effect, denial of service, can be produced by using ARP,
which can not be protected in any way.
The last factor is especially important since it may cause a false
sense of security if the user is using a hardened version of HSRP as an
attacker can still disrupt the network by using crafted ARP packets.
Another aspect of this issue is that in its current implementation, HSRP
doesn't seem to perform a validity check on the IP addresses. This is
under active investigation as Cisco Bug ID CSCdu38323.
Cisco HSRP documentation can be found at -
Product Security Incident Response Team (PSIRT) Incident Manager
Phone:+1 (408) 527-6087 (Land line and Mobile)
DH/DSS key Id: 0xC12A996C
Fingerprint: 9A93 2A20 43E5 7F01 2954 C427 1A81 A898 C12A 996C
Complete information on reporting security vulnerabilities in Cisco
products, obtaining assistance with security incidents, and registering
to receive security information from Cisco, is available on Cisco's
Worldwide Web site at http://www.cisco.com/go/psirt.
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