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15 November 2003
13 February 2007
Also Known As:
Win32.Skoob.B [Computer Associ, Downloader-DH.b [McAfee], Trojan.Win32.TalkStocks [Kaspe
Systems Affected:

Downloader.MSCache is a collection of several programs that download and install software from a predetermined Web site.

Antivirus Protection Dates

  • Initial Rapid Release version 17 November 2003
  • Latest Rapid Release version 23 March 2017 revision 037
  • Initial Daily Certified version 17 November 2003
  • Latest Daily Certified version 23 March 2017 revision 041
  • Initial Weekly Certified release date 17 November 2003
Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Downloader.MSCache consists of the following components:
  • A randomly named .dll file, which is 36,864 bytes in size. This component has been distributed as a .cab archive with a random file name. The archive contains the .dll and a .inf file, with matching random file names. When loaded, the .dll downloads the file, Randomiser.exe, which is 7,680 bytes in size. Randomizer.exe is often found in %System%, but may also be found in the Temporary Internet Files:
    C:\Documents and Settings\Owner\Local Settings\Temporary Internet Files\Content.IE5\[random_folder]

    %System% is a variable. By default, this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).

  • Randomiser.exe downloads mscache2.exe and mscache2.dll, and saves them in %Windir% with random names.
  • Mscache2.exe, which is 114,688 bytes in size. This component attempts to download and execute content from a Web site, which is not currently accessible.
  • Mscache2.dll, which is 122,880 or 131,072 bytes in size. This component is installed as a browser helper object that can download and install updates of itself.

NOTE: The contents of MScache2.exe and MScache2.dll may differ since they are being downloaded from a remote server.

When executed, the trojan performs the following actions:

1. Adds the following registry keys:
    • q1
    • q2
    • q3
    • q4
    • q5
    • q6
    • q7
    • q8
    • q8
    • q10
    • installed
    to HKLM\SOFTWARE\Microsoft\[random_reg_name]

2. Creates a text file, mslog.tmp, in %Windir%. This file contains the value of [random_reg_name].

Note: Depending on how a system was infected, all of these components will not necessarily be present.


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.

The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.
  1. Disable System Restore (Windows Me/XP).
  2. Update the virus definitions.
  3. Disconnect from the Internet.
  4. Restart the computer in Safe mode or VGA mode.
  5. Unregister the browser helper object.
  6. Remove any registry keys that may have been added.
  7. Run a full system scan and delete all the files detected as Downloader.MSCache.
  8. Clear the Temporary Internet Files folder, if required.
For specific details on each of these steps, read the following instructions.

1. To disable System Restore (Windows Me/XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.

Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.

Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.

For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:
Note: When you are completely finished with the removal procedure and are satisfied that the threat has been removed, re-enable System Restore by following the instructions in the aforementioned documents.

For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article, "Antivirus Tools Cannot Clean Infected Files in the _Restore Folder ," Article ID: Q263455.

2. To update the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:
  • Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
  • Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).

    The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.

3. To disconnect from the Internet
When the .dll file is unregistered, as suggested in step 5, it opens a Web site in a browser window. Depending on the content of the Web page, this could cause the computer to become re-infected. To prevent this, disconnect from the Internet before continuing.

4. To restart the computer in Safe mode or VGA mode
  1. Shut down the computer and turn off the power. Wait for at least 30 seconds, and then restart the computer in Safe mode or VGA mode.
  2. Do one of the following:
    • Windows 95/98/Me: If you see a log-on box, log on as usual. Then proceed to step 5.
    • Windows NT/2000/XP: When the log on box appears, log on as an Administrator.

    • If you are running Windows NT/2000/XP, do not skip this step. You must log on using an administrator account. If you log on using an account that does not have full permissions, you may not be able to delete all the infected files, especially if they are in the Temporary Internet Files folder.
    • If you are not sure how to log on as an Administrator, contact your Network Administrator (if you are on a corporate network), the computer vendor, or whomever set up your computer.

5. To unregister the browser helper object
Before performing this step, you will need the full path and file name of the .dll, which is installed as a browser helper object. It is usually found in the Windows folder (Windows 95/98/Me/XP) or the Winnt folder (Windows NT/2000).

The name of this file will have the form:

<6-8 random lower-case characters>.dll

The file size will be 122,880 or 131,072 bytes.

Note: There have been reports that the file has also been found in the Temporary Internet Files folder.

If you are not sure of the file name, first run a full system scan (see step 6) and record the path and file names, but do not delete the infected files when running the scan. (If you are not familiar with Windows file management, this may be the easiest way to do this.)

Once you know the path and file name to this file (or files, as there may be more than one), do the following for each one:
  1. Click Start, and then click Run. (The Run dialog box appears.)
  2. Type:

    regsvr32 /u "<path to dll>"

    For example:

    regsvr32 /u "c:\windows\zyxwabcd.dll"
  3. Click OK.
  4. At this point, an Internet Explorer dialog box may appear. Close it if it does.

Note: Symantec Security Response has received corrupted samples of the Downloader.MSCache dll. If you see an error message after attempting to unregister the dll, disregard it and proceed to step 6.

6. To edit the registry:
CAUTION : We strongly recommend that you back up the system registry before making any changes. Incorrect changes to the registry could result in permanent data loss or corrupted files. Please make sure you modify only the keys specified. Please see the document How to back up the Windows registry before proceeding.
  1. Open the file, %Windir%\mslog. This file contains the value of [random_reg_name].
  2. Click Start, and click Run. The Run dialog box appears.
  3. Type regedit and then click OK. The Registry Editor opens.
  4. Delete the following subkey:

  5. Click Registry, and then click Exit to save the changes.

7. To scan for and delete the infected files
  1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
  2. Run a full system scan.
  3. If any files are detected as Downloader.MSCache, click Delete.
  4. If your Symantec antivirus program detects any infected files that it cannot delete, record the location of the file and the file name. Then do one of the following:
    • If the file is in a location other than the Temporary Internet Files folder, using Windows Explorer, browse to and delete that particular file. When you have deleted the file, proceed to step e of this section.
    • If the file is in the Temporary Internet Files folder, write down the entire path and file name, and then follow the instructions in section 7.
  5. Restart the computer in Normal mode. For instructions, read the section on returning to Normal mode in the document, "How to start the computer in Safe Mode."

8. To clear the Temporary Internet Files folder, if required
  1. Restart the computer in Normal mode. For instructions, read the section on returning to Normal mode in the document, "How to start the computer in Safe Mode."
  2. Log onto the computer using the name that was shown in the path that you wrote down in step d.

    For example, if the path was:

    C:\Documents and Setting\Linda\Local Settings\Temporary Internet Files\qrwmqczd.dll

    log onto the computer as Linda.
  3. Start Internet Explorer.
  4. Click the Tools menu > Internet Options.
  5. In the Temporary Internet Files section, click the Delete Files button.
  6. Check "Delete all offline content," and then click OK.

Writeup By: Heather Shannon