1. Symantec/
  2. Security Response/
  3. Trojan.Ascesso


Risk Level 1: Very Low

August 28, 2007
August 28, 2007 9:29:58 PM
Infection Length:
32,768 bytes
Systems Affected:
When Trojan.Ascesso is executed it first tries to load its driver in Kernel memory.

To do that, the Trojan enumerates system services in the following registry subkey:

It looks for a service which runs at startup and is currently loaded in Kernel memory. (e.g. Beep.sys)

When a suitable service is found, the Trojan makes a backup copy of the original .SYS file in memory and temporarily overwrites the file on disk with its own malicious driver of 63 KB. Next, the Trojan restarts this service in order to load the dropped malicious driver and finally it restores the original file on disk to avoid detection.

If the driver gets loaded successfully, the threat creates the following file:
%System%\drivers\asc3550[RANDOM LETTER].sys

Next, it creates also the following registry subkeys for the driver:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\asc3550[RANDOM LETTER]\"ErrorControl" = "0x0"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\asc3550[RANDOM LETTER]\"Start" = "0x2"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\asc3550[RANDOM LETTER]\"Tag" = "0x55"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\asc3550[RANDOM LETTER]\"Type" = "0x1"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\asc3550[RANDOM LETTER]\"Group" = "SCSI miniport"

Where [RANDOM LETTER] is a single letter. E.g. "asc3550v.sys"

Note: The services "asc3550" and "asc3550p" are legitimate Windows services.

The Trojan uses the following rootkit techniques to hide itself in the system:
  • Hooks "iofCallDriver" function in Windows Kernel
  • Patches TCPIP.SYS and WANARP.SYS drivers to bypass local firewalls
  • Uses "CmRegisterCallback" to register a function that will hide its registry subkey

When the Trojan is active, the driver file is deleted from:
%System%\drivers\asc3550[RANDOM LETTER].sys

And is temporarily copied to the following location:

The Trojan changes the following registry key so that when the system starts,
it will copy the file %Windir%\smsys.dat back to %System%\drivers\asc3550[RANDOM LETTER].sys
again and the threat will be executed:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\"PendingFileRenameOperations" = "%SystemRoot%smsys.dat %SystemRoot%System32\drivers\asc3550[RANDOM LETTER].sys"

It may also create the following registry subkeys:

The Trojan injects a variable user-mode payload into SERVICES.EXE process space.

Depending of the payload variant, the Trojan may:
  • Download and execute a remote file into %System%\[RANDOM_DIGITS]ld.exe
  • Contact a remote server to get instructions and configurations
  • Update itself
  • Send spam and contact the following hosts on TCP port 25:
  • Send the following discovery packet over UDP port 1900 several times:
    M-SEARCH * HTTP/1.1
    ST: [VALUE]
    MAN: ssdp:discover
    MX: 3

Where [VALUE] could be

  • "urn:schemas-upnp-org:device:InternetGatewayDevice:1"
  • "urn:schemas-upnp-org:device:WANIPConnection:1"
  • "urn:schemas-upnp-org:device:WANPPPConnection:1"

May download a remote file and copy it into the download folder of the following P2P programs:

  • Kazaa
  • iMesh
  • Morpheus

The file is copied into the folder with one of the following names:

  • BeboTV_setup.exe
  • Myspace_setup.exe
  • Myspace_searcher.exe
  • Myspace_people.exe
  • MSDN_loader.exe
  • Opera_install.exe
  • IE7.exe
  • WinXP SP2 crack.exe
  • PGP serial code.exe
  • Opera_9.10_International_Setup.exe
  • Windows Vista keygen.exe
  • World cup viewer.exe
  • metacafe_submit.exe
  • metacafe_play_video.exe
  • Metacafe4Windows.exe
  • radioblog_join.exe
  • Wikipedia_setup.exe

It has been reported that the threat attempts to download the following files:
  • [http://]
  • [http://][REMOVED]
  • [http://][REMOVED]
  • [http://]www.gamers-basement.be/tmp/18.[REMOVED]
  • [http://][REMOVED]


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.
Writeup By: Elia Florio
Summary| Technical Details| Removal

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