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Intrusec 55808 Trojan Analysis
Jun 20 2003 10:59AM
David J. Meltzer (djm intrusec com)
Intrusec Alert: 55808 Trojan Analysis
Initial Release: 6/19/03 4:30PM EDT
Latest Update: 6/19/03 11:13PM EDT
- Corrected analysis regarding use of sequence numbers to change IP
- Added reference to alternate name "Stumbler" given to trojan by
Internet Security Systems subsequent to the release of Intrusec's
Intrusec has completed an initial analysis of a trojan that appears to
be one of several that is responsible for generating substantial
scanning traffic across the Internet with a TCP window size of 55808.
The trojan we have isolated appears to match many of the characteristics
that others in the security community have reported for this trojan.
However, we do not believe that the specific trojan we have identified
is the sole source of the traffic generated, and do not know that it is
a primary source.
The information we've been able to gather leads us to believe that the
trojan we have captured is not the original source of the 55808 traffic
that has been seen, but is rather a "copycat", created to mimic the
behavior of another trojan or worm. The behavior of this copycat appears
to be based on press releases, news articles, and mailing lists that
described its hypothetical behavior and known output. Nonetheless, this
copycat trojan appears to be actively deployed on systems across the
Internet and is something security professionals should be aware of.
Details contained in this analysis will be updated, and linked to linked
to numerous analyses that will be done by other security researchers, as
they become available.
Please visit and link to http://www.intrusec.com/55808.html to receive
information available regarding this trojan. There is apt to be great
discussion about the nature of this "trojan" and whether in fact it is
accurately characterized as a trojan, backdoor, zombie, or worm. While
the specific binaries we have captured are probably described as a
trojan or zombie, there is no assurance that other variants of this
trojan may not be far more malicious in nature and contain worm or
backdoor functionality. We are referring to the trojan we have captured,
and the presumed other existing trojans generating similar traffic as
"55808 Trojans," and the specific binary we have analyzed as "55808
Trojan - Variant A." All discussion in our analysis section refers
specifically to the 'A' variant we have captured. Internet Security
Systems subsequent to the release of this alert dubbed this "Stumbler",
and refers to this same trojan by that name.
This trojan aims to be a distributed port scanner whose presence is very
difficult to detect. It port scans random addresses across the IP
address space, with a random source address also spoofed. By spoofing
the source address, the trojan is able to avoid easy detection, but it
also means it can not receive the results of the TCP SYN that is sent.
However, since the trojan also sniffs the network it is on in
promiscuous mode, it is likely, over time, to pick up scans from other
installations of trojans that randomly selected a source address that
happened to be on its subnet. As the number of trojans installed across
the Internet grows, more spoofed packets will be sent out by each
trojan, and more of the spoofed source addresses will be captured by
Each time a reply to a trojan is seen, indicating an open port has been
found, it is written to a file and saved. Daily, the trojan will then
deliver the list of open ports it recorded while sniffing to a file and
deliver that file to a predefined IP address.
In addition, a specially crafted packet can be sent to the subnet the
trojan is listening on which contains in its sequence number the IP
address the trojan should deliver the open port list to daily. However,
in the current incarnations of this trojan this functionality appears to
Finally, the trojan contains a feature whereby if it fails to connect to
the IP address it is supposed to deliver its open ports list to, it will
automatically attempt to remove itself from the system.
The trojan we have identified has been a file named 'a' that resides in
/tmp/.../a on the filesystem. Its packet collection activity monitors
for any packet with a window size of 55808 and records all packets
matching that window size. The packet capture is written to its current
directory (/tmp/.../ typically) in a file named 'r'.
There is a default IP address of 18.104.22.168 that the trojan attempts
to make a standard connection (not spoofed) to on TCP port 22 and
deliver the packet capture after it has been running for 24 hours,
however this appears to have been randomly selected as it is not an
active system on the Internet, and it is potentially dynamically
modifiable by a packet that can be sent to the trojan.
The trojan appears to contain some functionality to change the IP
address it delivers its packet captures to, but this functionality is
not operational in the trojan we have obtained. It appears the stubbed
out code, if activated, would function as follows: If a packet is
captured that contains a window size of 55808 and a TCP option window
scale of 2, the trojan modifies the IP address packet captures are
delivered to based on the sequence number of that packet.
While a novel concept, this trojan seems largely to have been written as
a proof of concept relative to the ideas Lancope described as a '3rd
generation trojan.' Other than generating large amounts of network
traffic, it contains no self-replicating or malicious behavior, and a
few high-speed port scans from compromised host would be a far more
effective and efficient means to map open ports on the Internet than
this type of trojan.
We have only observed the trojan on Linux systems to date. However, the
program itself is quite portable to other unix variants, so it is
possible if not likely
that it may also exist on other unix distributions. It is also possible
that the 'original' trojan is Windows-based.
The trojan appears to be installed on a system either manually, or
through an external exploit that is unrelated to the trojan itself.
There is no exploit code or means to install itself on a host built-in
to the trojan itself. It is easy to identify that a system on your
network has been infected with this or a related trojan due to its
extremely noisy network activity it generates with TCP packets with a
window size of 55808. However, other legitimate services may
intentionally or incidentally also send packets with this same window
size, so do not solely rely upon the presence of such a packet as
guaranteeing the existence of such a trojan.
Security vendors who claim that identifying massive quantities of port
scanning originating from their network as a unique feature of their
software should be taken with a grain of salt. It is more difficult to
identify the specific system on your network that has been infected with
this trojan due to its spoofing activities other than for its daily
non-spoofed connection to remote port 22. Tools that can assist you in
locating the actual physical source of these spoofed packets (through
looking at MAC addresses and ARPs) may be quite useful. There is apt to
be a great deal of discussion in the general techniques that can be used
to locate it, a good starting resource for this is "Tracking Down the
Phantom Host" by John Payton available at
For Exposé Users:
Users of Exposé that take advantage of its SSH authenticated
differential signatures can detect new default installations of this
trojan on their systems by creating a custom SSH differential signature
that looks for the appearance of a /tmp/.../ directory on systems being
monitored. See the Exposé help for more information on using SSH
From the main user interface, select 'Configure App Layer Differentials'
from the Tools menu, click 'Add' under the checks box, and then enter a
new check with the following settings:
Name: 55808 Trojan
Type: SSH, Simple
Challenge Text: echo check;ls /tmp/.../
Port Range: 22
If that file appears on the filesystem of any of the hosts being
monitored by Exposé and SSH authentication configured, an alert will be
created. Note this is only useful for default installations of the
The best way to prevent intrusions is to find and eliminate
vulnerabilities before they can be exploited. Intrusec has been built on
the belief that continuous network change detection is a core technology
that will assist administrators in managing the security of their
networks and should be a part of any comprehensive security framework.
Utilizing Intrusec's product, along with those from other commercial and
free sources, can assist in limiting the breadth and time your network
may be exposed to the type of vulnerabilities being exploited to install
malicious software such as the 55808 Trojan.
Intrusec, Inc. was founded in January 2002 to build a new kind of
security software that provides continuous detection of changes
occurring on a network. Intrusec's first product, Exposé, brings this
technology vision to fruition. Using Intrusec's unique Differential
Detection Technology, Exposé can detect changes on a network at all of
the IP, application, and web services layers of today's modern networks
and works with existing vulnerability assessment products to help
administrators identify specific vulnerabilities. Exposé is currently in
beta testing and is available for download now.
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