Security companies warn of devious new trojan

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 Inc. (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 Inc. 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 and files named "a" and "r" in the "/tmp/.../" directory on host systems.

Join the newsletter!


Sign up to gain exclusive access to email subscriptions, event invitations, competitions, giveaways, and much more.

Membership is free, and your security and privacy remain protected. View our privacy policy before signing up.

Error: Please check your email address.
Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Paul Roberts

IDG News Service
Show Comments

Cool Tech

Toys for Boys

Family Friendly

Stocking Stuffer

SmartLens - Clip on Phone Camera Lens Set of 3

Learn more >

Christmas Gift Guide

Click for more ›

Brand Post

Most Popular Reviews

Latest Articles


PCW Evaluation Team

Aysha Strobbe

Microsoft Office 365/HP Spectre x360

Microsoft Office continues to make a student’s life that little bit easier by offering reliable, easy to use, time-saving functionality, while continuing to develop new features that further enhance what is already a formidable collection of applications

Michael Hargreaves

Microsoft Office 365/Dell XPS 15 2-in-1

I’d recommend a Dell XPS 15 2-in-1 and the new Windows 10 to anyone who needs to get serious work done (before you kick back on your couch with your favourite Netflix show.)

Maryellen Rose George

Brother PT-P750W

It’s useful for office tasks as well as pragmatic labelling of equipment and storage – just don’t get too excited and label everything in sight!

Cathy Giles

Brother MFC-L8900CDW

The Brother MFC-L8900CDW is an absolute stand out. I struggle to fault it.

Luke Hill


I need power and lots of it. As a Front End Web developer anything less just won’t cut it which is why the MSI GT75 is an outstanding laptop for me. It’s a sleek and futuristic looking, high quality, beast that has a touch of sci-fi flare about it.

Emily Tyson

MSI GE63 Raider

If you’re looking to invest in your next work horse laptop for work or home use, you can’t go wrong with the MSI GE63.

Featured Content

Product Launch Showcase

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?