Cisco Systems has been contacting internet service
providers and internet backbone providers around the world to warn
them of a fundamental vulnerability in its IOS operating system
which could make those devices vulnerable to a denial-of-service
attack.Cisco dominates the worldwide market for
networking gear such as switches and routers, and its products are
among the most commonly used on the internet and within
enterprises.
While critical systems such as those used by
internet backbone providers are likely to be patched quickly, it
may take some time for mid-sized and large enterprises to update
IOS on the hundreds or even thousands of Cisco devices that tie
their networks together, said Dan Ingevaldson, engineering director
for security company Internet Security Systems.
ISS is concerned about the sheer volume of
different IOS patches listed by Cisco in its 15-page advisory.
Cisco had to create different IOS updates for
the dozens of versions of the operating system that the company
supports. The complexity of Cisco's different upgrade paths is
likely to cause confusion among customers with multiple devices to
manage.
In addition, one or more of those patched
versions may contain a flaw that causes problems after the patch is
applied, Ingevaldson warned.
The flaw affects a wide range of Cisco
devices that run IOS and accept data packets using IPv4 (Internet
Protocol version 4), including Cisco's popular Catalyst family of
switches, 7300 series routers and Aironet family of wireless access
points.
Cisco devices are configured to accept IPv4
traffic by default, Cisco said. A specially crafted sequence of
IPv4 data packets sent to a device running a vulnerable version of
IOS can cause the device to stop processing traffic.
The unique sequence of IPv4 packets shuts down
the Cisco devices by causing them - incorrectly - to designate the
targeted device interface as full. Once flagged as full, the
interface - for example, Ethernet interface processing network
traffic - will stop processing incoming traffic.
The IPv4 packets could be sent, in sequence,
to each interface on an affected device, shutting those interfaces
down and rendering the device inaccessible to administrators who
need to access it remotely.
The packets can be sent directly to the
vulnerable device, without requiring authorisation by the attacker.
Following the attack, Cisco devices must be rebooted to clear the
blocked interface, Cisco said.
In addition, devices under attack will not
issue warnings or alarms, nor will they reboot automatically to
correct the problem when the targeted interface has been marked
full.
The lack of warning could enable an attacker
to cripple an organisation's network by taking out the key Cisco
hardware tying that network together.
That fact, coupled with the severity of the
security flaw, prompted the CERT Co-ordination Centre and security
companies to issue warnings about the IOS vulnerability on
Wednesday.
Cisco and CERT encouraged organisations with
Cisco hardware running the affected versions of IOS to obtain and
apply a software patch from Cisco. (See
http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml#fixes)
Organisations unable to apply the patch
immediately can use a number of workarounds.
For devices that have already been attacked,
administrators can raise the ceiling on the input queue for each
targeted interface to unblock those devices, Cisco said
Access control lists (Calls) can also be used
to block IPv4 traffic from unknown sources.
Paul Roberts writes for IDG News Service