Rumours of a troublesome new type of trojan program were confirmed last Thursday when two security companies issued alerts about a new threat known variously as "Trojan 55808" and "Stumbler."

The new program is not a worm and cannot spread itself or perform any malicious actions on hosts that it infects, said Dan Ingevaldson, engineering manager with X-Force at Internet Security Systems (ISS) of Atlanta, Georgia, which issued an alert on the trojan.

Instead, it must be installed manually on victim's computers running the Linux operating system. Once installed, the trojan runs quietly in the background and acts as a "distributed port scanner," randomly searching the Internet for machines with valid IP (Internet Protocol) addresses and open ports, according to Intrusec of Roswell, Georgia, which also issued an alert.

"It's the same as any port scanner, but distributed across the Internet," Ingevaldson said.

The new trojan uses "spoofing" techniques to disguise the source IP address and even hardware (MAC) address of the host that is running the trojan, the companies said.

That makes it very difficult for network administrators to determine the source of the network scanning traffic, even if they can narrow the traffic down to a small number of machines connected to a network hub, according to Ingevaldson.

However, the address spoofing also makes it impossible for the trojan to receive responses to its own scans, Intrusec said.

To counter the lack of direct communication, each infected host casts a wide net for acknowledgements, listening for a response to its own IP address while also sniffing its network subnet for responses to other addresses.

If other machines on the Internet that are infected with the trojan happen to use one of those addresses as a "spoof" address, the trojan will capture and save IP address and port information, though not for the scans it initiated, Intrusec said.

The passive network mapping technique generates huge volumes of wasted traffic and is ineffective given the small number of Internet hosts that are currently running the trojan, Ingevaldson said.

The high volume of Internet traffic it generates also makes the new trojan easy to spot, especially given the program's consistent use of a TCP window size of 55808 bytes, he said.

A TCP window specifies the amount of data one host can send before receiving an acknowledgement from the intended recipient that the data has been received.

In recent weeks, the spike in unusual TCP traffic caught the notice of network administrators worldwide and has been a topic of conversation in online discussion groups for a number of weeks, with some wondering whether a new worm or virus was loose on the Internet.

While ineffective as currently deployed, the passive scanning technique could be very effective at anonymously mapping a network of vulnerable machines when coupled with an effective worm and placed on a network of 200,000 or more infected hosts, according to Ingevaldson.

Those machines could then be used as a command and control network for distributing malicious software or as a platform for launching distributed denial of service attacks, Ingevaldson said.

The purpose of the release is probably as a proof of concept for the "passive scanning" technique, he said

ISS and Intrusec have isolated at least two versions of the trojan on the Internet, with a later release fixing bugs that were present in the first version of the Trojan, Ingevaldson said.

There may be other versions on the loose as well, but the relatively small number of infected computers coupled with the address spoofing features and the lack of self replicating code makes it difficult to determine how many variants of the trojan really exist, he said.

The current trojan could be a glimpse of future threats that may be joined to destructive payloads, Ingevaldson said.

ISS and Intrusec instructed individuals who believe that systems on their network may be infected to look for outbound connections to the IP address 12.108.65.76 and files named "a" and "r" in the "/tmp/.../" directory on host systems.