I thought this alert was worth passing along. It seems by turning off IPv6 you minimize exposure to this attack.
National Cyber Alert System
Technical Cyber Security Alert TA05-210A
Cisco IOS IPv6 Vulnerability
Original release date: July 29, 2005
Last revised: —
* Cisco IOS devices with IPv6 enabled
For specific information, please see the Cisco Advisory.
Cisco IOS IPv6 processing functionality contains a vulnerability that
could allow an unauthenticated, remote attacker to execute arbitrary
code or cause a denial of service.
Cisco IOS contains a vulnerability in the way IPv6 packets are
processed. US-CERT has not confirmed further technical details.
According to the Cisco Advisory, this vulnerability could be exploited
by an attacker on the same IP subnet:
Crafted packets from the local segment received on logical
interfaces (that is, tunnels including 6to4 tunnels) as well as
physical interfaces can trigger this vulnerability. Crafted packets
can not traverse a 6to4 tunnel and attack a box across the tunnel.
The crafted packet must be sent from a local network segment to
trigger the attack. This vulnerability can not be exploited one or
more hops from the IOS device.
US-CERT strongly recommends that sites running Cisco IOS devices
review the Cisco Advisory and upgrade as appropriate. We are tracking
this vulnerability as VU#930892.
This vulnerability could allow an unauthenticated, remote attacker on
the same IP subnet to execute arbitrary code or cause a denial of
service. The attacker may be able to take control of a vulnerable
Upgrade to a fixed version of IOS. Please see the Software Versions
and Fixes section of the Cisco Advisory for details.
From the Cisco Advisory:
In networks where IPv6 is not needed, disabling IPv6 processing on
an IOS device will eliminate exposure to this vulnerability. On a
router which supports IPv6, this must be done by issuing the
command "no ipv6 enable" and "no ipv6 address" on each interface.
Appendix A. Vendor Information
Cisco Systems, Inc.
Cisco Systems, Inc. has released a security advisory regarding a
vulnerability which was disclosed on July 27, 2005 at the Black Hat
security conference. Security advisory is available at:
For up-to-date information on security vulnerabilities in Cisco
Systems, Inc. products, visit http://www.cisco.com/go/psirt.
Appendix B. References
* US-CERT Vulnerability Note VU#930892 –
* Cisco Security Advisory: IPv6 Crafted Packet Vulnerability –
Information regarding this vulnerability was primarily provided by
Cisco Systems, who in turn acknowledge the disclosure of this
vulnerability at the Black Hat USA 2005 Briefings.
Feedback can be directed to US-CERT Technical Staff. Send mail to
<firstname.lastname@example.org> with "TA05-210A feedback VU#930892" in the subject.
The most recent version of this document is available at:
Produced 2005 by US-CERT, a government organization.
July 29, 2005: Initial release