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  2. Security Response/
  3. Trojan.Oshidor


Risk Level 1: Very Low

November 22, 2013
November 28, 2013 9:21:08 AM
Infection Length:
Systems Affected:
Windows 2000, Windows 7, Windows 95, Windows 98, Windows Me, Windows NT, Windows Server 2003, Windows Server 2008, Windows Vista, Windows XP
When the Trojan is executed, it creates the following mutex to ensure that only one copy of the Trojan is running on the compromised computer:

The Trojan then copies itself to the following location:
%UserProfile%\Application Data\[FIVE RANDOM CHARACTERS OR NUMBERS].exe

The Trojan then creates the following registry entry so that it runs every time Windows starts:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run\"EpsonPLJDriver" = "%UserProfile%\Application Data\[FIVE RANDOM CHARACTERS OR NUMBERS].exe"

The Trojan then deletes all registry entries under the following registry subkeys:
  • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SafeBoot\Minimal\
  • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SafeBoot\Network\

After this, the Trojan generates an encryption key, which is a string made up of 15 random characters or numbers. This key is needed to decrypt encrypted files.

Next, the Trojan will send the encryption key to the following command-and-control (C&C) server:

If the Trojan successfully connects to the C&C server, the server will return two strings. The Trojan then saves the two strings in the following registry entries:
  • HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\"email" = "[VALUE RECEIVED FROM C&C SERVER]"
  • HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\"id" = "[VALUE RECEIVED FROM C&C SERVER]"

The Trojan then creates the following file and writes the string "Completed" in the file:
%UserProfile%\Application Data\textnote.txt

This file is then encrypted to the following location:
%UserProfile%\Application Data\textnote.txt.oshit

Next, the Trojan searches for files with the following file extensions on the compromised computer in order to encrypt them:
  • .txt
  • .xls
  • .xlw
  • .docx
  • .doc
  • .cer
  • .key
  • .rtf
  • .xlsm
  • .xlsx
  • .xlc
  • .docm
  • .xlk
  • .htm
  • .chm
  • .text
  • .ppt
  • .djvu
  • .pdf
  • .lzo
  • .djv
  • .cdx
  • .cdt
  • .cdr
  • .bpg
  • .xfm
  • .dfm
  • .pas
  • .dpk
  • .dpr
  • .frm
  • .vbp
  • .php
  • .js
  • .wri
  • .css
  • .asm
  • .html
  • .jpg
  • .dbx
  • .dbt
  • .dbf
  • .odc
  • .mde
  • .mdb
  • .sql
  • .abw
  • .pab
  • .vsd
  • .xsf
  • .xsn
  • .pps
  • .lzh
  • .pgp
  • .arj
  • .md
  • .gz
  • .pst
  • .xl

Once the Trojan encrypts a file, the encrypted file is saved as the following file name:

The Trojan then deletes the original file. The Trojan also creates the following file in the folder of the original file:

After this, the Trojan displays a pop-up message that tells the user that their files are encrypted. The message asks the user to input a password in order to decrypt them.

The Trojan will also kill the following processes:
  • taskmgr.exe
  • regedit.exe


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: Liang Yuan
Summary| Technical Details| Removal

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