Cisco has warned users of a potential denial of service risk involving its popular Catalyst LAN switches.

A buffer overflow vulnerability in Telnet option handling can be used to crash the process and force a Catalyst switch to reload. This operation could be repeated by an attacker to produce a denial of service attack.

Cisco Catalyst switches, series 4000, 5000 and 6000 and also 2948G and 2900 models, running CatOS are affected by the flaw, which is been actively used.

In its notice, Cisco advises: "This vulnerability has been exploited to initiate denial of service (DoS) attacks."

Other Cisco LAN switches and routers are immune, it adds.

The networking giant is offering free software upgrades to affected by the flaw and published a schedule for the availability of fixed releases, which shows fixes will be available by February 4.

Cisco is advising users to upgrade software on affected switches as soon as is becomes available. In the meantime, workarounds to prevent attacks, such as using SSH instead of Telnet, or applying access control lists (ACLs) on routers and firewalls in front of vulnerable boxes, can be applied.

The Catalysts switch vulnerability is related to a flaw involving systems running versions of telnetd derived from BSD source, which was discovered in July last year. At the time this problem was thought to relate mainly to Unix systems, but it is now clear it also affects LAN switches. ®

External links

Cisco Security Advisory: Cisco CatOS Telnet Buffer Vulnerability