Security companies have issued alerts about a new threat
known 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 Inc. (ISS), 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, 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.
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.
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.
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.
Paul Roberts write for IDG News Service