Researchers have identified a new trojan that can tamper with a wide array of devices on a local network, an exploit that sends them to impostor websites even if they are hardened machines that are fully patched or run non-Windows operating systems.

The malware is a new variant of the DNSChanger, a trojan that has long been known to change the domain name system settings of PCs and Macs alike. According to researchers with anti-virus provider McAfee's Avert Labs, the update allows a single infected machine to pollute the DNS settings of potentially hundreds of other devices running on the same local area network by undermining its dynamic host configuration protocol, or DHCP, which dynamically allocates IP addresses.

"Systems that are not infected with the malware can still have the payload of communicating with the rogue DNS servers delivered to them," McAfee's Craig Schmugar writes here of the new variant. "This is achieved without exploiting any security vulnerability."

The scenario plays out something like this:

Jill connects a PC infected by the new DNSChanger variant to a coffee shop's WiFi hotspot or her employer's local network.

Steve connects to the same network using a fully-patched Linux box, which requests an IP address.

Jill's PC injects a DHCP offer command to instruct Steve's computer to rout all DNS requests through a booby-trapped DNS server.

Steve's Linux box can no longer be trusted to visit authoritative websites. Although the address bar on his browser may show he is accessing bankofamerica.com, he may in fact be at an impostor website.

The only way a user might know the attack is underway is by manually checking the DNS server his computer is using (e.g. by typing "ipconfig /all" at a Windows command prompt). There are several countermeasures users can take, Schmugar said, the easiest being hard-coding a DNS server in a machine's configuration settings.

(In Windows, this can be done by going to Start > Control Panel > Network Connections and right clicking on Local Area Connection and choosing properties. Scroll down to Internet Protocol (TCP/IP) and click the Properties button. Then type in the primary and secondary for your DNS service. We're partial to OpenDNS, whose settings are 208.67.222.222 and 208.67.220.220.)

In an interview, Schmugar said the DHCP attack doesn't exploit a vulnerability in either user machines or network hardware, allowing it to work with a wide variety of home and enterprise routers. It involves a ndisprot.sys driver that is installed on the infected box. Once there, it monitors network traffic for DHCP requests and responds with bogus offers that contain the IP address to the rogue DNS server.

DNSChanger has already been viewed exploiting router weaknesses to change DNS settings, but the ability to poison other machine's DHCP connections appears to be new, said Eric Sites, VP of research at Sunbelt Software. For the moment, the new variant doesn't appear to be widely circulated, but the prospect of a trojan that can poison other machines' DHCP connections suggests this one is worth watching. ®