The risk posed by a serious DoS vulnerability to a wide range of Cisco Systems routers and switches has been upgraded following the release of an exploit onto a full disclosure mailing list.

As we reported yesterday, The DoS vulnerability arises from a bug in Cisco's core IOS software which means vulnerable devices to stop processing inbound packets on receipt of maliciously constructed IPv4 packets. Normal service would be restored only with a manual reboot.

No alarms will be triggered, nor will the router reload to correct itself. Cisco IOS versions 11.x and 12.x prior to 12.3 are affected by the vulnerability. That means almost everybody is affected.

Cisco has released a fix but the concern is how much mischief could be wrought before admins get a chance to upgrade router software.

The situation was already bad and just got a whole lot worse with the release of exploit code into the wild.

In an update to its advisory, Cisco explains: "Since the initial posting of this document, the Cisco PSIRT has been made aware of public announcements of the vulnerabilities described in this advisory. Cisco PSIRT is aware that the exploit for this vulnerability has been published on a public mailing list."

Cisco has compiled a advisory detailing workarounds and available fixes, which network admins are urged in the strongest terms to promptly review. ®