Networking behemoth Cisco has issued an advisory explaining that certain security scanning utilities can cause its routers to crash.

Repeated exploitation of the defect can result in sustained denials of service, meaning that the scanners can conveniently be used to attack a system.

The company declined to identify the particular utilities in question, but said that they would typically be used on UNIX systems and would be "asserting the Telnet ENVIRON option, #36, before the router indicates that it is willing to accept it, and this causes the router to reload unexpectedly," which should be enough of a hint for most attackers to figure out which tools to use.

The vulnerability affects routers, series access servers and voice gateway products running vulnerable software. Users running the affected IOS software releases -- which include 11.3AA and several 12.0 releases through 12.0(6) -- are urged to upgrade to later versions which are not vulnerable.

Users running Cisco IOS software versions 11.3, 11.3T, 11.2 or lower, and 12.0(8) or 12.1 or higher are not affected.

Not all affected software releases have been patched as yet, Cisco notes. In that case, users can resort to one of the company's recommended workarounds. Full details of the defect, affected software versions, and the workarounds have been posted here. ®