GEN:SID 1:106
Message BACKDOOR ACKcmdC trojan scan
Summary Backdoor.AckCmd is a Trojan Horse that uses TCP ACK segments to
communicate. This Trojan may bypass firewalls that do not keep track of
the session state in a TCP transaction.
Impact Possible theft of data and control of the targeted machine leading to a
compromise of all resources the machine is connected to. This Trojan
also has the ability to delete data, steal passwords and disable the
machine. Other versions are capable of launching DDoS attacks.
Detailed Information This Trojan affects the following operating systems:

    Windows 95
    Windows 98
    Windows ME
    Windows NT
    Windows 2000
    Windows XP

No other systems are affected. This is a windows executable that does
not make changes to the system registry.

AckCmd is a "proof of concept" Trojan.

    SID    Message
    ---    -------
    106    ACKcmdC trojan scan

This event is indicative of an attacker attempting to locate AckCmd
servers.
Affected Systems  
Attack Scenarios This Trojan may be delivered to the target in a number of ways. This
event is indicative of an existing infection being activated. Initial
compromise can be in the form of a Win32 installation program that may
use the extension ".jpg" or ".bmp" when delivered via e-mail for
example.
Ease of Attack This is Trojan activity, the target machine may already be compromised.
Updated virus definition files are essential in detecting this Trojan.
Corrective Action Delete the file AckCmd.exe

A machine reboot is required to clear the existing process from running
in memory.

In addition, the use of a firewall that correctly checks the state of a
TCP session is recommended.
Additional References Whitehats arachNIDS
http://www.whitehats.com/info/IDS485

ntsecurity.nu
ACK Tunneling Trojans
http://ntsecurity.nu/papers/acktunneling/
Rule References arachnids: 445