Backdoor:Win32/Agent.FS is a backdoor trojan that is installed in the system by TrojanDropper:Win32/Agent.gen!I. It can gather Internet traffic information and perform commands issued by a remote attacker.

What to do now

Manual removal is not recommended for this threat. To detect and remove this threat and other malicious software that may have been installed, run a full-system scan with an up-to-date antivirus product such as Microsoft Security Essentials, or the Microsoft Safety Scanner. For more information about using antivirus software, see
Additional remediation instructions
This threat may make lasting changes to a computer’s configuration that are NOT restored by detecting and removing this threat. For more information on returning an infected computer to its pre-infected state, please see the following article/s:

Threat behavior

Backdoor:Win32/Agent.FS is a backdoor trojan that is installed in the system by TrojanDropper:Win32/Agent.gen!I. It can gather Internet traffic information and perform commands issued by a remote attacker.
Backdoor:Win32/Agent.FS arrives in the system as a dropped component of TrojanDropper:Win32/Agent.gen!I. It is usually installed as the following file:
  • <system folder>\nwagentsdt.dll
It is installed as a service with the creation of the following registry entries:
Adds value: "Description"
With data: "service advertising protocol."
To subkey: HKLM\SYSTEM\CurrentControlSet\Services\Nwsapagent
Adds value: "ServiceDll"
With data: "<system folder>\nwagentsdt.dll"
To subkey: HKLM\SYSTEM\CurrentControlSet\Services\Nwsapagent\Parameters
Note - <system folder> refers to a variable location that is determined by the malware by querying the Operating System. The default installation location for the System folder for Windows 2000 and NT is C:\Winnt\System32; and for XP, Vista, and 7 is C:\Windows\System32.
Copies system file
Backdoor:Win32/Agent.FS copies the system file "<system folder>\mtxex.dll" to another file with the name "<system folder>\msvatu.dll". The copied file is then used to store malware data.
Deletes files
Backdoor:Win32/Agent.FS deletes the currently-running copy of TrojanDropper:Win32/Agent.gen!I by running the following command:
rundll32 C:\WINDOWS\system32\nwagentsdt.dll NwcSvcGet <dropper path and file name>
Note that the NwcSvcGet module of Backdoor:Win32/Agent.FS is used to delete a file specified as its input parameter. Thus, Backdoor:Win32/Agent.FS can be used in theory to delete any file specified as its input parameter.
Connects to a remote server
Backdoor:Win32/Agent.FS connects to the remote server "" to download additional components and possibly to retrieve commands from an attacker.
Allows backdoor access and control
Backdoor:Win32/Agent.FS searches for a running Internet Explorer process. When found, it performs the following command:
netstat.exe -n ><system folder>\msasn.dll
This command displays TCP connections, addresses, and port numbers. The displayed information is saved in the file "<system folder>\msasn.dll", which Backdoor:Win32/Agent.FS creates.
Backdoor:Win32/Agent.FS then searches for connections to port 445 and 135 in the log file. If these ports are open, the trojan then uses these ports to allow backdoor access and control to the computer.
Backdoor:Win32/Agent.FS also searches for proxy servers that the computer connects to. It first checks if the proxy is enabled in the following registry subkeys:
  • HKU\S-1-5-18\Software\Microsoft\Windows\CurrentVersion\Internet Settings
  • HKU\S-1-5-19\Software\Microsoft\Windows\CurrentVersion\Internet Settings
  • HKU\S-1-5-20\Software\Microsoft\Windows\CurrentVersion\Internet Settings
If the value "ProxyEnable" does not have data equal to 0 (zero), then it retrieves data from the registry value "ProxyServer". Backdoor:Win32/Agent.FS then allows access to the infected computer through this proxy server.
Analysis by Elda Dimakiling


System changes
The following system changes may indicate the presence of this malware:
  • The presence of the following files:
    <system folder>\nwagentsdt.dll
    <system folder>\msvatu.dll
    <system folder>\msasn.dll 
  • The presence of the following service:


Alert level: Severe
First detected by definition:
Latest detected by definition: 1.77.911.0 and higher
First detected on: Jun 19, 2009
This entry was first published on: Mar 30, 2010
This entry was updated on: Apr 17, 2011

This threat is also detected as:
  • W32/Atraps.AGNL (Norman)
  • Agent2.ALTC (AVG)
  • TR/ATRAPS.Gen (Avira)