Cisco software flaw hits ISPs worldwide

Cisco Systems has been contacting internet service providers and internet backbone providers around the world to warn them of a...

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

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

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