Cisco last week warned of a widespread and serious flaw in its IOS operating system that could make devices vulnerable to a denial-of-service attack.Cisco\u00a0last week warned of a widespread and serious flaw in its\u00a0IOS\u00a0operating system that could make devices vulnerable to a denial-of-service attack.The flaw affects a range of Cisco devices that run IOS and accept data packets using IPv4, 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, the company says. 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 to incorrectly designate the targeted device interface as full. Once flagged as full, the interface - for example, an 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 down those interfaces and rendering the device inaccessible to administrators who need to access it remotely, the company says. The packets can be sent directly to the vulnerable device, without requiring authorization by the attacker. After the attack, Cisco devices must be rebooted to clear the blocked interface, Cisco says.In addition, devices under attack will not issue warnings or alarms, nor will they automatically reboot to correct the problem when the targeted interface has been marked full. The lack of warning could let an attacker silently cripple an organization's network by taking out the key Cisco hardware that ties that network together.Cisco dominates the worldwide market for network gear such as switches and routers, and its products are among the most commonly used on the Internet and within corporations. Cisco and CERT encouraged organizations with Cisco hardware running the affected versions of IOS to obtain and apply a\u00a0software patch\u00a0from Cisco.Security company\u00a0Internet Security Systems (ISS)\u00a0issued an alert to its customers notifying them of the flaw and recommending that they patch vulnerable systems as soon as possible, according to Dan Ingevaldson, engineering director for ISS X-Force. The company heard rumors about the flaw late last week, as Internet backbone providers and ISPs were contacted by Cisco about the flaw and announced unscheduled maintenance for their networks, he says.While critical systems such as those Internet backbone providers use are likely to be patched quickly, it might take some time for midsize and large companies to update IOS on the hundreds or even thousands of Cisco devices that tie together their networks, he says. ISS also is concerned about the sheer volume of different IOS patches Cisco listed in its 15-page advisory, Ingevaldson says.Cisco had to create different IOS updates for the dozens of versions of the operating system it supports.Roberts is a correspondent at IDG News Service's Boston bureau.